Poster – 13: Evaluation of an in-house CCD camera film dosimetry imaging system for small field deliveries

Purpose: Radiochromic film dosimetry is a standard technique used in clinics to verify modern conformal radiation therapy delivery, and sometimes in research to validate other dosimeters. We are using film as a standard for comparison as we improve high-resolution three-dimensional gel systems for small field dosimetry; however, precise film dosimetry can be technically challenging. We report here measurements for fractionated stereotactic radiation therapy (FSRT) delivered using volumetric modulated arc therapy (VMAT) to investigate the accuracy and reproducibility of film measurements with a novel in-house readout system. We show that radiochromic film can accurately and reproducibly validate FSRT deliveries and also benchmark our gel dosimetry work. Methods: VMAT FSRT plans for metastases alone (PTV_MET) and whole brain plus metastases (WB+PTV_MET) were delivered onto a multi-configurational phantom with a sheet of EBT3 Gafchromic film inserted mid-plane. A dose of 400 cGy was prescribed to 4 small PTV_MET structures in the phantom, while a WB structure was prescribed a dose of 200 cGy in the WB+PTV_MET iterations. Doses generated from film readout with our in-house system were compared to treatment planned doses. Each delivery was repeated multiple times to assess reproducibility. Results and Conclusions: The reproducibility of film optical density readout was excellent throughout all experiments. Doses measured from the film agreed well with plans for the WB+PTV_MET delivery. But, film doses for PTV_MET only deliveries were significantly below planned doses. This discrepancy is due to stray/scattered light perturbations in our system during readout. Corrections schemes will be presented.