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The effects of titanium mesh on passive-scattering proton dose
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Phys. Med. Biol. 59 N81 (http://iopscience.iop.org/0031-9155/59/10/N81) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 155.97.178.73 This content was downloaded on 28/05/2014 at 09:17
Please note that terms and conditions apply.
Institute of Physics and Engineering in Medicine Phys. Med. Biol. 59 (2014) N81–N89
Physics in Medicine and Biology
doi:10.1088/0031-9155/59/10/N81
Note
The effects of titanium mesh on passive-scattering proton dose H Lin, X Ding, L Yin, H Zhai, H Liu, A Kassaee, C Hill-Kayser, R A Lustig, J McDonough and S Both Department of Radiation Oncology, University of Pennsylvania, 3400 Civic Blvd, 4-314W TRC, PCAM, Philadelphia, PA 19104, USA E-mail: [email protected] Received 6 December 2013, revised 27 February 2014 Accepted for publication 18 March 2014 Published 28 April 2014 Abstract
High-density metallic implants can introduce considerable uncertainties in proton therapy treatment planning. These uncertainties eventually translate into proton range errors, which may cause significant underdosing to the target volume or overdosing to normal tissue beyond the target. This study investigated the dosimetric impact of a 0.6 mm titanium (Ti) mesh implant in passivescattering proton beam therapy through the study of the depth dose and output in water, and the dose profiles in solid water at various depths. The measurements were performed for a beam with a range of 8.5 cm and a modulation of 7.5 cm. The titanium mesh was placed at a depth of 1 cm below the surface of the phantom for all measurements. A range reduction of 0.5 ± 0.1 mm was observed for a beam perpendicular to the mesh, with no further reductions when the incident angle increased to 60◦ . We conclude that the dosimetric effect of a 0.6 mm titanium mesh implant is small for a passive scattering proton beam. With proper correction applied to metal artifacts, consistent results were observed in the phantom study in the treatment planning system. Keywords: titanium mesh, dosimetry, proton therapy (Some figures may appear in colour only in the online journal) 1. Introduction The use of proton therapy has expanded rapidly in recent years. The main advantage of proton therapy versus conventional photon external beam radiotherapy is its ability to deliver most of the energy to the target volume and virtually no exit dose beyond it. However, the proton beam exhibits increased sensitivity to changes in the material through which it passes, and treatment quality can be compromised due to range uncertainties and setup errors. One of the important sources of range uncertainty is the complex spatial distribution of heterogeneities. 0031-9155/14/100081+09$33.00
© 2014 Institute of Physics and Engineering in Medicine Printed in the UK & the USA
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Phys. Med. Biol. 59 (2014) N81
Metallic implants, such as dental implants and Ti mesh implants can cause spatial heterogeneity resulting in streak artifacts in three-dimensional (3D) computed tomography (CT) scans. These artifacts cause uncertainties in the dose distribution calculated by the treatment planning system (TPS) (Verburg et al 2010). High density metal implants such as titanium can also saturate the CT numbers of the CT calibration curve entered in the TPS. When the CT numbers of the implant or the artifact regions are not properly corrected and the image is used for treatment planning, the associated uncertainties may be further increased (Park et al 2013). For this reason, beam orientation should ideally be selected to avoid traversing such implants at the time of planning. However, to preserve organs at risk (OARs) and/or achieve better coverage of the target, some clinical cases require the treatment beam to pass through an implant. Titanium mesh implants are clinically appealing as they are strong, light-weight, foldable and magnetically and biologically compatible. Perhaps most importantly, these implants cause fewer artifacts on CT scans compared to other metal mesh implants, allowing improved clinical follow-up using comprehensive imaging. The dosimetric impact of Ti mesh on photon therapy has been widely studied by several groups (Patone et al 2006, Rakowski et al 2012, Shimozato et al 2010). In 2001, (Patone et al (2006)) studied the effect of Ti mesh in a patient treated with 6 and 18 MV photon beams and concluded that the dosimetric impact of a 0.4 mm Ti mesh is negligible and does not require modification in treatment parameters. (Rakowski et al (2012)) reported that due to the existence of a Ti mesh, the dose perturbations in MV photon beams immediately prior to and after the Ti mesh ranged from −18% to 23%, while less than 3% alteration was observed for the delivered Gamma Knife dose. To the best of our knowledge, the impact of Ti mesh on proton therapy has not yet been studied. About 10% of patients with brain tumor resection in our institution had Ti mesh implanted and most of them received radiotherapy subsequently. The clinical scenario highlighting these concerns is presented below. 2. Materials and methods 2.1. Clinical case
A clinical case of a brain boost using passive-scattering (PS) proton therapy is shown in figure 1. A 36 year old male patient was diagnosed with a malignant lesion of the cerebellum. Prior to radiation the patient underwent a craniotomy and resection of the tumor and the skull was reconstructed with Ti mesh. The patient underwent craniospinal irradiation to 36 Gy which was followed by a proton boost to the tumor bed to a total dose of 54 Gy. The clinical target volume (CTV) (4.5 × 4.5 × 4 cm3) is located between the mesh and the brain stem. Two oblique beams were used for the treatment. Range uncertainty concerns arose for the left-posterior oblique (LPO) field regarding the presence of a mesh in the beam path and the brainstem’s position at the distal end of the field. The purpose of this work was to investigate the dosimetry impact of Ti mesh to PS proton therapy when such an implant is present along the beam path. During surgical skull reconstructions at our institution, a 0.6 mm neurosurgical Ti mesh plate (FlexMeshTM , Medtronic Neurosurgery, Goleta, CA, USA) or Synthes (MatrixNEURO, Synthes Inc, West Chester, PA, USA) is typically used. As shown in figure 2(a), a sample mesh (11.3 × 7.7 cm2) from Medtronic was studied in this work. The mesh was made of 0.75 mm diameter Ti wire and the metal-to-hole ratio was approximately 0.85. Measurements were performed for a ∼160 MeV (energy entering the nozzle) PS proton beam on an IBA proton therapy system (Ion Beam Applications SA, Louvain-la-Neuve, Belgium) with a range (R) of 8.5 cm and a modulation (M) of 7.5 cm (R8.5M7.5). This selection of the proton N82
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Figure 1. A brain case with titanium mesh implanted. The target volume is located in between the mesh implant and the brainstem. Two beams were used in which an LPO beam passes through the implant and toward the brainstem. The artifacts from the implant were minimal due to the proximity of the mesh to the high density bone. In planning, they were overridden with HU of sampled fat and tissue in the artifact-free area.
