Dosimetric evaluation of 6 MV photon beams in small fields using PRIMO-penEasy Monte Carlo

Abstract

Dosimetric validation of clinical radiotherapy plans for small fields requires high-precision instrumentation to ensure accuracy and spatial resolution. This study proposes a combination of PRIMO and penEasy Monte Carlo simulators to address scenarios where direct measurements are impractical, challenging, or yield inconsistent results due to detector limitations. We compared three dosimetric tools: A Semiflex ionization chamber PTW (model 31010) within a water-filled acrylic tank; EBT3 radiochromic films positioned between solid water layers; and PRIMO-PenEasy simulations. Experiments were performed at San Ignacio University Hospital in Bogota, Colombia, with dose measurements in two small fields and one conventional field for an Axesse-type linear accelerator. Results indicate that EBT3 radiochromic film show better agreement with MC simulation than the Semiflex ionization chamber, likely due to the chamber dose averaging effect across its detection volume. Additionally, variations in off-axis ratios were noted in both crossplane and inplane directions with more pronounced discrepancies observed in small fields compared to the conventional field. Overall, PRIMO-PenEasy MC simulations demonstrated accurate dose distributions and can be used as a viable for dosimetric validation in small photon fields.