P Taddei

Monte Carlo study of neutron dose equivalent during passive scattering proton therapy

Stray radiation exposures are of concern for patients receiving proton radiotherapy and vary strongly with several treatment factors. The purposes of this study were to conservatively estimate neutron exposures for a contemporary passive scattering proton therapy system and to understand how they vary with treatment factors. We studied the neutron dose equivalent per therapeutic absorbed dose (H/D) as a function of treatment factors including proton energy, location in the treatment room, treatment field size, spread-out Bragg peak (SOBP) width and snout position using both Monte Carlo simulations and analytical modeling. The H/D value at the isocenter for a 250 MeV medium field size option was estimated to be 20 mSv Gy(-1). H/D values generally increased with the energy or penetration range, fell off sharply with distance from the treatment unit, decreased modestly with the aperture size, increased with the SOBP width and decreased with the snout distance from the isocenter. The H/D values from Monte Carlo simulations agreed well with experimental results from the literature. The analytical model predicted H/D values within 28% of those obtained in simulations; this value is within typical neutron measurement uncertainties.

Monte Carlo simulations of neutron spectral fluence, radiation weighting factor and ambient dose equivalent for a passively scattered proton therapy unit

Stray neutron exposures pose a potential risk for the development of secondary cancer in patients receiving proton therapy. However, the behavior of the ambient dose equivalent is not fully understood, including dependences on neutron spectral fluence, radiation weighting factor and proton treatment beam characteristics. The objective of this work, therefore, was to estimate neutron exposures resulting from the use of a passively scattered proton treatment unit. In particular, we studied the characteristics of the neutron spectral fluence, radiation weighting factor and ambient dose equivalent with Monte Carlo simulations. The neutron spectral fluence contained two pronounced peaks, one a low-energy peak with a mode around 1 MeV and one a high-energy peak that ranged from about 10 MeV up to the proton energy. The mean radiation weighting factors varied only slightly, from 8.8 to 10.3, with proton energy and location for a closed-aperture configuration. For unmodulated proton beams stopped in a closed aperture, the ambient dose equivalent from neutrons per therapeutic absorbed dose (H*(10)/D) calculated free-in-air ranged from about 0.3 mSv/Gy for a small scattered field of 100 MeV proton energy to 19 mSv/Gy for a large scattered field of 250 MeV proton energy, revealing strong dependences on proton energy and field size. Comparisons of in-air calculations with in-phantom calculations indicated that the in-air method yielded a conservative estimation of stray neutron radiation exposure for a prostate cancer patient.

SU‐GG‐AUD‐06: Stray Radiation Exposure During Proton Radiotherapy of the Prostate: The Influence of the Patient On Scatter and Production

Purpose: To characterize the scatter, production, and attenuation of secondary radiation in patients receiving passively‐scattered protonradiotherapy for prostate cancer.Methods and Materials: A proton therapytreatment was simulated using a Monte Carlo model of a double scattering treatment machine. Whole body effective dose (E) from secondary radiation was estimated from a weighted sum of doses to the major organs in an anthropomorphic phantom. The effect of the patient on secondary dose was quantified by comparing E with ambient dose equivalent, H*(10), which was based on free‐in‐air spectral fluence calculations at isocenter. Various treatment parameters (proton beam energy, range modulation width, field size, and snout position) were varied in order to study their influence on E and H*(10). Results: The calculated E for the simulated treatment was 7.8 mSv/Gy, while the calculated H*(10) at isocenter was 16 mSv/Gy. Both E and H*(10) approximately doubled over the range of modulation widths and energies studied. As field size increased from 0×0 to 15×15, E doubled, while H*(10) decreased by 30%. When the snout position was changed from 30 cm to 48 cm, E decreased by less than 20%, while H*(10) decreased by 44% over the same interval. Simulations revealed that, while E is predominated by neutrons generated in the nozzle, neutrons produced in the patient contributed significantly (up to 40%) to dose equivalent in near‐field organs. In most cases, H*(10) provided a conservative estimate of E. However — because H*(10) does not account for neutrons created in the patient — it did not conservatively estimate E for large field sizes, where neutron production in the patient becomes significant. Conclusions:Neutrons generated in the patient contribute significantly to exposures to organs near the irradiated volume. When evaluating stray radiation exposure, production, scatter, and attenuation in the patient should be taken into consideration.