(a)
(c)
(b)
Figure 2. (a) Titanium mesh implant (thickness of 0.6 mm, the area of the metal to air ratio ∼ 0.85); (b) setup for range, modulation and output measurements (0.05 cc, PPC05 chamber); (c) setup for profile measurements: red dashed lines indicate the profile of the open area versus the profile with mesh, shown in figure 5.
beam was based on the clinical case mentioned above. The compensator was not in place during measurements in order to reduce the sources of uncertainties. All measurements were performed with the Ti mesh placed at a depth of 1 cm in the phantom in a source-to-axial distance (SAD) geometry. 2.2. Proton range measurements
As shown in figure 2(b), the measurements were performed in a motorized Scanditronix– Wellh¨ofer water tank (Blue Phantom, IBA dosimetry, Bartlett, TN, USA) using a parallel plate N83
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ionization chamber (0.05 cc, PPC05 chamber, IBA dosimetry, Bartlett, TN, USA). To study the impact on proton range (depth at 90% of the distal fall-off), the percent depth dose (PDD) curve was collected at five random positions in the fields (at and off the center of the field) for both open and implant field alternately. During the measurement, a single proton beam was delivered and paused a couple of times during delivery to allow for mesh insertion and/or removal. The PDD of the open field was compared to the PDD of the field with the Ti implant. The measurements were repeated to explore the beam angle dependence for an incidence up to 60◦ to the mesh. In addition, three proton beams with different ranges were delivered to investigate the energy dependence. 2.3. Lateral dose profile measurements
Two-dimensional (2D) dose distributions within the water equivalent square phantom (30 cm × 30 cm2 Solid Water, Gammex, Middleton, WI) were measured using R Gafchromic films (EBT2, International Specialty Products MD-55(16)). All films came from the same batch and were placed in the high dose and low gradient regions, which were immediately above and below the mesh as well as at depths of 1.5, 2, and 4.5 cm (the middle of the spread-out Bragg peak (SOBP)) in the phantom. A 1.5 Gy (RBE) PS proton beam of R8.5M7.5 was delivered to the phantom. For the sake of comparison, the mesh plate was covered by half of the 15 × 15 cm2 field and the other half was left open as a reference, as R film, the dose could shown in figure 2(c). Due to the quenching effect of the Gafchromic be underestimated by up to 20% in the Bragg peak region (Reinhardt et al 2012); a relative comparison of the profile was conducted for film measurements. The films were scanned 24 h after irradiation under the same condition with an Epson Expression 10000 XL flatbed transmission scanner (Epson America Inc, Long Beach, CA, USA). The results were analyzed with Omnipro IMRT film analysis software (IBA dosimetry, Bartlett, TN, USA). Each profile was normalized to the averaged dose at the center area of the open reference field. 2.4. Phantom study in the Eclipse treatment planning system
The same phantom setup used in the lateral dose profile measurements was scanned with our Siemens Sensation Open CT scanner (Siemens Medical Solutions USA, Inc, Malvern, PA, USA) using our proton brain protocol. The CT images with 1.5 mm slice thickness were imported into the Eclipse (Varian Medical System Inc, Palo Alto, CA, USA) TPS. The proton convolution superposition (version 10.0.28) algorithm with a calculation grid size of 2.5 mm was used for dose calculation. All the planning related parameters were kept the same as those used in the clinical case. As shown in figure 3, a single proton beam was placed on the phantom and the mesh covered half of the proton field. The CT artifacts due to the mesh were observed above and below the implant. To assess the impact of CT metal artifacts on proton range, a comparison on range pullback was carried out with and without overrides of their CT number with the averaged CT number in the corresponding depth at the open area. The PDD and lateral profile were also investigated in order to understand the limitations of the TPS system. For consistency, all depths were converted to water equivalent thickness. 3. Results As shown in figure 4, a consistent range reduction of 0.5 ± 0.1 mm was observed for perpendicularly incident beams. When the incident angle increased up to 60◦ , no significant N84
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Figure 3. Phantom study on the effects of titanium mesh on PS proton dose.
