International Journal of
Radiation Oncology biology
physics
www.redjournal.org
Physics Contribution
Development and Clinical Implementation of a Universal Bolus to Maintain Spot Size During Delivery of Base of Skull Pencil Beam Scanning Proton Therapy Stefan Both, PhD,* Jiajian Shen, PhD,*,y Maura Kirk, MMP,* Liyong Lin, PhD,* Shikui Tang, PhD,* Michelle Alonso-Basanta, MD, PhD,* Robert Lustig, MD,* Haibo Lin, PhD,* Curtiland Deville, MD,* Christine Hill-Kayser, MD,* Zelig Tochner, MD,* and James McDonough, PhD* *Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania; and y Department of Radiation Oncology, Mayo Clinic, Phoenix, Arizona Received Nov 27, 2013, and in revised form Apr 30, 2014. Accepted for publication May 3, 2014.
Summary Range shifter significantly increases in-air spot sizes of proton pencil beams when used for the treatment of tumors at shallow depth. The clinical implementation of a universal bolus that is placed closer to the patient shows that the proton beam properties are preserved and provides improved dosimetric benefits for a variety of skull base tumors.
Purpose: To report on a universal bolus (UB) designed to replace the range shifter (RS); the UB allows the treatment of shallow tumors while keeping the pencil beam scanning (PBS) spot size small. Methods and Materials: Ten patients with brain cancers treated from 2010 to 2011 were planned using the PBS technique with bolus and the RS. In-air spot sizes of the pencil beam were measured and compared for 4 conditions (open field, with RS, and with UB at 2- and 8-cm air gap) in isocentric geometry. The UB was applied in our clinic to treat brain tumors, and the plans with UB were compared with the plans with RS. Results: A UB of 5.5 cm water equivalent thickness was found to meet the needs of the majority of patients. By using the UB, the PBS spot sizes are similar with the open beam (P>.1). The heterogeneity index was found to be approximately 10% lower for the UB plans than for the RS plans. The coverage for plans with UB is more conformal than for plans with RS; the largest increase in sparing is usually for peripheral organs at risk. Conclusions: The integrity of the physical properties of the PBS beam can be maintained using a UB that allows for highly conformal PBS treatment design, even in a simple geometry of the fixed beam line when noncoplanar beams are used. Ó 2014 Published by Elsevier Inc.
Reprint requests to: Stefan Both, PhD, Department of Radiation Oncology, University of Pennsylvania, 3400 Civic Center Blvd, TRC 2
Int J Radiation Oncol Biol Phys, Vol. 90, No. 1, pp. 79e84, 2014 0360-3016/$ - see front matter Ó 2014 Published by Elsevier Inc. http://dx.doi.org/10.1016/j.ijrobp.2014.05.005
West, Philadelphia, PA 19104. Tel: (215) 662-3694; E-mail: Stefan.Both@ uphs.upenn.edu Conflict of interest: none.
Both et al.
International Journal of Radiation Oncology Biology Physics
Introduction
efficiently, the minimum proton energy in our system is limited to 100 MeV, which is equivalent to a proton range of 7.79 cm water equivalent thickness (WET) (4). A Lexanmade range shifter (RS) with a measured WET of 7.41 cm attached on the end of the snout is designed for treating shallow targets. The use of RS, however, results in an increase of the spot size and decrease in the dose conformality for complex targets such as base of skull (BOS) tumors, which may compromise the advantage of PBS over intensity modulated radiation therapy in terms of high-dose conformality. It is reported that the distance of the air gap between the RS and the patient has a strong influence on the achievable lateral spot sizes (5, 6). The cause of this relationship is that the proton’s angular distribution at the downstream surface of the RS translates into a geometric spread of the beam’s cross-sections at the patient surfaces as a function of distance (6). To achieve the smallest spot size, it is desirable to minimize the distance between the RS and the patient surface. In this article we report our investigation on the design and clinical implementation of a universal bolus (UB) based on patient-specific data to replace the RS. By using a UB to treat shallow targets, we can minimize the air gap between the patient and the bolus, and so the integrity of the spot size can be maintained. This UB has been successfully used in our clinic for targets located in brain, head and neck, and spine level.
