TH-D-BRB-03: The Application of a Novel Three Dimensional Dosimetry System to the Acquisition of Commissioning Data of Small Photon Fields

Abstract

Purpose: To investigate the application of a radiochromic plastic 3D dosimeter to the measurement of commissioning data in radiosurgery fields. Method and Materials: In order to measure total scatter factors, a single cylinder of a radiochromic plastic (PRESAGE) was irradiated with five different radiosurgery fields uniformly spaced in a circular geometry. These square fields were delivered with a Novalis Tx system equipped with a high definition multileaf collimator (HDMLC) and ranged from 40 mm to 5 mm. The radiation‐induced change in optical density within the dosimeter was digitized using a first generation laser based optical CT system. Additionally, a cylinder of PRESAGE was irradiated in a water bath with one 10 mm beam to measure percent depth dose. This dosimeter was digitized using a novel in‐house high‐resolution CCD based optical CT system. Gafchromic EBT film was used to validate the assumption of negligible cross‐field scatter associated with the combined field geometry. It also served as an independent check of all PRESAGE measurements. Results: The total scatter factor measurements made with PRESAGE agreed with radiosurgery mini‐ion chamber commissioning measurements with a percent error of less than 3.2% for 30, 20, and 5 mm fields. The percent error for the 10 mm field was 5.5%. The scatter measurements made with radiochromic film confirmed the validity of the combined field geometry and agreed with the commissioning data within 2%. PDD measurements showed good agreement with the high‐resolution diode commissioning measurements with a competitive level of resolution. Conclusion: The PRESAGE and optical CT dosimetrysystem presents a new approach for the commissioning of radiosurgery fields, with many advantages. These include high‐resolution 3D beam data measured with energy, dose rate, and directional independence, as well as a reduction in placement and volume averaging errors.