(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: During high-energy electron-beam therapy, beam shaping components, and other parameters such as beam energy, may contribute secondary neutron dose to the patient. This study identifies the effects of these factors on the spectrum of neutrons generated.
Methods: In this study, the impact of various irradiation parameters on neutron production were studied, including the collimator size, applicator size, cerrobend cut-out size, beam energy, measurement location, and presence of a phantom. Each irradiation was performed at a dose rate of 1000 MU/min using a Varian TrueBeam™ STx linac, during which one of the aforementioned parameters was varied while the others remained unchanged. Our nominal setup consisted of a 6 x 6 cm² applicator and an open cut-out, irradiated with a beam energy of 20 MeV and measured at 1 m away from the isocentre on the couch. Three repeat neutron spectrum measurements were made per set-up using a Nested Neutron Spectrometer™ (NNS). Spectra were determined by iteratively unfolding the measurements and subsequently compared with the spectrum of our nominal setup.
Results: The neutron fluence spectrum was not affected significantly by varying the applicator or cut-out size, nor by introducing a solid water phantom in the beam. On the other hand, a large dependence on collimator size, measurement location and electron beam energy was observed. Regarding the beam energy, the neutron fluence increased in a non-linear fashion while the fluence showed an inverse dependence on collimator size.
Conclusion: Our investigation revealed that the introduction of an applicator, cut-out, or phantom does not contribute significantly to the secondary neutron fluence in high-energy electron therapy. This also holds true for various sizes of these components. However, the beam energy, measurement location, and the collimator each play a significant role in neutron dose contribution to the patient.
Funding Support, Disclosures, and Conflict of Interest: We acknowledge the support provided by the Natural Sciences and Engineering Research Council of Canada (NSERC) through the Discovery Grant (J.Kildea) and CREATE Medical Physics Research Training Network grant (Grant number: 432290). Partial support for this research was provided by the Canadian Nuclear Safety Commission (CNSC).
Keywords
Electron Therapy, Radiation Protection, Fast Neutrons
Taxonomy
TH- External Beam- Electrons: out of field dosimetry/risk analysis
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