Monte Carlo simulations are widely used to calculate secondary neutron doses in proton therapy. When using the passive scattering technique, a small variation in the tumor size or location requires a full adjustment of the beamline configuration. This work thus focuses on modeling all possible beamline elements of the local facility with the MCNPX code while covering the entire clinical range of proton beam energies. This paper documents the experimental validation of realistic beamline configurations dedicated to intracranial tumor treatments, with different sources and beamline elements. Simulations were compared against measurements and treatment planning system (TPS) data considering percentage depth dose distributions (PDD) and relative lateral dose profiles. The results show that simulated and measured PDD distributions differ by no more than 1.6 mm on the proton beam range (tolerance set at 2 mm/2%) for the 162 MeV configuration. Meanwhile, for the modulated lateral dose profiles, the 219 MeV configuration presented the largest difference for the in-plane field width at 90% with 2 mm difference in measurements. The modeled proton sources and beamline elements were validated, and the new realistic geometries can be used to calculate secondary neutron doses to healthy tissues.
Purpose:
Measure stray radiation inside a passive scattering proton therapy facility, compare values to Monte Carlo (MC) simulations and identify the actual needs and challenges.
Methods:
Measurements and MC simulations were considered to acknowledge neutron exposure associated with 75 MeV ocular or 180 MeV intracranial passively scattered proton treatments. First, using a specifically‐designed high sensitivity Bonner Sphere system, neutron spectra were measured at different positions inside the treatment rooms. Next, measurement‐based mapping of neutron ambient dose equivalent was fulfilled using several TEPCs and rem‐meters. Finally, photon and neutron organ doses were measured using TLDs, RPLs and PADCs set inside anthropomorphic phantoms (Rando, 1 and 5‐years‐old CIRS). All measurements were also simulated with MCNPX to investigate the efficiency of MC models in predicting stray neutrons considering different nuclear cross sections and models.
Results:
Knowledge of the neutron fluence and energy distribution inside a proton therapy room is critical for stray radiation dosimetry. However, as spectrometry unfolding is initiated using a MC guess spectrum and suffers from algorithmic limits a 20% spectrometry uncertainty is expected. H*(10) mapping with TEPCs and rem‐meters showed a good agreement between the detectors. Differences within measurement uncertainty (10–15%) were observed and are inherent to the energy, fluence and directional response of each detector. For a typical ocular and intracranial treatment respectively, neutron doses outside the clinical target volume of 0.4 and 11 mGy were measured inside the Rando phantom. Photon doses were 2–10 times lower depending on organs position. High uncertainties (40%) are inherent to TLDs and PADCs measurements due to the need for neutron spectra at detector position. Finally, stray neutrons prediction with MC simulations proved to be extremely dependent on proton beam energy and the used nuclear models and cross sections.
Conclusion:
This work highlights measurement and simulation limits for ion therapy radiation protection applications.
Purpose:
Development of a parametric equation suitable for a daily use in routine clinic to provide estimates of stray neutron doses in proton therapy.
Methods:
Monte Carlo (MC) calculations using the UF‐NCI 1‐year‐old phantom were exercised to determine the variation of stray neutron doses as a function of irradiation parameters while performing intracranial treatments. This was done by individually changing the proton beam energy, modulation width, collimator aperture and thickness, compensator thickness and the air gap size while their impact on neutron doses were put into a single equation. The variation of neutron doses with distance from the target volume was also included in it. Then, a first step consisted in establishing the fitting coefficients by using 221 learning data which were neutron absorbed doses obtained with MC simulations while a second step consisted in validating the final equation.
Results:
The variation of stray neutron doses with irradiation parameters were fitted with linear, polynomial, etc. model while a power‐law model was used to fit the variation of stray neutron doses with the distance from the target volume. The parametric equation fitted well MC simulations while establishing fitting coefficients as the discrepancies on the estimate of neutron absorbed doses were within 10%. The discrepancy can reach ∼25% for the bladder, the farthest organ from the target volume. Finally, the validation showed results in compliance with MC calculations since the discrepancies were also within 10% for head‐and‐neck and thoracic organs while they can reach ∼25%, again for pelvic organs.
Conclusion:
The parametric equation presents promising results and will be validated for other target sites as well as other facilities to go towards a universal method.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.