PRIMO-PenEasy Monte Carlo simulation compared with dosimetric results for 6MV photon beams in small fields

Dosimetric validation of clinically prescribed plans for small fields requires adapted instrumentation that ensures high accuracy and spatial resolution. The objective of this study is to propose a solution by employing PRIMO-penEasy MC simulation for scenarios where obtaining direct measurements is impractical, poses challenges, or leads to inconsistencies in measurements from various detectors when assessing treatments involving small fields. This study presents the application of three distinct tools: the Semiflex ionization chamber PTW, model 31010, placed in an acrylic tank filled with water; EBT3 radiochromic films positioned between layers of a solid water phantom; and PRIMO-PenEasy Monte Carlo (MC) simulations for small photon beams. The experimental dosimetric outcomes were acquired at San Ignacio University Hospital in Bogota, Colombia, involving two small fields and one conventional field. These fields were generated using an Axesse-type linear accelerator. Radiochromic film show better agreement with MC simulation compared to the semiflex ionization chamber data, due to the fact that measurements with the 1 ionization chamber involve averaging the dose across the detection volume. Additionally, variations in off-axis ratios are observed in both crossplane and inplane directions. This difference is more evident in the case of two small fields compared to a conventional field. This observation can be attributed to the specific opening direction of the multi-blade collimation system and XY-jaws. Our study demonstrated that PRIMO-PenEasy MC simulation provides precise dose distributions and can be used as an alternative computational method to obtain the characteristics of small photon beams from an Axesse-type linear accelerator. The simulation results offer a means to reconcile disparities observed in values measured with different detectors. The values computed by PRIMO-PenEasy Monte Carlo (MC) can serve as a reliable indicator of accuracy and, consequently, be employed for dosimetric validation of prescribed clinical plans.