TU‐C‐AUD‐02: Study of Neutron Exposure During Passively Scattered Proton Therapy

Purpose: Stray radiation exposures are of concern for patients receiving protonradiotherapy and vary strongly with several treatment factors such as proton energy, field size and modulation width. The purposes of this study were to conservatively estimate neutron exposures for a contemporary passive scatteringprotontreatment unit and to understand how they vary with treatment factors. Method and Materials: We simulated all 24 options (each range modulator and second scatterer combination is accounted for one option) for a passive scatteringproton therapy unit with MCNPX. Spectral neutron fluence from simulations was then converted to neutrondose equivalent using corresponding dose conversion factors. We studied the neutrondose equivalent per therapeutic absorbed dose (H/D) as a function of treatment factors including proton energy, location in the treatment room, treatmentfield size, and spread‐out Bragg peak (SOBP) width using Monte Carlo simulation.Results: The H/D value at isocenter for a 250‐MeV medium field size option was estimated to be 20 mSv Gy−1. H/D decreased to about 20% from 250 Mev to 160 MeV. H/D fell off sharply with distance from the treatment unit, approximately following a power law; H/D was about 10% higher for a large field option than a medium field option for the same energy. H//D almost doubled when SOBP width was increased from a pristine peak to 16 cm. An analytical model was developed, which predicted H/D values within 28% of those obtained in simulations; this value is within typical neutron measurement uncertainties. Conclusion: The results quantified how treatment factors influence H/D values. The in‐air method with a closed aperture presented here provides a simple and straightforward approach that could be adopted for facility inter‐comparisons. In addition, an analytical model was developed to quickly estimate H/D values.

SU‐FF‐T‐25: A Monte‐Carlo Based Dose Engine for Proton Radiotherapy Treatment Planning

Purpose: To develop a fast Monte Carlo (MC)dose engine for protonradiation treatment planning calculations and research studies. Method and Materials: We developed a complete MC simulation system for calculating the dose distributions in patients. The system uses a commercial treatment planning system with an analytical dose algorithm to design the treatment plans. A DICOM‐RT‐ION interface was developed to allow automated data transfer between the treatment planning system and the MC system. The MCmodel included all major components of a passively‐scattered protontreatment unit and a CT‐based patient model. A suite of programs converted the prescriptive data (e.g., range, modulation width, field size) and patient CT data into a MC input file, including coordinate system transformations for arbitrary treatment beam orientations. The radiation transport calculations are performed with the MCNPX Monte Carlo system running on a cluster of 512 64‐bit CPUs. For each treatment field, multiple simulation output files were postprocessed and the resulting MCdose matrix was written to the DICOM‐RT‐ION plan. The plan was then imported into the commercial planning system for visualization. Results: The MC simulations and pencil beam dose distributions are in good agreement for a two‐field prostate plan and a three‐field lung plan. The prostate plan required 1.6 hours and the lung plan required 14 hours using 512 CPUs to achieve < 2 % statistical uncertainty in the total dose at isocenter. The computing time was directly related to the number of voxels in the patient model. Timing studies revealed that the simulation speed for this system scales almost linearly with the inverse number of CPUs. Conclusion: The results of this study strongly suggest that it is feasible to implement a fast and easy‐to‐use MCtreatment planningdose engine with currently available computing technologies and resources.

SU‐FF‐T‐305: Monte Carlo Investigation of Local Shielding to Reduce Stray Radiation Doses to Patients Receiving Proton Therapy

Purpose: The purpose of this study was to quantify the effectiveness of local shielding in reducing the stray radiation exposure to a patient receiving curative proton therapy for prostate cancer by varying the thickness and material of the shielding in the treatment head. Method and Materials: Effective dose,E, was predicted using Monte Carlo simulations of a 76‐Gy prostate treatment with protons. A passive‐scattering treatment nozzle was modeled together with a detailed anthropomorphic phantom. Protons,neutrons, and photons were tracked throughout the geometry. Tallies of energy deposition and fluence were made for these particles in selected organs. Eleven different compositions of shielding material were tested, and the thickness of additional shielding was varied from 0 to 35 cm. A comparison was made between using brass and tungsten alloy for the field‐defining collimator.Results: E from stray radiation for the unmodified nozzle was 583 mSv for the prostate treatment. Additional shielding of the treatment head reduced E by up to 66 mSv. Using tungsten alloy instead of brass for the collimator material reduced E by 50 mSv. Implementing both techniques together reduced E by 87 mSv. Conclusion: This study suggests that a significant reduction in effective dose from stray radiation may be reasonably achievable for patients receiving proton therapy. By adding local shielding to the treatment head and exchanging the brasscollimator with a tungsten alloy replacement, the effective dose to proton therapy patients from stray radiation was reduced by 15%.

SU‐E‐T‐283; Risks of Cardiac Toxicity in Pediatric Patients Receiving Photon Or Proton Radiotherapy

