Angélica Pérez-Andújar

Neutron production from beam-modifying devices in a modern double scattering proton therapy beam delivery system

In this work the neutron production in a passive beam delivery system was investigated. Secondary particles including neutrons are created as the proton beam interacts with beam shaping devices in the treatment head. Stray neutron exposure to the whole body may increase the risk that the patient develops a radiogenic cancer years or decades after radiotherapy. We simulated a passive proton beam delivery system with double scattering technology to determine the neutron production and energy distribution at 200 MeV proton energy. Specifically, we studied the neutron absorbed dose per therapeutic absorbed dose, the neutron absorbed dose per source particle and the neutron energy spectrum at various locations around the nozzle. We also investigated the neutron production along the nozzles central axis. The absorbed doses and neutron spectra were simulated with the MCNPX Monte Carlo code. The simulations revealed that the range modulation wheel (RMW) is the most intense neutron source of any of the beam spreading devices within the nozzle. This finding suggests that it may be helpful to refine the design of the RMW assembly, e.g., by adding local shielding, to suppress neutron-induced damage to components in the nozzle and to reduce the shielding thickness of the treatment vault. The simulations also revealed that the neutron dose to the patient is predominated by neutrons produced in the field defining collimator assembly, located just upstream of the patient.

An analytic model of neutron ambient dose equivalent and equivalent dose for proton radiotherapy.

Stray neutrons generated in passively scattered proton therapy are of concern because they increase the risk that a patient will develop a second cancer. Several investigations characterized stray neutrons in proton therapy using experimental measurements and Monte Carlo simulations, but capabilities of analytical methods to predict neutron exposures are less well developed. The goal of this study was to develop a new analytical model to calculate neutron ambient dose equivalent in air and equivalent dose in phantom based on Monte Carlo modeling of a passively scattered proton therapy unit. The accuracy of the new analytical model is superior to a previous analytical model and comparable to the accuracy of typical Monte Carlo simulations and measurements. Predictions from the new analytical model agreed reasonably well with corresponding values predicted by a Monte Carlo code using an anthropomorphic phantom.

Maximum proton kinetic energy and patient‐generated neutron fluence considerations in proton beam arc delivery radiation therapy

Several compact proton accelerator systems for use in proton therapy have recently been proposed. Of paramount importance to the development of such an accelerator system is the maximum kinetic energy of protons, immediately prior to entry into the patient, that must be reached by the treatment system. The commonly used value for the maximum kinetic energy required for a medical proton accelerator is 250 MeV, but it has not been demonstrated that this energy is indeed necessary to treat all or most patients eligible for proton therapy. This article quantifies the maximum kinetic energy of protons, immediately prior to entry into the patient, necessary to treat a given percentage of patients with rotational proton therapy, and examines the impact of this energy threshold on the cost and feasibility of a compact, gantry-mounted proton accelerator treatment system. One hundred randomized treatment plans from patients treated with IMRT were analyzed. The maximum radiological pathlength from the surface of the patient to the distal edge of the treatment volume was obtained for 180 degrees continuous arc proton therapy and for 180 degrees split arc proton therapy (two 90 degrees arcs) using CT# profiles from the Pinnacle (Philips Medical Systems, Madison, WI) treatment planning system. In each case, the maximum kinetic energy of protons, immediately prior to entry into the patient, that would be necessary to treat the patient was calculated using proton range tables for various media. In addition, Monte Carlo simulations were performed to quantify neutron production in a water phantom representing a patient as a function of the maximum proton kinetic energy achievable by a proton treatment system. Protons with a kinetic energy of 240 MeV, immediately prior to entry into the patient, were needed to treat 100% of patients in this study. However, it was shown that 90% of patients could be treated at 198 MeV, and 95% of patients could be treated at 207 MeV. Decreasing the proton kinetic energy from 250 to 200 MeV decreases the total neutron energy fluence produced by stopping a monoenergetic pencil beam in a water phantom by a factor of 2.3. It is possible to significantly lower the requirements on the maximum kinetic energy of a compact proton accelerator if the ability to treat a small percentage of patients with rotational therapy is sacrificed. This decrease in maximum kinetic energy, along with the corresponding decrease in neutron production, could lower the cost and ease the engineering constraints on a compact proton accelerator treatment facility.