80
Proton therapy is rapidly increasing in use worldwide owing to its well-known advantage of low integrated dose and ability to spare healthy tissues (1). Even simple proton beam arrangements can provide precise and conformal dose distributions to targets while sparing adjacent organs at risk (OARs). Passive scattering and pencil beam scanning (PBS) are the 2 commonly used techniques for delivering proton beams. In the PBS technique, the target is “painted” by sequential superposition of many small-size PBS spots, which are magnetically directed to the target. The intensity of each spot in PBS can be optimized to achieve a highly conformal target dose and to spare the OARs to a greater degree than with passive scattering techniques. Moreover, the PBS technique does not need patient-specific devices, such as aperture and compensator, which are necessary in passive scattering. Thus, the proton PBS technique is particularly appealing. Because each PBS spot is a building element of the desired dose distribution, the PBS dosimetry is determined by the basic parameters of the PBS spots. The lateral spot size is one of these basic parameters that determine the slope of the dose falloff in the penumbra regions. It has been reported that smaller spot sizes could significantly reduce the penumbra (2, 3), which would give more room for sparing OARs while maintaining the target coverage. This is imperative for brain, skull base, and head and neck cancers in close proximity to critical OARs. The Roberts Proton Therapy Center at the Hospital of University of Pennsylvania has 4 gantries and 1 fixed beam line treatment room, whose source is a cyclotron with a fixed energy of 230 MeV at its exit (Ion Beam Applications, Louvain-La-Neuve, Belgium). An energy degrader is used to reduce the proton beam energy to clinically required ranges. The efficiency of the beam is reduced when a degrader is used because it scatters the proton beam and spreads in energy space. For proton energies
Radiation Oncology biology
physics
www.redjournal.org
Physics Contribution
Development and Clinical Implementation of a Universal Bolus to Maintain Spot Size During Delivery of Base of Skull Pencil Beam Scanning Proton Therapy Stefan Both, PhD,* Jiajian Shen, PhD,*,y Maura Kirk, MMP,* Liyong Lin, PhD,* Shikui Tang, PhD,* Michelle Alonso-Basanta, MD, PhD,* Robert Lustig, MD,* Haibo Lin, PhD,* Curtiland Deville, MD,* Christine Hill-Kayser, MD,* Zelig Tochner, MD,* and James McDonough, PhD* *Department of Radiation Oncology, University of Pennsylvania, Philadelphia, Pennsylvania; and y Department of Radiation Oncology, Mayo Clinic, Phoenix, Arizona Received Nov 27, 2013, and in revised form Apr 30, 2014. Accepted for publication May 3, 2014.
Summary Range shifter significantly increases in-air spot sizes of proton pencil beams when used for the treatment of tumors at shallow depth. The clinical implementation of a universal bolus that is placed closer to the patient shows that the proton beam properties are preserved and provides improved dosimetric benefits for a variety of skull base tumors.
Purpose: To report on a universal bolus (UB) designed to replace the range shifter (RS); the UB allows the treatment of shallow tumors while keeping the pencil beam scanning (PBS) spot size small. Methods and Materials: Ten patients with brain cancers treated from 2010 to 2011 were planned using the PBS technique with bolus and the RS. In-air spot sizes of the pencil beam were measured and compared for 4 conditions (open field, with RS, and with UB at 2- and 8-cm air gap) in isocentric geometry. The UB was applied in our clinic to treat brain tumors, and the plans with UB were compared with the plans with RS. Results: A UB of 5.5 cm water equivalent thickness was found to meet the needs of the majority of patients. By using the UB, the PBS spot sizes are similar with the open beam (P>.1). The heterogeneity index was found to be approximately 10% lower for the UB plans than for the RS plans. The coverage for plans with UB is more conformal than for plans with RS; the largest increase in sparing is usually for peripheral organs at risk. Conclusions: The integrity of the physical properties of the PBS beam can be maintained using a UB that allows for highly conformal PBS treatment design, even in a simple geometry of the fixed beam line when noncoplanar beams are used. Ó 2014 Published by Elsevier Inc.
