Journal of X-Ray Science and Technology 23 (2015) 135–140 DOI 10.3233/XST-150476 IOS Press

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Evaluation of the dose perturbation around gold and steel fiducial markers in a medical linac through Geant4 Monte Carlo simulation Antonio Pontorieroa,b , Ernesto Amatoa,∗ , Giuseppe Iatìb , Costantino De Renzisa,b and Stefano Pergolizzia,b a Department

of Biomedical Sciences and of Morphologic and Functional Imaging, University of Messina, Messina, Italy b Radiotherapy Unit, University Hospital “G. Martino”, Messina, Italy Received 3 April 2014 Revised 4 January 2015 Accepted 20 January 2015 Abstract. OBJECTIVE: Purpose of this work was to study the dose perturbation within the target volume of a external MV radiation therapy when using metal fiducials. METHODS: We developed a Monte Carlo simulation in Geant4 of a cylindrical fiducial made either of gold or of steel and simulated the photon irradiation beam originating from a medical Linac operating at 6, 10 or 15 MV. For each energy, two different irradiation schemes were simulated: a single 5 × 5-cm square field in the −x direction, and five 5 × 5-cm fields at 0◦ , 80◦ , 165◦ , 195◦ and 280◦ . RESULTS: In a single beam irradiation scheme, we observed a dose reduction behind fiducials varying from −20% for gold at 6 MV to −5% for steel at 15 MV, and a dose increment in front of the fiducial ranging from +33% for gold at 15 MV to +10% for steel at 6 MV. When five beams were employed, a dose increment ranging from +28% to +46% has been found around gold. Around a steel fiducial, an average increment of +17% was found, irrespective of the photon energy. CONCLUSIONS: When using a single beam, the decrement of dose behind both steel and gold markers increases with the photon energy. This effect vanishes when a multifield treatment is delivered; in this instance there is a dose increment around fiducials, according to both fiducial material and photon energy, with lower values for steel and 6 MV. This energy represents the best choice when fiducial markers are present inside the irradiated volume. Keywords: Radiotherapy, Monte Carlo, dose, fiducial marker x-rays

1. Introduction Fiducial markers are thin metal rods currently used as a positional reference in radiotherapeutic treatments employing image-guided systems. Nowadays, advanced technologies are adopted to deliver high ∗

Corresponding author: Ernesto Amato, Department of Biomedical Sciences and of Morphologic and Functional Imaging, University of Messina, via Consolare Valeria, 1 – Pad. E, I-98125 Messina, Italy. Tel.: +39 090 2212942; Fax: +39 090 2212843; E-mail: [email protected]. c 2015 – IOS Press and the authors. All rights reserved 0895-3996/15/$35.00 

