Room: ePoster Forums
Purpose: To investigate how variation of proton spot properties (i.e., spot size, spot divergence, mean energy, and energy spread) influences dose reconstructions of patients in pencil beam scanning (PBS) treatment.
Methods: Organ/tissue averaged absorbed doses of the 1-and 5-year-old male NCI computational phantoms for intracranial and craniospinal irradiations were calculated by performing Monte Carlo (MC) simulations with the TOPAS (TOol for PArticle Simulation) MC code using the spot properties matching the beam commissioning data at the Maryland Proton Treatment Center (MPTC). The calculated organ/tissue doses were then compared with the values calculated using the spot properties assumed as an extremely different case (i.e., the mean energy is the nominal energy and the other spot properties are zero). The intracranial and craniospinal irradiations were planned on the pediatric phantoms by a professional dosimetrist at the MPTC. The proton RBE of 1.1 and the neutron RBE of 25 were applied to obtain the RBE-weighted absorbed doses. 10 billion protons were transported for each simulation to keep the statistical errors for the calculated organ/tissue doses mostly less than 1%.
Results: For the intracranial irradiations, most organs/tissues showed the dose differences were less than 5%, while only three organs/tissues (i.e., active marrow, muscle, and endosteum) for the 1-year-old phantom showed relatively large differences up to ~35%. For the craniospinal irradiations, the differences tended to be larger than those for the intracranial irradiations, but still were less than 10% for most cases.
Conclusion: The results of the present study indicate that the influence of the PBS spot properties on organ/tissue dose calculations is generally insignificant although only intracranial and craniospinal irradiations were considered. The influence might be even smaller in practice considering differences of the spot properties between proton centers should be less than considered in the present study.
Funding Support, Disclosures, and Conflict of Interest: This work was funded by the intramural program of the National Institutes of Health, National Cancer Institute, Division of Cancer Epidemiology and Genetics. The contents are solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health