Figure 4. Range comparisons between open field (black) versus with mesh (red) from measurements and range comparison from eclipse TPS for open field (gray), with mesh with (green) and without corrected artifacts (cyan). The overridden HU was taken from the averaged HU in the corresponding volumes of the open field.
further reduction was found. No significant energy dependence was found for range reduction (0.43 ± 0.11 mm). Profiles with the implant are compared to the profiles of an open area in figure 5. No notable dose perturbation was observed immediately above (not shown) or below the implant (figure 5(a)). Although a small dose enhancement of 1–2% was detected at a depth of 1.5 cm in the phantom, the difference became negligible at a depth of 2 cm. The dose heterogeneity caused by the implant became more pronounced as depth in tissue increased. The maximal amplitude of the dose fluctuation was found to be around 6% at the middle of the SOBP and the periodic dose fluctuation was caused by the regular pattern of the implant. The range reductions were studied in the TPS and the results are presented in figure 4. Due to artifacts from the metal implant, a range reduction of 1.1 mm was observed. When the CT numbers of the artifacts were corrected with averaged CT numbers of corresponding N85
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(a)
(b)
(c)
(d)
Figure 5. (a)–(d) Profile comparison between open area and with mesh at depth of 1 cm. The films were located at the depth of (a) 1 cm (below mesh), (b) 1.5 cm, (c) 2 cm and (d) 4.5 cm (middle of the SOBP) from the surface of the phantom. Two profiles were measured with films and indicated as red dashed lines in figure 2(c).
volume in open field, the calculated range reduction of 0.6 mm matched well with measured values. To compare the output change at the middle of the SOBP with and without mesh, the lateral dose profile across mesh and open area was plotted in figure 6. The change in output was found to be negligible. As indicated by figure 3, the 100% isodose line (yellow) was perturbed into vertical strips by the mesh. A pullback effect was observed on the 90% isodose line (green) at both the distal and proximal ends. Due to the limitations of the calculation grid size, a dose averaging effect was observed for dose fluctuations caused by mesh at various depths. A ∼1% calculated dose enhancement was revealed, which is similar to what was found via film measurement at a shallow depth (figure 5(b)).
4. Discussion The presence of metal implants near the target volume can cause local spatial heterogeneity, which may affect the dosimetric accuracy of proton therapy. Studies have been performed to understand all the effects that will impact the particle’s range (Fredriksson et al 2011, Nichiporov et al 2011). The purpose of this study was to investigate the dosimetric effect in clinical proton therapy due to the presence of a Ti mesh for skull reconstruction and identify the necessary steps to be performed during the treatment planning phase. N86
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Figure 6. Lateral profile at the middle of the SOBP across the mesh implant and open
area. The vertical dotted line indicates the border between mesh and open area. This was a partial lateral profile of 15 × 15 cm2 PS proton field shown in figure 3.
(Moskvin et al (2012)) proposed a semi-empirical model to estimate the therapeutic range shift caused by inhomogeneities in proton beam therapy. Although the range shift was calculated based on the thickness, atomic number (Z) and density of the high-Z implant without the knowledge of proton energy or the location of the implant, the authors claimed that the predicted range shift was within 2.5% accuracy compared to the FLUKA Monte Carlo results. According to this model, the range reductions from a 0.6 mm solid Ti slab in the beam path was about 1.3 mm for a 160 MeV proton beam. For a given mesh plate of 0.6 mm, the range reduction could be between 0 and 1.3 mm depending on the hole pattern design (Moskvin et al 2012). Due to the fact that this mesh plate was manufactured with holes in a dominant pattern (metal:hole ∼0.85), a range reduction of 0.5 ± 0.1 mm may be expected. The thickness of the cranial Ti mesh varied, but it was generally equal to or less than 0.7 mm (Rakowski et al 2012), which corresponds to a range reduction of up to 1.6 mm. Depending on the designed hole pattern, the range reduction for any skull reconstruction Ti mesh with the same thickness was less than 1.6 mm. In general, adequate lateral margins were added during the treatment planning process to accommodate possible geometric misses. Most importantly, the finite range of proton particles potentially led to geometrical misses at the distal end of the target volume. Thus, distal margins were introduced to ensure sufficient target coverage. In our institution, we used a calculated distal margin of 3.5% of the proton range plus 3 mm for all PS proton planning. The 3.5% was to account for uncertainties during the conversion of CT number to relative proton linear stopping power (Moyers et al 2001) and recently it was proved to be still valid (Yang et al 2012). The 3 mm was added to correct for range uncertainty due to the manufacture of compensator and proton energy calibration. Depending on the complexity of the case, the distal margins were adjusted to allow enough coverage of the target or to be more sparing for deep-seated OARs, e.g., instead of 3.5%, 5% would be used for a case with pronounced artifacts. Thus, the typical distal margins for brain planning are 6–8 mm. The distal margin for brain treatment was generally several times that of the range reduction of a thin Ti mesh, but the impact of the metal artifacts and the limitation of the N87