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conformal dose to the tumour target while sparing normal tissues [1]. In particular, during treatments of prostate cancer, the internal motion of the prostate during a high conformational radiation treatment is compensated by means of ultrasound or x-ray imaging localization [2,3] of intra-prostatic metallic fiducial markers [4–6]. Usually, three to four gold seeds are implanted into the prostate through a trans-rectal procedure. Since the seed has a photon mass attenuation coefficient larger than that of normal tissue, the absorbed dose distribution in the tissue surrounding the seed is modified. As a consequence, it is necessary to quantify such a change in dose [7], since it would affect the iso-dose distribution and the dose volume histogram employed to determine both the tumor control probability and normal tissue complication probability. Monte Carlo simulation of the radiation transport and interaction had been previously employed by other research groups, in order to evaluate the dose perturbation around gold fiducials. In 2006, Chow and Grigorov [8] used a EGSnrc code to evaluate the dose profiles near a gold fiducial implanted in soft tissue, when irradiated by a 6-MV or 18-MV single beam of photons. They found doses at the upstream surface of the fiducial 1.64 and 1.56 times higher than that evaluated at 5 mm of distance, for 6-MV and 18-MV beams, respectively. They also reported a significant shadowing effect behind the fiducial, with both upstream and downstream dose perturbations extending up to about 3 mm of distance from the center of the fiducial. In 2012, Vassiliev et al. [9], using another EGSnrc simulation, confirmed the results of Chow and Grigorov at the same energies, and studied the case of two opposite beams. They also simulated the effects of a carbon fiducial, obtaining dose changes within 1%. More recently, Shiinoki et al. [10] carried out other EGSnrc simulations, aimed to study the dose perturbation induced by a gold seed in lung tissue at 6 MV, using a single beam or two opposite beams. Concerning dose increments at the upstream surface in a single beam regime, they found values higher for lung tissue (1.91) with respect to water data (1.63). In our study we evaluated, through a Geant4 Monte Carlo simulation, the dose perturbation induced by the presence of gold or steel fiducials implanted in a simplified abdomen model, for 6, 10 and 15 MV photon spectra generated by a medical linac. The steel fiducial was characterized for the first time to our knowledge, and we studied, besides the single beam irradiation geometry, also the five fields setup, typical of the prostate treatments. 2. Materials and methods We developed a Monte Carlo simulation based on Geant4 release 9.4.p04. Geant4 [11] is a simulation toolkit originally developed for high energy physics, currently applied also in the field of medical radiation physics [11–15]. The simulation reproduced a cylindrical fiducial made either of gold or of stainless steel, placed at the centre of a simplified abdomen model, represented by an elliptical cylinder made of soft tissue with a density of 1.0 g/cm3 [16]. The metallic fiducial was 1 mm in diameter and 5 mm in length, while the phantom had a size of 16 cm, 30 cm and 40 cm along x, y and z axis, respectively. The phantom was immersed in a cubic world volume of air with 2 m side. The simulated geometry is represented in Fig. 1. We simulated the photon irradiation originating from a medical linac equipped with a flattening filter and operating at 6, 10 or 15 MV. Photon emission spectra were taken from Ref. [17]. For each energy, two different irradiation schemes were simulated: a single 5 × 5-cm square field in the −x direction, and five 5 × 5-cm fields at 0◦ , 80◦ , 165◦, 195◦ and 280◦ with respect to the x axis, in the xy plane. Figure 2 represents the tracks of photons in the second irradiation regime when they interact with the phantom.

A. Pontoriero et al. / Evaluation of the dose perturbation around fiducial markers

Fig. 1. Trans-axial view of the simulated geometry.

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Fig. 2. Trans-axial view of the 5-field irradiation scheme.

The Geant4 code used in this work has been derived from the ones developed for previous works dealing with the interaction of photons and electrons with matter [18,19]. The Low Energy Electromagnetic Physics Package, using the Livermore evaluated data libraries, was adopted as it is specifically addressed to low energy physics domain. The range cut was chosen equal to 10 μm, since it ensures a satisfactory accuracy in the estimation of the volume distribution of the dose, taking into account the smallest physical structure simulated. i.e. the fiducial marker. We tracked 5 × 108 events for each run, so that the statistical uncertainties (2σ ) associated with the presented results were below 2%, a value lower than the uncertainties on the parameterizations of the experimental crosssections employed by Geant4 in the energy range explored in our study [20,21]. We expressed our results in terms of relative per cent dose variation, defined as: ΔD = 100 × (Dfid − D0 )/D

(1)

Where Dfid is the dose in presence of fiducials, and D0 is the dose in the same point in absence of a fiducial. Concerning the first irradiation regime, in which a single beam was directed in the −x direction, the dose was scored in parallelepiped voxels of 0.2 × 1 × 4 mm of dimension, while for the second irradiation regime, consisting in the above described five beams, the dose was scored in cylindrical layers, with a thickness of 50 μm and a height of 4 mm. 3. Results In Figs 3–5 the dose profiles along x direction for a single beam on gold and steel fiducials are presented for 6, 10 and 15 MV photons, respectively. The dose profile in absence of a metal fiducial is reported, as a reference. The first three columns of Table 1 report the maximum and minimum values in the dose variations, as defined in Eq. (1), observed respectively in front and behind each fiducial. The dose increment in front of the fiducial, i.e. backwards with respect to the beam direction, is due to the backscatter of electrons from the entrance surface of the metal, and increases with the photon energy, ranging from +23% at 6 MV to +33% at 15 MV for gold and from +10% at 6 MV to +13% at 15 MV for steel. The dose decrement behind the fiducial is due to the shielding effect, resulting in a shadow of

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Table 1 The minimum and maximum per cent dose variations as a function of fiducial material, source energy and beams configuration. Material Gold Steel

Dose var. Max Min Max Min

6 MV 23 −20 10 −13

1 beam 10 MV 30 −14 12 −8

15 MV 33 −6 13 −5

6 MV 28 − 17 −

5 beams 10 MV 38 − 17 −

15 MV 46 − 17 −

Fig. 3. Dose profiles along the x direction of a 6-MV single beam irradiation. The beam is directed towards the negative x values.