Reprint requests to: Stefan Both, PhD, Department of Radiation Oncology, University of Pennsylvania, 3400 Civic Center Blvd, TRC 2
Int J Radiation Oncol Biol Phys, Vol. 90, No. 1, pp. 79e84, 2014 0360-3016/$ - see front matter Ó 2014 Published by Elsevier Inc. http://dx.doi.org/10.1016/j.ijrobp.2014.05.005
West, Philadelphia, PA 19104. Tel: (215) 662-3694; E-mail: Stefan.Both@ uphs.upenn.edu Conflict of interest: none.
Both et al.
International Journal of Radiation Oncology Biology Physics
Introduction
efficiently, the minimum proton energy in our system is limited to 100 MeV, which is equivalent to a proton range of 7.79 cm water equivalent thickness (WET) (4). A Lexanmade range shifter (RS) with a measured WET of 7.41 cm attached on the end of the snout is designed for treating shallow targets. The use of RS, however, results in an increase of the spot size and decrease in the dose conformality for complex targets such as base of skull (BOS) tumors, which may compromise the advantage of PBS over intensity modulated radiation therapy in terms of high-dose conformality. It is reported that the distance of the air gap between the RS and the patient has a strong influence on the achievable lateral spot sizes (5, 6). The cause of this relationship is that the proton’s angular distribution at the downstream surface of the RS translates into a geometric spread of the beam’s cross-sections at the patient surfaces as a function of distance (6). To achieve the smallest spot size, it is desirable to minimize the distance between the RS and the patient surface. In this article we report our investigation on the design and clinical implementation of a universal bolus (UB) based on patient-specific data to replace the RS. By using a UB to treat shallow targets, we can minimize the air gap between the patient and the bolus, and so the integrity of the spot size can be maintained. This UB has been successfully used in our clinic for targets located in brain, head and neck, and spine level.
80
Proton therapy is rapidly increasing in use worldwide owing to its well-known advantage of low integrated dose and ability to spare healthy tissues (1). Even simple proton beam arrangements can provide precise and conformal dose distributions to targets while sparing adjacent organs at risk (OARs). Passive scattering and pencil beam scanning (PBS) are the 2 commonly used techniques for delivering proton beams. In the PBS technique, the target is “painted” by sequential superposition of many small-size PBS spots, which are magnetically directed to the target. The intensity of each spot in PBS can be optimized to achieve a highly conformal target dose and to spare the OARs to a greater degree than with passive scattering techniques. Moreover, the PBS technique does not need patient-specific devices, such as aperture and compensator, which are necessary in passive scattering. Thus, the proton PBS technique is particularly appealing. Because each PBS spot is a building element of the desired dose distribution, the PBS dosimetry is determined by the basic parameters of the PBS spots. The lateral spot size is one of these basic parameters that determine the slope of the dose falloff in the penumbra regions. It has been reported that smaller spot sizes could significantly reduce the penumbra (2, 3), which would give more room for sparing OARs while maintaining the target coverage. This is imperative for brain, skull base, and head and neck cancers in close proximity to critical OARs. The Roberts Proton Therapy Center at the Hospital of University of Pennsylvania has 4 gantries and 1 fixed beam line treatment room, whose source is a cyclotron with a fixed energy of 230 MeV at its exit (Ion Beam Applications, Louvain-La-Neuve, Belgium). An energy degrader is used to reduce the proton beam energy to clinically required ranges. The efficiency of the beam is reduced when a degrader is used because it scatters the proton beam and spreads in energy space. For proton energies
