Secondary Neutron Dose Research Articles

Reply to “Comment on ‘Comparison of secondary neutron dose in proton therapy resulting from the use of a tungsten alloy MLC or a brass collimator system’” [Med. Phys. 38(11), 6248–6256 (2011)

Reply to “Comment on ‘Comparison of secondary neutron dose in proton therapy resulting from the use of a tungsten alloy MLC or a brass collimator system’” [Med. Phys. 38(11), 6248–6256 (2011)

Comment on “Comparison of secondary neutron dose in proton therapy resulting from the use of a tungsten alloy MLC or a brass collimator system” [Med. Phys. 38(11), 6248–6256 (2011)

Comment on “Comparison of secondary neutron dose in proton therapy resulting from the use of a tungsten alloy MLC or a brass collimator system” [Med. Phys. 38(11), 6248–6256 (2011)

Pitfalls of tungsten multileaf collimator in proton beam therapy

Particle beam therapy is associated with significant startup and operational cost. Multileaf collimator (MLC) provides an attractive option to improve the efficiency and reduce the treatment cost. A direct transfer of the MLC technology from external beam radiation therapy is intuitively straightforward to proton therapy. However, activation, neutron production, and the associated secondary cancer risk in proton beam should be an important consideration which is evaluated. Monte Carlo simulation with FLUKA particle transport code was applied in this study for a number of treatment models. The authors have performed a detailed study of the neutron generation, ambient dose equivalent [H∗(10)], and activation of a typical tungsten MLC and compared with those obtained from a brass aperture used in a typical proton therapy system. Brass aperture and tungsten MLC were modeled by absorber blocks in this study, representing worst-case scenario of a fully closed collimator. With a tungsten MLC, the secondary neutron dose to the patient is at least 1.5 times higher than that from a brass aperture. The H∗(10) from a tungsten MLC at 10 cm downstream is about 22.3 mSv/Gy delivered to water phantom by noncollimated 200 MeV beam of 20 cm diameter compared to 14 mSv/Gy for the brass aperture. For a 30-fraction treatment course, the activity per unit volume in brass aperture reaches 5.3 × 10⁴ Bq cm(-3) at the end of the last treatment. The activity in brass decreases by a factor of 380 after 24 h, additional 6.2 times after 40 days of cooling, and is reduced to background level after 1 yr. Initial activity in tungsten after 30 days of treating 30 patients per day is about 3.4 times higher than in brass that decreases only by a factor of 2 after 40 days and accumulates to 1.2 × 10⁶ Bq cm(-3) after a full year of operation. The daily utilization of the MLC leads to buildup of activity with time. The overall activity continues to increase due to (179)Ta with a half-life of 1.82 yr and thus require prolonged storage for activity cooling. The H∗(10) near the patient side of the tungsten block is about 100 μSv/h and is 27 times higher at the upstream side of the block. This would lead to an accumulated dose for therapists in a year that may exceed occupational maximum permissible dose (50 mSv/yr). The value of H∗(10) at the upstream surface of the tungsten block is about 220 times higher than that of the brass. MLC is an efficient way for beam shaping and overall cost reduction device in proton therapy. However, based on this study, tungsten seems to be not an optimal material for MLC in proton beam therapy. Usage of tungsten MLC in clinic may create unnecessary risks associated with the secondary neutrons and induced radioactivity for patients and staff depending on the patient load. A careful selection of material for manufacturing of an optimal MLC for proton therapy is thus desired.

Comparison of secondary neutron dose in proton therapy resulting from the use of a tungsten alloy MLC or a brass collimator system