PURPOSE To compare the predicted risk of developing radiogenic cardiac toxicity after photon versus proton radiotherapies for a pediatric patient with Hodgkin disease (HD) and a pediatric patient with medulloblastoma (MB). METHODS In the treatment plans, each patient heart was contoured in fine detail, including substructures of the pericardium and myocardium. Risk calculations took into account both therapeutic and stray radiation doses. We calculated the relative risk (RR) of cardiac toxicity using a linear risk model and the normal tissue complication probability (NTCP) values using relative seriality and Lyman models. Uncertainty analyses were also performed Results: The RR values of cardiac toxicity for the HD patient were 7.27 (95% confidence interval (CI), 3.09 to 27.12) (proton) and 8.37 (95% CI, 3.46 to 31.70) (photon), respectively; the RR values for the MB patient were 1.28 (95% CI, 1.09 to 2.18) (proton) and 8.39 (95% CI, 3.46 to 31.78) (photon), respectively. The predicted NTCP values for the HD patient were 2.17% (proton) and 2.67% (photon) for the myocardium, and were 2.11% (proton) and 1.92% (photon) for the whole heart. The predicted ratios of NTCP values (proton/photon) for the MB patient were much less than unity. Uncertainty analyses revealed that the predicted ratio of risk between proton and photon therapies was sensitive to uncertainties in the NTCP model parameters and the mean radiation weighting factor for neutrons, but was not sensitive to heart structure contours. The qualitative findings of the study were not sensitive to uncertainties in these factors. CONCLUSION We conclude that proton and photon radiotherapies confer similar predicted risks of cardiac toxicity for the HD patient and that proton therapy reduced the predicted risk for the MB patient relative to photon therapy.

Poster — Thur Eve — 36: Out‐of‐Field dose in craniospinal irradiation

The risk of radiotherapy induced secondary cancer depends on the integral dose delivered to the patient where the dose delivered within the radiation field is accounted for, as well as dose to out-of-field organs from scattered and leakage radiation. While commercial treatment planning systems allow accurate determination of in-field dose, they are generally not capable of accurate out-of-field dose prediction. Secondary cancer risk is especially an issue in craniospinal treatments where involved patients are often children or young adults. In this work we therefore propose a mathematical model that accurately predicts out-of-field dose for patients treated by craniospinal irradiation at the American University of Beirut Medical Center. An anthropomorphic phantom was imaged, planned and treated, with thermoluminescent dosimeters inserted in the phantom at in-field and out-of-field locations. The measurements showed that our treatment planning system calculated accurately (within 2%) dose inside the field, but did not perform well at points just outside the field edge and consistently underestimated the dose at points further away from the field edge. From the out-of-field measured data, a model was developed that predicts out-of-field dose at a point in the patient based on the distance of that point to the treatment field edge. The developed model is of the double-gaussian type; it contains parameters that can be tuned to make it applicable in other centers where linac geometry and treatment techniques may differ.

SU‐E‐T‐257: Risk of Radiogenic Second Cancer after Photon and Proton Craniospinal Irradiation

PURPOSE To compare proton and photon therapies in terms of the risks of second cancers for a pediatric medulloblastoma patient receiving craniospinal irradiation (CSI). METHODS Two CSI treatment plans with 23.4 Gy or Gy (RBE) prescribed dose were computed for a 4-year-old boy withmedulloblastoma: a three-field 6-MV photon therapy plan and a four-field proton therapy plan. The primary doses for both plans were determined using a commercial treatment planning system. Stray radiation doses for proton therapy were determined from Monte Carlo simulations, and stray radiation doses for photon therapy were determined from measured data. The dose-risk model based on Biological Effects of Ionization Radiation VII report was used to estimate risk of second cancer. RESULTS Baseline predictions of the relative risk of each organ were always less for proton CSI than for photon CSI after various follow-up years for the patient. The lifetime risks of the incidence of second cancer after proton CSI and photon CSI were 7.7% and 92%, respectively, and the ratio of lifetime risk was 0.083. Uncertainty analysis revealed the qualitative findings of this study were insensitive to any plausible changes of dose-risk models and mean neutron radiation weighting factor. CONCLUSIONS Proton therapy confers lower predicted risk of second cancer for the pediatric medulloblastoma patient compared with photon therapy.

MO‐G‐BRC‐01: Comparison of the Risk of Second Malignant Neoplasm in a Developed Country versus a Developing Country for a 13‐Year‐Old Girl Receiving Craniospinal Irradiation

Purpose: Childhood cancer is a global health problem that affects nations of every socioeconomic status. The incidence of treatment‐induced second malignant neoplasms (SMNs) for these children is high and will increase with years of follow up. Advanced radiotherapy techniques may reduce the risk of SMN, but these techniques are not available in developing countries. The purpose of this study was to compare the predicted SMN risk for a 13‐ year‐old girl who received craniospinal irradiation (CSI) in a developed country versus that if she had been treated in a developing country. Methods: Treatment plans were created for the girl on the basis of the standards of care in each country, comprised of 4 proton fields in the developed country and 4 6‐MV photon fields in the developing country. Mean organ equivalent dose, HT, values from primary radiation fields were calculated using commercial treatment planning systems in the clinics of the respective countries while HT from stray radiation were determined based on Monte Carlo simulations in the case of proton therapy and on thermoluminescent dosimeter measurements in an anthropomorphic phantom in the case of photon therapy. An organ‐, age‐, and sex‐specific risk model was applied to predict the risk of SMN incidence for each standard of care. Results: The predicted risk of SMN incidence was almost a factor of two higher for the standard of care of a developing country using photons versus that of a developed country using protons. The absolute risks were predominated by second thyroid, lung, and other solid cancers. Conclusion: Our findings suggest that SMN incidence of children undergoing CSI in developing countries may be improved if they are treated with advanced radiotherapy techniques currently available only in developed countries.