TH‐AB‐201‐11: Small‐Field Beam Quality Corrections and Their Clinical Impact for Flattened and Unflattened Fields of the TrueBeam

Abstract

Purpose:To determine small‐field beam quality correction factors for several detectors used with flattened and unflattened beams of the Varian TrueBeam accelerator, and to quantify the clinical impact of these corrections through a treatment planning study.Methods:Small‐field beam quality correction factors were measured and simulated in EGSnrc for several microchambers and diodes. The Exradin W1 plastic scintillation detector was used as a reference for the measured results, and its response was validated using alanine measurements traceable to a primary standard. Symmetric, jaw‐defined field sizes ranging from 6 mm to 10 cm were used. Twelve hypofractionated clinical cases were then recalculated and/or reoptimized using beam models that were modified based on the resulting output factors. Three‐dimensional dose distributions and cumulative dose‐volume‐histogram plots were compared between the models. The impact of these measurements on the flattening‐filter‐scatter‐source (FFSS) model, and its subsequent effect on treatment plans, was also investigated.Results:The diodes exhibited over‐response in small fields, whereas the microchambers tended to under‐respond as field size decreased. Field‐size‐dependent effects were less significant for the flattening‐filter‐free beams (i.e., corrections were closer to unity) than for flattened fields of the same size. Recalculated and reoptimized plans displayed varying levels of deviation from the original dose distributions, with larger discrepancies at effective jaw settings of 3×3 cm2 or smaller. Dose distributions calculated using uncorrected microchamber output factors and/or modified FFSS showed large discrepancies in the low‐dose regions, which manifested as DVH changes for the organs at risk (OAR).Conclusion:Field‐size‐dependent beam quality corrections were measured and simulated for ionization chambers and diodes typically used in small fields. Plans calculated with models based on these measured data showed dosimetric deviations mainly out of field, which affected OAR dose metrics. Accordingly, these beam quality corrections should be accounted for during commissioning to ensure greater treatment planning accuracy.