To apply the dual ionization chamber method for mixed radiation fields to an accurate comparison of the secondary neutron dose arising from the use of a tungsten alloy multileaf collimator (MLC) as opposed to a brass collimator system for defining the shape of a therapeutic proton field. Hydrogenous and nonhydrogenous ionization chambers were constructed with large volumes to enable measurements of absorbed doses below 10(-4) Gy in mixed radiation fields using the dual ionization chamber method for mixed-field dosimetry. Neutron dose measurements were made with a nominal 230 MeV proton beam incident on a closed tungsten alloy MLC and a solid brass block. The chambers were cross-calibrated against a (60)Co-calibrated Farmer chamber in water using a 6 MV x-ray beam and Monte Carlo simulations were performed to account for variations in ionization chamber response due to differences in secondary neutron energy spectra. The neutron and combined proton plus γ-ray absorbed doses are shown to be nearly equivalent downstream from either a closed tungsten alloy MLC or a solid brass block. At 10 cm downstream from the distal edge of the collimating material the neutron dose from the closed MLC was (5.3 ± 0.4) × 10(- 5) Gy/Gy. The neutron dose with brass was (6.4 ± 0.7) × 10(- 5) Gy/Gy. Further from the secondary neutron source, at 50 cm, the neutron doses remain close for both the MLC and brass block at (6.9 ± 0.6) × 10(- 6) Gy/Gy and (6.3 ± 0.7) × 10(- 6) Gy/Gy, respectively. The dual ionization chamber method is suitable for measuring secondary neutron doses resulting from proton irradiation. The results of measurements downstream from a closed tungsten alloy MLC and a brass block indicate that, even in an overly pessimistic worst-case scenario, secondary neutron production in a tungsten alloy MLC leads to absorbed doses that are nearly equivalent to those seen from brass collimators. Therefore, the choice of tungsten alloy in constructing the leaves of a proton MLC is appropriate, and does not lead to a substantial increase in the secondary neutron dose to the patient compared to that generated in a brass collimator.

Evaluation of Secondary Neutron Dose in Particle and High Energy X-ray Therapy

Evaluation of Secondary Neutron Dose in Particle and High Energy X-ray Therapy

Epidemiological analysis of patients with prostate cancer treated with proton beams at LLUMC

Abstract The James M. Slater, M.D. Proton Therapy Treatment and Research Center of the Loma Linda University Medical Center (LLUMC) has treated more than 15,000 patients with proton beams since its inception in 1990, and it is responsible for the majority of patients treated with protons in a hospital setting. Moreover, it is one of only a few institutions with approval by the Federal Drug Administration to treat patients with dynamically scanned proton beams with such beams available presently for experimentation at LLUMC. These features place LLUMC in a unique position for determining improved estimates of outcomes and risks associated with proton therapy including more precise risk estimates for second cancers in patients treated with active or passive proton delivery systems An ancillary benefit in the context of establishing the risks of doses in medical applications is that it would provide a much more solid basis for the estimation of effective doses and the application of microdosimetric spectral analyses to such risk estimations. Such results are important as both the number of institutions treating with protons and the number of patients treated with protons will be increasing rapidly over the next few years. The LLUMC prostate cancer group is the largest and most homogeneously treated proton patient group at the facility and has the potential for yielding risk estimates of secondary cancers with reasonable uncertainty, as will be discussed in this presentation. The total dose and dose per fraction for all patients treated to date have a relatively restricted range from 74 Gy to 81 Gy. In addition, the primary treatment planning and delivery technique (opposing laterals with one field treated per day, proton energy range 225–250 MeV) has not been changed from inception. In particular, this group represents a single-institution homogeneous cohort in terms of treatment parameters important for, for example, secondary neutron dose outside the treatment volume. The potential clinical and epidemiological benefit from such treatments has provided the impetus for the development of a comprehensive patient database. A projected accrual has been calculated based upon the current number of proton patients already being followed, their observed survival, and the censoring rate because of loss to follow-up. The analysis is presented and discussed for a patient population restricted to those treated with protons only, i.e., not including those who received combined proton and X-ray treatments.

TU-C-BRB-03: Hazards of a Tungsten Multi-Leaf Collimator in Proton Beam Therapy

Purpose: Charged particle therapy, especially proton therapy is a growing treatment modality worldwide. A direct transfer of the MLC technology to proton therapy seems intuitively straightforward as leakage radiation is not a concern. On the other hand, neutron production and induced radioactivity in tungsten MLC are critical issues that are the subjects of the present study. Methods: We have performed a study with FLUKA Monte Carlo simulation, of the neutron generation, ambient dose equivalent (H*(10)), and activation of a typical tungsten MLC and compared with those obtained from a brass aperture used for proton therapy. Results: With the tungsten MLC, the secondary neutron dose to patient is at least 1.6 times higher than that from the brass aperture. This may lead to an increase on the risk of secondary cancer. The buildup of activity in MLC (30 patients per day) is about 3.4 times higher than that for the brass aperture at the end of 40-day treatment. The residual activity decreases by a factor of 2 after 40 days of MLC cooling, but still is about 7600 times higher than the activity for the brass aperture. The utilization of the MLC at the end of the operation cycle will require prolonged storage for cooling due to W-181 and Ta-179. There are also risks of MLC electronic malfunctioning associated with the soft error caused by secondary neutrons. The tritium production in tungsten MLC as a result of its daily use in the clinic is also estimated. Conclusions: MLC could be an efficient way for beam shaping, an efficient cost reduction option for proton therapy. However, tungsten MLC does not seem to be suitable for proton therapy. Usage of tungsten MCL in clinic will create unnecessary risks associated with the secondary neutrons and induced radioactivity for patients and staff.