Fig. 4. Dose profiles along the x direction of a 10-MV single beam irradiation. The beam is directed towards the negative x values.

Fig. 5. Dose profiles along the x direction of a 15-MV single beam irradiation. The beam is directed towards the negative x values.

Fig. 6. Radial dose profiles using five beams: comparison between gold and steel fiducials.

dose, casted by the bulk metal cylinder behind it. Such an effect reduces with photon energy, ranging from −20% at 6 MV to −6% at 15 MV for gold and from −13% at 6 MV to −5% at 15 MV for steel. In all cases, the dose perturbations extend up to about 3 mm from the center of the fiducial. In Fig. 6, the radial dose profiles in the case of five irradiation field on gold and steel fiducials are reported for 6, 10 and 15 MV photon energies. The results, summarized in the last three columns of Table 1 exhibit an average dose increment ranging from +28% to +46% around gold and of +17% around steel.

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4. Discussion and conclusions The radiation dose escalation to the prostate gland is one method of attempting to increase local control rate in prostate cancer. Using advanced techniques some authors reported on the ability to deliver up to 85 Gy [22]. The use of fiducial markers to account for organ motion has been widely reported, including a few reports on the dose perturbation caused by fiducials [7–10]. In agreement with previously reported results, we calculated a relevant dose perturbation around a metal fiducial, when using a single perpendicular radiation beam. In particular, Vassiliev et al. [9] reported significant dose increments in the upstream surface of a gold fiducial exposed to 6-MV and 18-MV single beams, together with significant dose reductions at the downstream surface. Concerning gold fiducials, our results at 6 MV confirm the data measured by Chow and Grigorov [7], who reported 21% of dose increase above the fiducial, and 22% of dose reduction below it. This beam-target geometry, however, is not applied in clinical radiation therapy, where multifield techniques are used instead. The dose perturbation near the metal fiducial becomes less relevant when more beams are delivered. Indeed, Vassiliev et al. [9] observed that the dose reduction was within 5% at 6 MV and 2% at 18 MV, when using two parallel and opposed beams. At the best of our knowledge, the present study is the first report on dose perturbation using a common clinical beam setup (five beams) and confirms a dose increment around a gold fiducial which increases with photon energy, while a 17% average dose increment is observed around a steel fiducial, irrespective of photon energy. However, it is to be pointed out explicitly that steel fiducials are contraindicated when MRI is required. Dose increment around both steel and gold markers may cause undesirable dose “hot spots” into the treated volume. Theoretically, when fiducials are into the GTV (gross tumor volume) this increment could be desirable. In general, a dose “hot spot” should be considered significant, following the approach proposed in the ICRU Report Nr. 50 [23], when the diameter exceeds 1.5 cm, i.e. spherical volumes greater than 1.8 cm3 . However, when small organs at risk have to be taken into account, Ref. [23] points out the need to take care of dose inhomogeneities at smaller scales. Therefore, even if, generally, the hot spot around a fiducial irradiated by five fields is not significant due to its small dimension (about 6 mm of diameter), such a perturbation can become significant near small organs at risk, as the urethra in the case of prostate treatments. As a consequence, according to our results, the fiducials should be placed far from small organs at risk, and within the constraints required by the IGRT system. In conclusion, the knowledge of perturbation dose values for different fiducials, as well as their size and orientation, may help to a better marker positioning with respect to the tumor site.

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Evaluation of the dose perturbation around gold and steel fiducial markers in a medical linac through Geant4 Monte Carlo simulation.

Purpose of this work was to study the dose perturbation within the target volume of a external MV radiation therapy when using metal fiducials...
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