Abstract
We evaluate a photon convolution‐superposition algorithm used to model a fast neutron therapy beam in a commercial treatment planning system (TPS). The neutron beam modeled was the Clinical Neutron Therapy System (CNTS) fast neutron beam produced by 50 MeV protons on a Be target at our facility, and we implemented the Pinnacle3 dose calculation model for computing neutron doses. Measured neutron data were acquired by an IC30 ion chamber flowing 5 cc/min of tissue equivalent gas. Output factors and profile scans for open and wedged fields were measured according to the Pinnacle physics reference guide recommendations for photon beams in a Wellhofer water tank scanning system. Following the construction of a neutron beam model, computed doses were then generated using 100 monitor units (MUs) beams incident on a water‐equivalent phantom for open and wedged square fields, as well as multileaf collimator (MLC)‐shaped irregular fields. We compared Pinnacle dose profiles, central axis doses, and off‐axis doses (in irregular fields) with 1) doses computed using the Prism treatment planning system, and 2) doses measured in a water phantom and having matching geometry to the computation setup. We found that the Pinnacle photon model may be used to model most of the important dosimetric features of the CNTS fast neutron beam. Pinnacle‐calculated dose points among open and wedged square fields exhibit dose differences within 3.9 cGy of both Prism and measured doses along the central axis, and within 5 cGy difference of measurement in the penumbra region. Pinnacle dose point calculations using irregular treatment type fields showed a dose difference up to 9 cGy from measured dose points, although most points of comparison were below 5 cGy. Comparisons of dose points that were chosen from cases planned in both Pinnacle and Prism show an average dose difference less than 0.6%, except in certain fields which incorporate both wedges and heavy blocking of the central axis. All clinical cases planned in both Prism and Pinnacle were found to be comparable in terms of dose‐volume histograms and spatial dose distribution following review by the treating clinicians. Variations were considered minor and within clinically acceptable limits by the treating clinicians. The Pinnacle TPS has sufficient computational modeling ability to adequately produce a viable neutron model for clinical use in treatment planning.PACS numbers: 87.53 Bn, 28.20.Pr, 87.53.Bn
Highlights
Introduction134 Kalet et al.: Pinnacle neutrons sensitivity, oxygen enhancement ratio, and sublethal damage repair parameters compared to low linear energy transfer (LET) standard X-ray treatments.[1,2] To date, the promise of a higher radiobiological effectiveness of neutrons on tumors relative to normal tissues has been clinically proven only for a few tumor types that are radioresistant to low LET radiation, such as salivary gland malignancies[3] and high-risk soft tissue sarcomas.[4,5] Clinical trials for advanced prostate cancer illustrated the need for a fully rotational gantry and multileaf collimator in order to avoid higher levels of complications.[6,7] Otherwise, published clinical trials have shown little benefit from fast neutron radiotherapy
25 MeV of energy is lost in the Be target, and the remainder in a copper moderator used to reduce the beam’s low energy neutron component.[16,17] The beam is delivered through a rotating gantry with a source-to-axis distance (SAD) of 150 cm
We present two separate comparisons: 1) a dose point comparison between Prism and Pinnacle doses both computed on the same full computed tomography (CT)-based structure sets derived from a historic clinical patient database, and 2) dose point comparisons of clinically planned beams computed in Pinnacle compared to Prism-computed doses and measured doses in phantom conditions
Summary
134 Kalet et al.: Pinnacle neutrons sensitivity, oxygen enhancement ratio, and sublethal damage repair parameters compared to low LET standard X-ray treatments.[1,2] To date, the promise of a higher radiobiological effectiveness of neutrons on tumors relative to normal tissues has been clinically proven only for a few tumor types that are radioresistant to low LET radiation, such as salivary gland malignancies[3] and high-risk soft tissue sarcomas.[4,5] Clinical trials for advanced prostate cancer illustrated the need for a fully rotational gantry and multileaf collimator in order to avoid higher levels of complications.[6,7] Otherwise, published clinical trials have shown little benefit from fast neutron radiotherapy These results, coupled with the substantial cost of such facilities, have left it as a niche treatment, with very few operating facilities worldwide available for clinical use. Differences in the magnitudes of scattering and radiation transport meant that the algorithm designed for photons was not as accurate for fast neutrons
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