MO-F-BRA-03: Secondary Neutron Dose Measurements Using CR-39 Detectors in Photon and Proton Radiotherapy

Purpose: It is assumed that modern radiation treatment techniques such as intensity‐modulated radiotherapy(IMRT), volumetric‐modulated arc therapy (VMAT) and protonradiotherapy are increasing the cancer cure rates and simultaneously reducing side effects. But there are also concerns about radiation‐induced malignancies caused by secondary neutronradiation, in particular, for younger patients. The aim of this study was to measure secondary neutrondose for a radiotherapy patient during typical treatments. Methods: In order to evaluate the neutrondose for a patient, an anthropomorphic Alderson‐Rando phantom was used. The measurements were performed using CR‐39 etched track detectors, placed inside and on the surface of the phantom, giving a three dimensional dose distribution and the organ doses. The neutrondose was compared with respect to treatment technique, therapy machine and radiation quality. The CR‐39 detectors were calibrated using an Am‐Be neutron source and an ICRU cubic phantom. The series of measurements included photon irradiations delivered by treatment machines of the manufacturers Varian, Elekta and Siemens and the treatment techniques 3D‐conformal, IMRT and VMAT. The proton irradiations were performed using active spot scanning and passive scattering techniques. Results: The doses were determined in terms of equivalent dose. In addition, effective dose was calculated using organ specific weighting factors of the ICRP. The effective doses for the complete course of treatment were 3.8 mSv, 3.0 mSv, 3.4 mSv, 0.7 mSv, 2.8 mSv, 20.3 mSv and 26.8 mSv for Varian 3D‐conformal, Varian IMRT, Varian VMAT, Elekta IMRT, Siemens IMRT, spot scanning protons and passively scattered protons, respectively. Conclusions: The differences in neutrondose between the different treatment techniques for photons were not large. However, Elekta machines produce considerably lower neutrondose than Varian and Siemens machines. For protontreatments,neutrondose is higher in general and apparently higher for scattered compared to spot scanned protons.

Analytic estimates of secondary neutron dose in proton therapy

Proton beam losses in various components of a treatment nozzle generate secondary neutrons, which bring unwanted out of field dose during treatments. The purpose of this study was to develop an analytic method for estimating neutron dose to a distant organ at risk during proton therapy. Based on radiation shielding calculation methods proposed by Sullivan, we developed an analytical model for converting the proton beam losses in the nozzle components and in the treatment volume into the secondary neutron dose at a point of interest. Using the MCNPx Monte Carlo code, we benchmarked the neutron dose rates generated by the proton beam stopped at various media. The Monte Carlo calculations confirmed the validity of the analytical model for simple beam stop geometry. The analytical model was then applied to neutron dose equivalent measurements performed on double scattering and uniform scanning nozzles at the Midwest Proton Radiotherapy Institute (MPRI). Good agreement was obtained between the model predictions and the data measured at MPRI. This work provides a method for estimating analytically the neutron dose equivalent to a distant organ at risk. This method can be used as a tool for optimizing dose delivery techniques in proton therapy.

SU‐GG‐T‐413: Comparison of Out‐Of‐Field Neutron Equivalent Doses in Scanning Carbon and Proton Therapies for Cranial Fields