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TPS were not considered. One of the most important sources of range error was from the CT artifacts caused by the metal implants. The magnitude of this kind of error could be clinically significant and is highly dependent on the geometry of the implant and the orientation of the proton beam relative to the artifacts. In recent years, techniques have been developed to reduce the different CT artifacts (Hertanto et al 2012, Li et al 2012, Wei et al 2006) or quantify their dosimetric influence (Jakel and Reiss 2007). Many challenges still exist. For example, the complex patient specific anatomy and geometry makes it difficult to determine the correct CT number for artifacts. For the case studied in this report, due to the shallow depth and the simple anatomy surrounding the mesh, the artifacts were local and minimal. Our phantom study indicated that the artifacts from a mesh had almost equal power for range pullback as the mesh itself and therefore artifacts must be corrected to improve the quality of the plan. Thus, an averaged CT number of sampled brain tissue or fat was assigned to artifacts based on their locations. Although we know the mesh was made of titanium, its CT number should not be overridden simply because the dimension of the mesh was exaggerated in the CT image. Improper correction of the CT number of the metal device can cause significant range and dosimetry errors. It is recommended that any CT number correction on a high density implant should be evaluated individually for each clinical case. For proton therapy, due to the finite range of the proton beam in tissue, the dose at the distal end of each field should be carefully evaluated. During the planning stage, we recommend that the beam angle be selected to minimize uncertainties or errors that could arise within the beam path. However, other factors may dictate that the optimal beam pass through a metal implant, e.g. LPO field in figure 1. Our study indicated that the dose impact from the thin Ti mesh is minimal for the deep-seated targets and therefore should allow them to be treated without under or over dosage. Concerns arose regarding the brainstem located next to the distal end of the target along the LPO field. To reduce the potential dose uncertainties to the brainstem, a second field RPO was chosen so both the mesh and the brainstem were within the lateral penumbra area of the field. Supported by our study, the patient proceeded to treatment without alteration of the planning parameters except for overriding the metal artifacts. The dose inhomogeneity caused by the implant may need to be evaluated for each individual case. The method used in this study can be used to evaluate other implants for proton therapy. 5. Conclusions The results indicated that the impact of the 0.6 mm Ti mesh implant on dosimetry was within 1%, and less than 0.5 mm on the proton range. Therefore, the decision regarding whether or not to override the mesh should be carefully evaluated during the planning process, while the CT artifacts caused by the implant need to be overridden in order to reduce the level of uncertainty. References Fredriksson A, Forsgren A and Hardemark B 2011 Minimax optimization for handling range and setup uncertainties in proton therapy Med. Phys. 38 1672–84 Hertanto A, Zhang Q H, Hu Y C, Dzyubak O, Rimmer A and Mageras G S 2012 Reduction of irregular breathing artifacts in respiration-correlated CT images using a respiratory motion model Med. Phys. 39 3070–9 Jakel O and Reiss P 2007 The influence of metal artefacts on the range of ion beams Phys. Med. Biol. 52 635–44 Li H, Giles W, Ren L, Bowsher L and Yin F F 2012 Implementation of dual-energy technique for virtual monochromatic and linearly mixed CBCTs Med. Phys. 39 6056–64 N88
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Moskvin V, Cheng C W, Fanelli L, Zhao L and Das I J 2012 A semi-empirical model for the therapeutic range shift estimation caused by inhomogeneities in proton beam therapy J. Appl. Clin. Med. Phys. 13 3–12 Moyers M F, Miller D W, Bush D A and Slater J D 2001 Methodologies and tools for proton beam design for lung tumors Int. J. Radiat. Oncol. Biol. Phys. 49 1429–38 Nichiporov D, Moskvin V, Fanelli L and Das I J 2011 Range shift and dose perturbation with high-density materials in proton beam therapy Nucl. Instrum. Methods Phys. Res. B 269 2685–92 Park P C et al 2013 Statistical assessment of proton treatment plans under setup and range uncertainties Int. J. Radiat. Oncol. Biol. Phys. 86 1007–13 Patone H, Barker J and Roberge D 2006 Effects of neurosurgical titanium mesh on radiation dose Med. Dosim. 31 298–301 Rakowski J T, Chin K and Mittal S 2012 Effects of titanium mesh implant on dosimetry during gamma knife radiosurgery J. Appl. Clin. Med. Phys. 13 54–61 Reinhardt S, Hillbrand M, Wilkens J J and Assmann W 2012 Comparison of Gafchromic EBT2 and EBT3 films for clinical photon and proton beams Med. Phys. 39 5257–62 Shimozato T, Yasui K and Kawanami R 2010 Dose distribution near thin titanium plate for skull fixation irradiated by a 4-MV photon beam J. Appl. Clin. Med. Phys. 35 81–87 Verburg J M, Joshi M C, Madden T M, Kooy H M and Seco J C 2010 Impact of CT metal artifacts on proton radiotherapy treatment planning Int. J. Radiat. Oncol. Biol. Phys. 78 S809–10 Wei J K, Sandison G A, His W C, Ringor M and Lu X Y 2006 Dosimetric impact of a CT metal artefact suppression algorithm for proton, electron and photon therapies Phys. Med. Biol. 51 5183–97 Yang M, Zhu X R, Park P C, Titt U, Mohan R, Virshup G, Clayton J E and Dong L 2012 Comprehensive analysis of proton range uncertainties related to patient stopping-power-ratio estimation using the stoichiometric calibration Phys. Med. Biol. 57 4095–115
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The effects of titanium mesh on passive-scattering proton dose
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2014 Phys. Med. Biol. 59 N81 (http://iopscience.iop.org/0031-9155/59/10/N81) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 155.97.178.73 This content was downloaded on 28/05/2014 at 09:17
Please note that terms and conditions apply.
