2449 - Dose and Dose-Weighted Linear Energy Transfer Calculations In a Cohort of Pediatric Passive Scattering Proton Therapy Patients

Dose and Dose-Weighted Linear Energy Transfer Calculations in a Cohort of Pediatric Passive Scattering Proton Therapy Patients

Purpose/Objective(s): The multi-institutional Pediatric Proton and Photon Therapy Comparison Cohort study will evaluate and compare risk of second malignancies in patients treated with proton versus photon radiotherapy (RT). Retrospective calculations of normal tissue doses are planned for certain patients using Monte Carlo (MC) methods; however, late effects are influenced not only by physical dose but also by radiation quality. In proton therapy, the primary proton beam and secondary particles deposit dose through charged particle interactions. As the relative biological effectiveness for cancer induction may be associated with the density of ionization events, or the linear energy transfer (LET), evaluation of this quantity may support future analyses of treatment-related outcomes. This pilot study aims to develop a workflow to estimate LET distributions in patients within this cohort study who were treated with passive scattering proton therapy (PSPT).

Materials/Methods: A MC workflow built around the TOPAS simulation package was applied to data from a single institution for thirty patients treated with the passive scattering modality of proton RT. For patients with limited imaging, the workflow extended partial body scans to whole body anatomy. Normal tissue dose estimates were reconstructed for fifteen patients receiving intracranial irradiation and fifteen receiving craniospinal irradiation (CSI). Additionally, dose-averaged linear energy transfer (LETd) distributions were calculated using the built-in ProtonLET scorer.

Results: Normal tissues located near or outside the treatment field typically received higher LETd than those partially or fully within the field. For example, in a patient treated with CSI, the near-field esophagus received a mean absorbed dose of 3.9 Gy and the partially in-field right kidney received 3.2 Gy. Mean LETd values were 10.3 keV/um and 4.8 keV/um, respectively. Among patients treated with intracranial irradiation, doses to out-of-field normal tissues were typically on the order of a few cGy, with mean LETd estimates ranging from approximately 9 to 24 keV/um.

Conclusion: This study presents a MC-based workflow for calculating patient specific LETd estimates for patients treated with PSPT. The resulting data will allow future cohort analyses to account for radiation quality when assessing treatment-related health risks. This supports The Pediatric Proton and Photon Therapy Comparison Cohort aiming to inform clinical decisions on radiation therapy.