Purpose: The purpose of this analysis is to compare the secondary neutron lateral doses from scanning carbon and proton beam therapies. Method and Materials: We simulated secondary neutron doses for out‐of‐field organs in an 11‐year old male patient. Scanned carbon and proton beams were simulated separately using Monte Carlo techniques. We have used circular aperture field of 6 cm in diameter as a representative field. The tumor was assumed to be in the cranium. The range and modulation width for both carbon and proton beams were set to 15 cm and 10 cm, respectively. Results: In carbon therapy, absorbed neutron doses to tonsils and pharynx close to the field‐edge were found to be 5×10−4 mSv/GyE and 4×10−4 mSv/GyE, respectively. Whereas, neutron equivalent doses to tonsils and pharynx were estimated to be 0.57 mSv/GyE and 0.55 mSv/GyE in scanned proton therapy, respectively. In heavy ion carbon beams neutrons produced inside the patient are emitted at small angles, predominantly in the forward direction, whereas in proton therapy, neutrons are emitted more isotropic. Therefore the absorbed neutron doses in carbon therapy lateral to the field edge are smaller compared to the corresponding proton fields. In forward direction though, the neutron doses from carbon ion therapy can be expected to be higher compared to proton therapy. Conclusions: Secondary neutron doses received by tonsils (out‐of‐field organ) in scanned carbon and proton therapies are found to be 5×10−4 mSv/GyE and 0.6 mSv/GyE, respectively. Organs located laterally away from the field‐edge receive much lower doses (thousand times smaller) in carbon therapy as compared to neutron doses in proton therapy. Whereas, on the distal side the neutron equivalent doses in scanned carbon therapy are a factor of 2 higher compared to similar scanned proton therapy fields.

Influence of beam efficiency through the patient‐specific collimator on secondary neutron dose equivalent in double scattering and uniform scanning modes of proton therapy

Conventional proton therapy facilities use double scattering nozzles, which are optimized for delivery of a few fixed field sizes. Similarly, uniform scanning nozzles are commissioned for a limited number of field sizes. However, cases invariably occur where the treatment field is significantly different from these fixed field sizes. The purpose of this work was to determine the impact of the radiation field conformity to the patient-specific collimator on the secondary neutron dose equivalent. Using a WENDI-II neutron detector, the authors experimentally investigated how the neutron dose equivalent at a particular point of interest varied with different collimator sizes, while the beam spreading was kept constant. The measurements were performed for different modes of dose delivery in proton therapy, all of which are available at the Midwest Proton Radiotherapy Institute (MPRI): Double scattering, uniform scanning delivering rectangular fields, and uniform scanning delivering circular fields. The authors also studied how the neutron dose equivalent changes when one changes the amplitudes of the scanned field for a fixed collimator size. The secondary neutron dose equivalent was found to decrease linearly with the collimator area for all methods of dose delivery. The relative values of the neutron dose equivalent for a collimator with a 5 cm diameter opening using 88 MeV protons were 1.0 for the double scattering field, 0.76 for rectangular uniform field, and 0.6 for the circular uniform field. Furthermore, when a single circle wobbling was optimized for delivery of a uniform field 5 cm in diameter, the secondary neutron dose equivalent was reduced by a factor of 6 compared to the double scattering nozzle. Additionally, when the collimator size was kept constant, the neutron dose equivalent at the given point of interest increased linearly with the area of the scanned proton beam. The results of these experiments suggest that the patient-specific collimator is a significant contributor to the secondary neutron dose equivalent to a distant organ at risk. Improving conformity of the radiation field to the patient-specific collimator can significantly reduce secondary neutron dose equivalent to the patient. Therefore, it is important to increase the number of available generic field sizes in double scattering systems as well as in uniform scanning nozzles.

? Measurement of secondary neutron dose generated during proton beam therapy for craniospinal irradiation.

We measured the secondary neutron dose from proton craniospinal irradiaton and assessed ways to reduce the secondary neutron dose. A polymethyl methacrylate phantom was irradiated with 159.5MeV proton beams, and CR-39 etch detectors were used to measure the neutron dose equivalent to the proton absorbed dose. We compared the neutron doses generated in cases with and without the neutron absorbing blocks. The neutron dose equivalent to the proton dose (H(10)/D) absorbed on the surface of the phantom showed a decrease from 7.24 to 4.76 mSv/Gy with a change in the displacement from the primary field edge and a decrease from 7.24 to 1.06 mSv/Gy with a change in the depth at a 5-cm displacement from the field edge. When 4.32-mm-thick neutron absorbing blocks were added, the neutron dose decreased by about 30%. We conclude that ratio of the neutron dose to the proton absorbed dose for a proton craniospinal irradiation treatment beam on the surface is less than 1% of the prescription dose. We also observed a 30% decrease in the neutron dose on adding 4.32-mm-thick neutron-absorbing blocks. The contribution of secondary neutrons to the integral dose was found to be very low, and it can be reduced further by using high hydrogen-boron containing blocks.