Institute of Physics and Engineering in Medicine Phys. Med. Biol. 59 (2014) N81–N89
Physics in Medicine and Biology
doi:10.1088/0031-9155/59/10/N81
Note
The effects of titanium mesh on passive-scattering proton dose H Lin, X Ding, L Yin, H Zhai, H Liu, A Kassaee, C Hill-Kayser, R A Lustig, J McDonough and S Both Department of Radiation Oncology, University of Pennsylvania, 3400 Civic Blvd, 4-314W TRC, PCAM, Philadelphia, PA 19104, USA E-mail: [email protected] Received 6 December 2013, revised 27 February 2014 Accepted for publication 18 March 2014 Published 28 April 2014 Abstract
High-density metallic implants can introduce considerable uncertainties in proton therapy treatment planning. These uncertainties eventually translate into proton range errors, which may cause significant underdosing to the target volume or overdosing to normal tissue beyond the target. This study investigated the dosimetric impact of a 0.6 mm titanium (Ti) mesh implant in passivescattering proton beam therapy through the study of the depth dose and output in water, and the dose profiles in solid water at various depths. The measurements were performed for a beam with a range of 8.5 cm and a modulation of 7.5 cm. The titanium mesh was placed at a depth of 1 cm below the surface of the phantom for all measurements. A range reduction of 0.5 ± 0.1 mm was observed for a beam perpendicular to the mesh, with no further reductions when the incident angle increased to 60◦ . We conclude that the dosimetric effect of a 0.6 mm titanium mesh implant is small for a passive scattering proton beam. With proper correction applied to metal artifacts, consistent results were observed in the phantom study in the treatment planning system. Keywords: titanium mesh, dosimetry, proton therapy (Some figures may appear in colour only in the online journal) 1. Introduction The use of proton therapy has expanded rapidly in recent years. The main advantage of proton therapy versus conventional photon external beam radiotherapy is its ability to deliver most of the energy to the target volume and virtually no exit dose beyond it. However, the proton beam exhibits increased sensitivity to changes in the material through which it passes, and treatment quality can be compromised due to range uncertainties and setup errors. One of the important sources of range uncertainty is the complex spatial distribution of heterogeneities. 0031-9155/14/100081+09$33.00
© 2014 Institute of Physics and Engineering in Medicine Printed in the UK & the USA
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Phys. Med. Biol. 59 (2014) N81
Metallic implants, such as dental implants and Ti mesh implants can cause spatial heterogeneity resulting in streak artifacts in three-dimensional (3D) computed tomography (CT) scans. These artifacts cause uncertainties in the dose distribution calculated by the treatment planning system (TPS) (Verburg et al 2010). High density metal implants such as titanium can also saturate the CT numbers of the CT calibration curve entered in the TPS. When the CT numbers of the implant or the artifact regions are not properly corrected and the image is used for treatment planning, the associated uncertainties may be further increased (Park et al 2013). For this reason, beam orientation should ideally be selected to avoid traversing such implants at the time of planning. However, to preserve organs at risk (OARs) and/or achieve better coverage of the target, some clinical cases require the treatment beam to pass through an implant. Titanium mesh implants are clinically appealing as they are strong, light-weight, foldable and magnetically and biologically compatible. Perhaps most importantly, these implants cause fewer artifacts on CT scans compared to other metal mesh implants, allowing improved clinical follow-up using comprehensive imaging. The dosimetric impact of Ti mesh on photon therapy has been widely studied by several groups (Patone et al 2006, Rakowski et al 2012, Shimozato et al 2010). In 2001, (Patone et al (2006)) studied the effect of Ti mesh in a patient treated with 6 and 18 MV photon beams and concluded that the dosimetric impact of a 0.4 mm Ti mesh is negligible and does not require modification in treatment parameters. (Rakowski et al (2012)) reported that due to the existence of a Ti mesh, the dose perturbations in MV photon beams immediately prior to and after the Ti mesh ranged from −18% to 23%, while less than 3% alteration was observed for the delivered Gamma Knife dose. To the best of our knowledge, the impact of Ti mesh on proton therapy has not yet been studied. About 10% of patients with brain tumor resection in our institution had Ti mesh implanted and most of them received radiotherapy subsequently. The clinical scenario highlighting these concerns is presented below. 2. Materials and methods 2.1. Clinical case
A clinical case of a brain boost using passive-scattering (PS) proton therapy is shown in figure 1. A 36 year old male patient was diagnosed with a malignant lesion of the cerebellum. Prior to radiation the patient underwent a craniotomy and resection of the tumor and the skull was reconstructed with Ti mesh. The patient underwent craniospinal irradiation to 36 Gy which was followed by a proton boost to the tumor bed to a total dose of 54 Gy. The clinical target volume (CTV) (4.5 × 4.5 × 4 cm3) is located between the mesh and the brain stem. Two oblique beams were used for the treatment. Range uncertainty concerns arose for the left-posterior oblique (LPO) field regarding the presence of a mesh in the beam path and the brainstem’s position at the distal end of the field. The purpose of this work was to investigate the dosimetry impact of Ti mesh to PS proton therapy when such an implant is present along the beam path. During surgical skull reconstructions at our institution, a 0.6 mm neurosurgical Ti mesh plate (FlexMeshTM , Medtronic Neurosurgery, Goleta, CA, USA) or Synthes (MatrixNEURO, Synthes Inc, West Chester, PA, USA) is typically used. As shown in figure 2(a), a sample mesh (11.3 × 7.7 cm2) from Medtronic was studied in this work. The mesh was made of 0.75 mm diameter Ti wire and the metal-to-hole ratio was approximately 0.85. Measurements were performed for a ∼160 MeV (energy entering the nozzle) PS proton beam on an IBA proton therapy system (Ion Beam Applications SA, Louvain-la-Neuve, Belgium) with a range (R) of 8.5 cm and a modulation (M) of 7.5 cm (R8.5M7.5). This selection of the proton N82
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Figure 1. A brain case with titanium mesh implanted. The target volume is located in between the mesh implant and the brainstem. Two beams were used in which an LPO beam passes through the implant and toward the brainstem. The artifacts from the implant were minimal due to the proximity of the mesh to the high density bone. In planning, they were overridden with HU of sampled fat and tissue in the artifact-free area.