Inter-Institutional Comparison of Personalized Risk Assessments for Second Malignant Neoplasms for a 13-Year-Old Girl Receiving Proton versus Photon Craniospinal Irradiation

Children receiving radiotherapy face the probability of a subsequent malignant neoplasm (SMN). In some cases, the predicted SMN risk can be reduced by proton therapy. The purpose of this study was to apply the most comprehensive dose assessment methods to estimate the reduction in SMN risk after proton therapy vs. photon therapy for a 13-year-old girl requiring craniospinal irradiation (CSI). We reconstructed the equivalent dose throughout the patient’s body from therapeutic and stray radiation and applied SMN incidence and mortality risk models for each modality. Excluding skin cancer, the risk of incidence after proton CSI was a third of that of photon CSI. The predicted absolute SMN risks were high. For photon CSI, the SMN incidence rates greater than 10% were for thyroid, non-melanoma skin, lung, colon, stomach, and other solid cancers, and for proton CSI they were non-melanoma skin, lung, and other solid cancers. In each setting, lung cancer accounted for half the risk of mortality. In conclusion, the predicted SMN risk for a 13-year-old girl undergoing proton CSI was reduced vs. photon CSI. This study demonstrates the feasibility of inter-institutional whole-body dose and risk assessments and also serves as a model for including risk estimation in personalized cancer care.

Microdosimetric measurements for neutron-absorbed dose determination during proton therapy

This work presents microdosimetric measurements performed at the Midwest Proton Radiotherapy Institute in Bloomington, Indiana, USA. The measurements were done simulating clinical setups with a water phantom and for a variety of stopping targets. The water phantom was irradiated by a proton spread out Bragg peak (SOBP) and by a proton pencil beam. Stopping target measurements were performed only for the pencil beam. The targets used were made of polyethylene, brass and lead. The objective of this work was to determine the neutron-absorbed dose for a passive and active proton therapy delivery, and for the interactions of the proton beam with materials typically in the beam line of a proton therapy treatment nozzle. Neutron doses were found to be higher at 45° and 90° from the beam direction for the SOBP configuration by a factor of 1.1 and 1.3, respectively, compared with the pencil beam. Meanwhile, the pencil beam configuration produced neutron-absorbed doses 2.2 times higher at 0° than the SOBP. For stopping targets, lead was found to dominate the neutron-absorbed dose for most angles due to a large production of low-energy neutrons emitted isotropically.

Quality assurance of stereotactic alignment and patient positioning mechanical accuracy for robotized Gamma Knife radiosurgery

The automatic patient positioning system and its alignment is critical and specified to be less than 0.35 mm for a radiosurgical treatment with the latest robotized Gamma Knife Perfexion (GKPFX). In this study, we developed a quantitative QA procedure to verify the accuracy and robustness of such a system. In particular, we applied the test to a unit that has performed >1000 procedures at our institution. For the test, a radiochromic film was first placed inside a spherical film phantom and then irradiated with a sequence of linearly placed shots of equal collimator size (e.g. 4 mm) via the Leksell Gamma Knife Perfexion system (PFX). The shots were positioned with either equal or unequal gaps of approximately 8 mm both at center and off-center positions of the patient positioning system. Two independent methods of localizing the irradiation shot center coordinates were employed to measure the gap spacing between adjacent shots. The measured distance was then compared with the initial preset values for the test. On average, the positioning uncertainty for the PFX delivery system was found to be 0.03 ± 0.2 mm (2σ). No significant difference in the positioning uncertainty was noted among measurements in the x-, y- and z-axis orientations. In conclusion, a simple, fast, and quantitative test was developed and demonstrated for routine QA of the submillimeter PFX patient positioning system. This test also enables independent verification of any patient-specific shot positioning for a critical treatment such as a tumor in the brainstem.

Contribution to Neutron Fluence and Neutron Absorbed Dose from Double Scattering Proton Therapy System Components

Abstract Proton therapy offers low integral dose and good tumor comformality in many deep-seated tumors. However, secondary particles generated during proton therapy, such as neutrons, are a concern, especially for passive scattering systems. In this type of system, the proton beam interacts with several components of the treatment nozzle that lie along the delivery path and can produce secondary neutrons. Neutron production along the beam’s central axis in a double scattering passive system was examined using Monte Carlo simulations. Neutron fluence and energy distribution were determined downstream of the nozzle’s major components at different radial distances from the central axis. In addition, the neutron absorbed dose per primary proton around the nozzle was investigated. Neutron fluence was highest immediately downstream of the range modulator wheel (RMW) but decreased as distance from the RMW increased. The nozzle’s final collimator and snout also contributed to the production of high-energy neutrons. In fact, for the smallest treatment volume simulated, the neutron absorbed dose per proton at isocenter increased by a factor of 20 due to the snout presence when compared with a nozzle without a snout. The presented results can be used to design more effective local shielding components inside the treatment nozzle as well as to better understand the treatment room shielding requirements.