Monte Carlo study on secondary neutrons in passive carbon‐ion radiotherapy: Identification of the main source and reduction in the secondary neutron dose

Recent successful results in passive carbon-ion radiotherapy allow the patient to live for a longer time and allow younger patients to receive the radiotherapy. Undesired radiation exposure in normal tissues far from the target volume is considerably lower than that close to the treatment target, but it is considered to be non-negligible in the estimation of the secondary cancer risk. Therefore, it is very important to reduce the undesired secondary neutron exposure in passive carbon-ion radiotherapy without influencing the clinical beam. In this study, the source components in which the secondary neutrons are produced during passive carbon-ion radiotherapy were identified and the method to reduce the secondary neutron dose effectively based on the identification of the main sources without influencing the clinical beam was investigated. A Monte Carlo study with the PHITS code was performed by assuming the beamline at the Heavy-Ion Medical Accelerator in Chiba (HIMAC). At first, the authors investigated the main sources of secondary neutrons in passive carbon-ion radiotherapy. Next, they investigated the reduction in the neutron dose with various modifications of the beamline device that is the most dominant in the neutron production. Finally, they investigated the use of an additional shield for the patient. It was shown that the main source is the secondary neutrons produced in the four-leaf collimator (FLC) used as a precollimator at HIAMC, of which contribution in the total neutron ambient dose equivalent is more than 70%. The investigations showed that the modification of the FLC can reduce the neutron dose at positions close to the beam axis by 70% and the FLC is very useful not only for the collimation of the primary beam but also the reduction in the secondary neutrons. Also, an additional shield for the patient is very effective to reduce the neutron dose at positions farther than 50 cm from the beam axis. Finally, they showed that the neutron dose can be reduced by approximately 70% at any position without influencing the primary beam used in treatment. This study was performed by assuming the HIMAC beamline; however, this study provides important information for reoptimizing the arrangement and the materials of beamline devices and designing a new facility for passive carbon-ion radiotherapy and probably passive proton radiotherapy.

Reduction of the secondary neutron dose in passively scattered proton radiotherapy, using an optimized pre-collimator/collimator

Proton radiotherapy represents a potential major advance in cancer therapy. Most current proton beams are spread out to cover the tumor using passive scattering and collimation, resulting in an extra whole-body high-energy neutron dose, primarily from proton interactions with the final collimator. There is considerable uncertainty as to the carcinogenic potential of low doses of high-energy neutrons, and thus we investigate whether this neutron dose can be significantly reduced without major modifications to passively scattered proton beam lines. Our goal is to optimize the design features of a patient-specific collimator or pre-collimator/collimator assembly. There are a number of often contradictory design features, in terms of geometry and material, involved in an optimal design. For example, plastic or hybrid plastic/metal collimators have a number of advantages. We quantify these design issues, and investigate the practical balances that can be achieved to significantly reduce the neutron dose without major alterations to the beamline design or function. Given that the majority of proton therapy treatments, at least for the next few years, will use passive scattering techniques, reducing the associated neutron-related risks by simple modifications of the collimator assembly design is a desirable goal.

SU-FF-T-462: Treatment of Retinoblastoma Using Different Proton Delivery Methods: Scattering, Rectangular Scanning, and Circular Scanning

Purpose: The high risk of radiation induced malignancy in retinoblastoma with conventional radiotherapy makes treatment with proton beams advantageous since it minimizes exposure of normal tissue in brain and orbit. Until recently, only scattered proton beams were used. In this study we compare contaminating neutron dose from three different beam delivery methods: passive scattering, uniform scanning with a rectangular scan pattern, and uniform scanning with a circular scan pattern adjusted to the target size. In all three scenarios the same brass aperture was used. Methods: CT scans of three patients with retinoblastoma were selected for treatment planning. Patients were treated under anesthesia using an ocular suction cup. Two patients had bilateral, one had unilateral treatment. The plans were reproduced in a phantom using the three delivery methods described above. Dose distributions were measured with MAGIC gel placed inside the phantom with readout performed using MRI. Neutron doses to the phantom were measured using Bubble detectors (Bubble Technology Industries™) and a SWENDI-II (Thermo Electron Corporation™) detector. Results: In all three patients dose to the ethmoid bone and the frontal lobe was minimized by protons. When the optimized circular beam scanning pattern was used, secondary neutron dose was lower compared to both the rectangular scanning pattern and the passively scattered beam. For a 3 cm circular aperture, the secondary neutron component was 0.23 mSv/Gy for a 12×12 cm rectangular pattern. It was 80% lower for a 4cm optimized circular scan pattern and 15% higher for a 12cm diameter double scattered field. Conclusions: Delivery of proton beams using a scan pattern adjusted to the shape of the treatment field yields comparable dose distributions to conventional proton beam delivery but has the advantage of delivering less contaminating neutron dose to the patient.