(a)
(c)
(b)
Figure 2. (a) Titanium mesh implant (thickness of 0.6 mm, the area of the metal to air ratio ∼ 0.85); (b) setup for range, modulation and output measurements (0.05 cc, PPC05 chamber); (c) setup for profile measurements: red dashed lines indicate the profile of the open area versus the profile with mesh, shown in figure 5.
beam was based on the clinical case mentioned above. The compensator was not in place during measurements in order to reduce the sources of uncertainties. All measurements were performed with the Ti mesh placed at a depth of 1 cm in the phantom in a source-to-axial distance (SAD) geometry. 2.2. Proton range measurements
As shown in figure 2(b), the measurements were performed in a motorized Scanditronix– Wellh¨ofer water tank (Blue Phantom, IBA dosimetry, Bartlett, TN, USA) using a parallel plate N83
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Phys. Med. Biol. 59 (2014) N81
ionization chamber (0.05 cc, PPC05 chamber, IBA dosimetry, Bartlett, TN, USA). To study the impact on proton range (depth at 90% of the distal fall-off), the percent depth dose (PDD) curve was collected at five random positions in the fields (at and off the center of the field) for both open and implant field alternately. During the measurement, a single proton beam was delivered and paused a couple of times during delivery to allow for mesh insertion and/or removal. The PDD of the open field was compared to the PDD of the field with the Ti implant. The measurements were repeated to explore the beam angle dependence for an incidence up to 60◦ to the mesh. In addition, three proton beams with different ranges were delivered to investigate the energy dependence. 2.3. Lateral dose profile measurements
Two-dimensional (2D) dose distributions within the water equivalent square phantom (30 cm × 30 cm2 Solid Water, Gammex, Middleton, WI) were measured using R Gafchromic films (EBT2, International Specialty Products MD-55(16)). All films came from the same batch and were placed in the high dose and low gradient regions, which were immediately above and below the mesh as well as at depths of 1.5, 2, and 4.5 cm (the middle of the spread-out Bragg peak (SOBP)) in the phantom. A 1.5 Gy (RBE) PS proton beam of R8.5M7.5 was delivered to the phantom. For the sake of comparison, the mesh plate was covered by half of the 15 × 15 cm2 field and the other half was left open as a reference, as R film, the dose could shown in figure 2(c). Due to the quenching effect of the Gafchromic be underestimated by up to 20% in the Bragg peak region (Reinhardt et al 2012); a relative comparison of the profile was conducted for film measurements. The films were scanned 24 h after irradiation under the same condition with an Epson Expression 10000 XL flatbed transmission scanner (Epson America Inc, Long Beach, CA, USA). The results were analyzed with Omnipro IMRT film analysis software (IBA dosimetry, Bartlett, TN, USA). Each profile was normalized to the averaged dose at the center area of the open reference field. 2.4. Phantom study in the Eclipse treatment planning system
The same phantom setup used in the lateral dose profile measurements was scanned with our Siemens Sensation Open CT scanner (Siemens Medical Solutions USA, Inc, Malvern, PA, USA) using our proton brain protocol. The CT images with 1.5 mm slice thickness were imported into the Eclipse (Varian Medical System Inc, Palo Alto, CA, USA) TPS. The proton convolution superposition (version 10.0.28) algorithm with a calculation grid size of 2.5 mm was used for dose calculation. All the planning related parameters were kept the same as those used in the clinical case. As shown in figure 3, a single proton beam was placed on the phantom and the mesh covered half of the proton field. The CT artifacts due to the mesh were observed above and below the implant. To assess the impact of CT metal artifacts on proton range, a comparison on range pullback was carried out with and without overrides of their CT number with the averaged CT number in the corresponding depth at the open area. The PDD and lateral profile were also investigated in order to understand the limitations of the TPS system. For consistency, all depths were converted to water equivalent thickness. 3. Results As shown in figure 4, a consistent range reduction of 0.5 ± 0.1 mm was observed for perpendicularly incident beams. When the incident angle increased up to 60◦ , no significant N84
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Figure 3. Phantom study on the effects of titanium mesh on PS proton dose.