SU‐GG‐T‐526: Energy Thresholds in 180 Degree Arc Delivery Proton Therapy

Purpose: To quantify the maximum initial proton kinetic energy necessary to treat a given percentage of patients with rotational proton therapy and to examine the impact of this energy threshold on the cost and feasibility of a compact, gantry‐mounted proton accelerator treatment system. Method and Materials: One hundred randomized treatment plans from patients treated with IMRT were analyzed. The maximum radiological pathlength from the surface of the patient to the distal edge of the treatment volume was obtained for 180° continuous arc proton therapy and for 180° split arc proton therapy (two 90° arcs) using CT♯ histograms from the Pinnacle treatment planning system. In each case, the maximum energy necessary to treat a patient with protons was calculated using proton range tables for various media. In addition, Monte Carlo simulations were performed to quantify neutron production in the patient as a function of maximum initial proton energy. Results: The widely accepted value of 250 MeV needed to treat 100 percent of patients with protons was confirmed. However, it was shown that 90 percent of patients could be treated at 198 MeV, and 95 percent of patients could be treated at 207 MeV. Decreasing the maximum proton energy from 250 MeV to 200 MeV decreases the total neutron energy fluence created in the patient by a factor of 2.3. Conclusion: It is possible to significantly lower the requirements on the maximum energy of a compact proton accelerator if the ability to treat a small percentage of patients with rotational therapy is sacrificed. This decrease in maximum energy, along with the corresponding decrease in neutron production, could lower the cost and ease the engineering constraints on a compact proton accelerator treatment facility. Conflict of Interest: An author of this study has financial interest in Tomotherapy, Inc., which has licensed proton accelerator technology.

SU‐GG‐T‐356: Benchmark of Geant4 and MCNPX for Proton Radiation Therapy (E<70 MeV): Total Dose and Secondary Particle Production

Purpose: The validity of the Monte Carlo(MC) for protondose calculation has been proved by the fact that the simulation matches measurement. The secondary particles generated by hadronic interactions, e.g. neutron, alpha, can give high effective dose even if the production rate is low. This will potentially spoil the gain of using protonradiotherapy. Challenged by the measurement, attempts have been made to use MC to predict the secondary particle dose. Different code using different physics models or cross section data may lead to quite different spectrum. The comparison should be made among those MC codes to document the difference. Method and Materials: The ocular‐beamline from the Harvard Cyclotron Laboratory (Cambridge, USA) as well as measured data was used. Two MC codes, Geant4 (4.9.1) and MCNPX, were compared. Since the physics process can be customized in Geant4, different physics module combinations were generated.Proton nozzle, scoring region, material, and lateral spread out of the initial beam were kept same in all the simulations. In the first step, different simulations were tuned differently by changing the energy spread of the initial beam and the thickness of the range shifter to match the measurement. Then, the neutron flux as well as the dose from different particles within the phantom was compared between the simulations. Results: All the simulations match the protonbeamion chamber measurements providing different energy spreads and range shifter thickness. The neutron flux and specific particle dose within the phantom varies between different simulations in the preliminary result, which likely represents differences between physics module. Conclusion: Both Geant4 and MCNPX are good for protondose calculation. However, for the secondary particle dose, the preliminary result shows that quantitative evaluation is suspect. Conflict of Interest: Thomas Mackie has a commercial relationship with protonradiotherapy.

TH‐D‐AUD A‐03: Central Axis Neutron Production Determination for a Double Scatterer Passive System

Purpose: The purpose of this work was to determine which components along the central axis of a passive beam delivery system for proton therapy contributes the most to the production of secondary neutrons.Method and Materials: In this work a passive beam delivery system was modeled based on the MD Anderson Cancer Center treatment nozzle. We performed Monte Carlo simulations with Los Alamos code MCNPX. In these simulations a 200 MeV proton beam is shaped by a rotational modulator wheel (RMW), a secondary scatterer and by a collimating system including a variable snout. Cylindrical volumes were placed along the beam central axis to determine the radial distribution of the neutrons produced. The volumes were made of concentric cylinders with radius ranging from 50 mm to 20 mm. The volumes were placed after the RMW, the secondary scatterer, before and after the snout. The neutron flux and energy spectra were determined for each volume radii and for three treatment volumes. Results: After the RMW the neutron flux was higher for all treatment volumes diminishing as the distance along the central axis increased. The flux increased slightly just before the final snout for the smaller field sizes indicating a backscatter contribution as the proton beam is finally collimated. At the end of the nozzle the flux was lower than after the RMW. The larger neutron flux with energies ranging from 130 MeV to almost 200 MeV was found at smaller radii. As the radial distance increased the flux of energetic neutrons diminished. Conclusion: We found that the RMW was the major source of neutrons in the treatment nozzle. The flux diminished as the distance increased indicating a 1/r2 dependency. The other shaping components contribute to the neutron production but it is difficult to differentiate between contributors after the RMW.