SU‐FF‐T‐457: Second Cancer Risk From Secondary Neutrons for a Boy Who Received Proton Craniospinal Irradiation

Purpose: Proton fields used in radiotherapy expose healthy tissue to secondary neutrons emanating from the treatment unit (or “external neutrons”) and produced within the patient (or “internal neutrons”), which provide no known benefit to the patient and may increase a patient's risk of developing a second cancer. The objectives of this study were to calculate equivalent doses to organs and tissues and to estimate second cancer risk from secondary neutrons for a boy who received craniospinal irradiation (CSI) with proton beams. Method and Materials: Using Monte Carlo simulations, equivalent dose from secondary neutrons was calculated in organs and tissues for the 10‐year‐old boy. In order to maximize the accuracy and realism of the simulations, the geometric model comprised a detailed passive‐scattering proton treatment unit and a voxelized phantom that was created from the actual CT images of the patient. All treatment fields were included. The proton treatment included CSI at 30.6 Gy plus a boost of 23.4 Gy to the clinical target volume in the brain. Based on the equivalent dose values, effective dose and second cancer risk from secondary neutrons were predicted. Results: The effective dose from secondary neutrons was 418 mSv, of which 344 mSv was from external neutrons and 74 mSv was from internal neutrons. The lifetime attributable risks of second cancer incidence and mortality were 6.2% and 3.4%, respectively. The cancer sites that carried the highest risks were the lungs, colon, and thyroid. The risks were predominated by the two spinal fields; very little risk was associated with the boost fields. Conclusion: The results of this study provide an estimate of the secondary neutron dose and corresponding risk for a pediatric patient undergoing proton CSI and support the suitability of passively‐scattered proton beams for the treatment of tumors of the central nervous system in children.

SU‐FF‐T‐459: The Effect of Proton Therapy Beam Scanning Pattern Size On Secondary Neutron Dose Equivalent

Purpose: One way to deliver a proton therapy beam is to actively scan a uniform dose across a treatment field using a fast scanning magnet. In this study, we examine how proton therapy beam scanning patterns affect the scattered neutron dose equivalent in an active scanning proton therapy beam delivery nozzle. Method and Materials: The nominal scanning pattern size is a 324 cm2 rectangular pattern which projects a flat proton field of 12 × 12 cm at isocenter for a 12 cm snout size. However, it is possible to reduce the size of this scanning pattern for field sizes smaller than 12 cm and maintain beam flatness while minimizing the scattered neutron dose equivalent. Neutron measurements were taken using a SWENDI‐II (Thermo Electron Corporation™) neutron detector for a treatment field defined by apertures of 10 cm and 5 cm in diameter using protons of approximately 160 MeV. In each case, the field modulation in depth was 10 cm. A Scanditronix Matrixx™ panel was used to determine field flatness. Results: The maximum neutron dose equivalent was measured to be 0.65 mSv/Gy for the 10 cm field and 0.97 mSv/Gy for the 5 cm field 40 cm laterally from isocenter. By minimizing the rectangular scanning pattern, it was possible to reduce the secondary neutron dose equivalent by approximately 20% and 60% for the 10 cm and 5 cm diameter field sizes respectively and maintain acceptable treatment parameters. Conclusion: The efficiency of a proton treatment beam through the final patient collimator has a large influence on the creation of unwanted secondary neutrons. Increasing this efficiency using different scanning patterns minimizes neutron dose equivalent and maintains acceptable treatment parameters for the therapeutic proton beam.