Figure 4. Range comparisons between open field (black) versus with mesh (red) from measurements and range comparison from eclipse TPS for open field (gray), with mesh with (green) and without corrected artifacts (cyan). The overridden HU was taken from the averaged HU in the corresponding volumes of the open field.
further reduction was found. No significant energy dependence was found for range reduction (0.43 ± 0.11 mm). Profiles with the implant are compared to the profiles of an open area in figure 5. No notable dose perturbation was observed immediately above (not shown) or below the implant (figure 5(a)). Although a small dose enhancement of 1–2% was detected at a depth of 1.5 cm in the phantom, the difference became negligible at a depth of 2 cm. The dose heterogeneity caused by the implant became more pronounced as depth in tissue increased. The maximal amplitude of the dose fluctuation was found to be around 6% at the middle of the SOBP and the periodic dose fluctuation was caused by the regular pattern of the implant. The range reductions were studied in the TPS and the results are presented in figure 4. Due to artifacts from the metal implant, a range reduction of 1.1 mm was observed. When the CT numbers of the artifacts were corrected with averaged CT numbers of corresponding N85
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(a)
(b)
(c)
(d)
Figure 5. (a)–(d) Profile comparison between open area and with mesh at depth of 1 cm. The films were located at the depth of (a) 1 cm (below mesh), (b) 1.5 cm, (c) 2 cm and (d) 4.5 cm (middle of the SOBP) from the surface of the phantom. Two profiles were measured with films and indicated as red dashed lines in figure 2(c).
volume in open field, the calculated range reduction of 0.6 mm matched well with measured values. To compare the output change at the middle of the SOBP with and without mesh, the lateral dose profile across mesh and open area was plotted in figure 6. The change in output was found to be negligible. As indicated by figure 3, the 100% isodose line (yellow) was perturbed into vertical strips by the mesh. A pullback effect was observed on the 90% isodose line (green) at both the distal and proximal ends. Due to the limitations of the calculation grid size, a dose averaging effect was observed for dose fluctuations caused by mesh at various depths. A ∼1% calculated dose enhancement was revealed, which is similar to what was found via film measurement at a shallow depth (figure 5(b)).
4. Discussion The presence of metal implants near the target volume can cause local spatial heterogeneity, which may affect the dosimetric accuracy of proton therapy. Studies have been performed to understand all the effects that will impact the particle’s range (Fredriksson et al 2011, Nichiporov et al 2011). The purpose of this study was to investigate the dosimetric effect in clinical proton therapy due to the presence of a Ti mesh for skull reconstruction and identify the necessary steps to be performed during the treatment planning phase. N86
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Figure 6. Lateral profile at the middle of the SOBP across the mesh implant and open
area. The vertical dotted line indicates the border between mesh and open area. This was a partial lateral profile of 15 × 15 cm2 PS proton field shown in figure 3.
(Moskvin et al (2012)) proposed a semi-empirical model to estimate the therapeutic range shift caused by inhomogeneities in proton beam therapy. Although the range shift was calculated based on the thickness, atomic number (Z) and density of the high-Z implant without the knowledge of proton energy or the location of the implant, the authors claimed that the predicted range shift was within 2.5% accuracy compared to the FLUKA Monte Carlo results. According to this model, the range reductions from a 0.6 mm solid Ti slab in the beam path was about 1.3 mm for a 160 MeV proton beam. For a given mesh plate of 0.6 mm, the range reduction could be between 0 and 1.3 mm depending on the hole pattern design (Moskvin et al 2012). Due to the fact that this mesh plate was manufactured with holes in a dominant pattern (metal:hole ∼0.85), a range reduction of 0.5 ± 0.1 mm may be expected. The thickness of the cranial Ti mesh varied, but it was generally equal to or less than 0.7 mm (Rakowski et al 2012), which corresponds to a range reduction of up to 1.6 mm. Depending on the designed hole pattern, the range reduction for any skull reconstruction Ti mesh with the same thickness was less than 1.6 mm. In general, adequate lateral margins were added during the treatment planning process to accommodate possible geometric misses. Most importantly, the finite range of proton particles potentially led to geometrical misses at the distal end of the target volume. Thus, distal margins were introduced to ensure sufficient target coverage. In our institution, we used a calculated distal margin of 3.5% of the proton range plus 3 mm for all PS proton planning. The 3.5% was to account for uncertainties during the conversion of CT number to relative proton linear stopping power (Moyers et al 2001) and recently it was proved to be still valid (Yang et al 2012). The 3 mm was added to correct for range uncertainty due to the manufacture of compensator and proton energy calibration. Depending on the complexity of the case, the distal margins were adjusted to allow enough coverage of the target or to be more sparing for deep-seated OARs, e.g., instead of 3.5%, 5% would be used for a case with pronounced artifacts. Thus, the typical distal margins for brain planning are 6–8 mm. The distal margin for brain treatment was generally several times that of the range reduction of a thin Ti mesh, but the impact of the metal artifacts and the limitation of the N87