Secondary Neutron Doses for Several Beam Configurations for Proton Therapy

To compare possible neutron doses produced in scanning and scattering modes, with the latter assessed using a newly built passive-scattering proton beam line. A 40 x 30.5 x 30-cm water phantom was irradiated with 230-MeV proton beams using a gantry angle of 270 degrees , a 10-cm-diameter snout, and a brass aperture with a diameter of 7 cm and a thickness of 6.5 cm. The secondary neutron doses during irradiation were measured at various points using CR-39 detectors, and these measurements were cross-checked using a neutron survey meter with a 22-cm range and a 5-cm spread-out Bragg peak. The maximum doses due to secondary neutrons produced by a scattering beam-delivery system were on the order of 0.152 mSv/Gy and 1.17 mSv/Gy at 50 cm from the beam isocenter in the longitudinal (0 degrees ) and perpendicular (90 degrees ) directions, respectively. The neutron dose equivalent to the proton absorbed dose, measured from 10 cm to 100 cm from the isocenter, ranged from 0.071 mSv/Gy to 1.96 mSv/Gy in the direction of the beam line (i.e., phi = 0 degrees ). The largest neutron dose, of 3.88 mSv/Gy, was observed at 135 degrees and 25 cm from the isocenter. Although the secondary neutron doses in proton therapy were higher when a scattering mode rather than a scanning mode was used, they did not exceed the scattered photon dose in typical photon treatments.

Influence of photon beam energy on IMRT plan quality for radiotherapy of prostate cancer

Summary Background Intensity-modulated radiation therapy (IMRT) has been widely used for prostate cancer treatments. 6MV photon beams were found to be an effective energy choice for most IMRT cases. The use of high-energy photons raise concerns about increased leakage and secondary neutron dose for the patients. Aim In this work, the effect of beam energy on the quality of IMRT plans for prostate radiotherapy was systematically studied for competing IMRT plans optimized for delivery with either 6 or 10MV beams. Materials and Methods A cohort of 20 prostate cases was selected for this study. All patients received full-course IMRT treatments to a dose of 79.2Gy to PTV in 44 fractions. For all of the cases we developed treatment plans using 6 MV and 10MV intensity-modulated beams with identical dose volume constraints. Results Percentage of doses received by the percentage volume of PTV was higher for 6MV photons compared to 10MV photons for 12 patients, less than or equal to 1% for 6 patients and 2.6%, 3.6% for the remaining 2 patients irrespective of the PTV volume. Percentage doses received by 15% of bladder volume were higher for 10 MV photons. Percentage doses received by 15% of rectum volume were also higher for 10 MV photons. Conclusions Since there is no greater advantage from 10MV photons as compared with 6MV photons in large volume pelvic IMRT dosimetry and also 10MV photons lie on the threshold energy border for the induction of photo neutrons from the accelerator components, we recommend the use of 6MV photons for IMRT of prostate cancer to achieve better results in tumour control and acceptable probability of complication rate.

SU‐GG‐T‐523: Benchmarking of An MMLC for Intensity‐Modulated Proton Radiotherapy, with Emphasis On Secondary Neutron Dose

Purpose. We discuss the results of a feasibility study for multileaf collimator driven intensity modulated proton radiotherapy. The main focus is directed towards secondary neutron dose when using a miniature multileaf collimator. We also present a dosimetric comparison with conformal brass apertures. Method and Materials. Mechanical properties of an Integra Radionics MMLC were tested. The dosimetry of the MMLC in a fixed intra‐cranial proton beamline was assessed by comparing depth and lateral dose profiles, as well as the conformity of 2D dose distributions. Neutron dose to the patient is a consequence of protons impinging on beam modifying devices, such as scatterers and collimators. Neutron dose was measured for both brass apertures and MMLC shaping at different distances from the aperture edge for varying proton energies, airgaps and field sizes. Activation of the MMLC leaves was quantified. Results. Mechanical reproducibility and centering of the MMLC leaves are excellent. As expected we observe differences in dose distributions shaped by brass aperture and MMLC with regards to lateral fall‐off, conformity and the field size dependence of the Bragg peak‐to‐entrance ratio. These differences are clinically negligible. Neutron dose depends strongly on particle energy, airgap, distance from isocenter and collimator material. It is increased for the tungsten MMLC, but the required increased airgap reduces the difference between MMLC and brass aperture. Because of the design of our intra‐cranial fixed beamline the neutron dose is a factor of 10 to 20 smaller than in our passive‐scattered IBA gantry system. Conclusion. Extensive measurements proved that secondary neutrons do not prevent the implementation of intensity modulated proton therapy in our fixed beamline. Our results will furthermore be used as basis for the development of an MMLC dedicated for proton therapy.