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TPS were not considered. One of the most important sources of range error was from the CT artifacts caused by the metal implants. The magnitude of this kind of error could be clinically significant and is highly dependent on the geometry of the implant and the orientation of the proton beam relative to the artifacts. In recent years, techniques have been developed to reduce the different CT artifacts (Hertanto et al 2012, Li et al 2012, Wei et al 2006) or quantify their dosimetric influence (Jakel and Reiss 2007). Many challenges still exist. For example, the complex patient specific anatomy and geometry makes it difficult to determine the correct CT number for artifacts. For the case studied in this report, due to the shallow depth and the simple anatomy surrounding the mesh, the artifacts were local and minimal. Our phantom study indicated that the artifacts from a mesh had almost equal power for range pullback as the mesh itself and therefore artifacts must be corrected to improve the quality of the plan. Thus, an averaged CT number of sampled brain tissue or fat was assigned to artifacts based on their locations. Although we know the mesh was made of titanium, its CT number should not be overridden simply because the dimension of the mesh was exaggerated in the CT image. Improper correction of the CT number of the metal device can cause significant range and dosimetry errors. It is recommended that any CT number correction on a high density implant should be evaluated individually for each clinical case. For proton therapy, due to the finite range of the proton beam in tissue, the dose at the distal end of each field should be carefully evaluated. During the planning stage, we recommend that the beam angle be selected to minimize uncertainties or errors that could arise within the beam path. However, other factors may dictate that the optimal beam pass through a metal implant, e.g. LPO field in figure 1. Our study indicated that the dose impact from the thin Ti mesh is minimal for the deep-seated targets and therefore should allow them to be treated without under or over dosage. Concerns arose regarding the brainstem located next to the distal end of the target along the LPO field. To reduce the potential dose uncertainties to the brainstem, a second field RPO was chosen so both the mesh and the brainstem were within the lateral penumbra area of the field. Supported by our study, the patient proceeded to treatment without alteration of the planning parameters except for overriding the metal artifacts. The dose inhomogeneity caused by the implant may need to be evaluated for each individual case. The method used in this study can be used to evaluate other implants for proton therapy. 5. Conclusions The results indicated that the impact of the 0.6 mm Ti mesh implant on dosimetry was within 1%, and less than 0.5 mm on the proton range. Therefore, the decision regarding whether or not to override the mesh should be carefully evaluated during the planning process, while the CT artifacts caused by the implant need to be overridden in order to reduce the level of uncertainty. References Fredriksson A, Forsgren A and Hardemark B 2011 Minimax optimization for handling range and setup uncertainties in proton therapy Med. Phys. 38 1672–84 Hertanto A, Zhang Q H, Hu Y C, Dzyubak O, Rimmer A and Mageras G S 2012 Reduction of irregular breathing artifacts in respiration-correlated CT images using a respiratory motion model Med. Phys. 39 3070–9 Jakel O and Reiss P 2007 The influence of metal artefacts on the range of ion beams Phys. Med. Biol. 52 635–44 Li H, Giles W, Ren L, Bowsher L and Yin F F 2012 Implementation of dual-energy technique for virtual monochromatic and linearly mixed CBCTs Med. Phys. 39 6056–64 N88
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Moskvin V, Cheng C W, Fanelli L, Zhao L and Das I J 2012 A semi-empirical model for the therapeutic range shift estimation caused by inhomogeneities in proton beam therapy J. Appl. Clin. Med. Phys. 13 3–12 Moyers M F, Miller D W, Bush D A and Slater J D 2001 Methodologies and tools for proton beam design for lung tumors Int. J. Radiat. Oncol. Biol. Phys. 49 1429–38 Nichiporov D, Moskvin V, Fanelli L and Das I J 2011 Range shift and dose perturbation with high-density materials in proton beam therapy Nucl. Instrum. Methods Phys. Res. B 269 2685–92 Park P C et al 2013 Statistical assessment of proton treatment plans under setup and range uncertainties Int. J. Radiat. Oncol. Biol. Phys. 86 1007–13 Patone H, Barker J and Roberge D 2006 Effects of neurosurgical titanium mesh on radiation dose Med. Dosim. 31 298–301 Rakowski J T, Chin K and Mittal S 2012 Effects of titanium mesh implant on dosimetry during gamma knife radiosurgery J. Appl. Clin. Med. Phys. 13 54–61 Reinhardt S, Hillbrand M, Wilkens J J and Assmann W 2012 Comparison of Gafchromic EBT2 and EBT3 films for clinical photon and proton beams Med. Phys. 39 5257–62 Shimozato T, Yasui K and Kawanami R 2010 Dose distribution near thin titanium plate for skull fixation irradiated by a 4-MV photon beam J. Appl. Clin. Med. Phys. 35 81–87 Verburg J M, Joshi M C, Madden T M, Kooy H M and Seco J C 2010 Impact of CT metal artifacts on proton radiotherapy treatment planning Int. J. Radiat. Oncol. Biol. Phys. 78 S809–10 Wei J K, Sandison G A, His W C, Ringor M and Lu X Y 2006 Dosimetric impact of a CT metal artefact suppression algorithm for proton, electron and photon therapies Phys. Med. Biol. 51 5183–97 Yang M, Zhu X R, Park P C, Titt U, Mohan R, Virshup G, Clayton J E and Dong L 2012 Comprehensive analysis of proton range uncertainties related to patient stopping-power-ratio estimation using the stoichiometric calibration Phys. Med. Biol. 57 4095–115
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