Purpose: We investigated the potential use of the built-in electronic portal imaging device (EPID) in the small animal radiation research platform (SARRP) as a dosimeter in the kV energy range. To this end, we developed a method for converting portal images to a two-dimensional (2D) dose maps at the detector plane and object's exit surface and validated them against empirical dose measurements.
Methods: We calibrated the SARRP's EPID to measure transit dose. The transit dose map was back-projected to calculate 2D dose distribution at the object's exit surface. The accuracy of transit and exit dose distributions was independently validated with a PinPoint ion chamber (IC) and Gafchromic EBT3 film measurements for a range of radiation dose rates (0.43-2.78 cGy/s), cone sizes (5-40 mm), in a homogeneous phantom of varying thickness (0-50 mm) and in an inhomogeneous phantom containing graphite, cork, air, and aluminum.
Results: In-air central axis (CAX) transit dose values measured with the EPID showed close agreement with film and IC measurements. The maximum differences in EPID in-air transit measurements with film or IC measurements were 1%. The EPID was capable of accurately measuring phantom transit dose independently of the attenuating phantom thickness, with average discrepancies of 0.5% and 2.9% with IC and film, respectively, where the maximum difference between the EPID and the IC was 1.8%. The results were slightly worse for film, with maximum differences in 4.9%. Output factor measurements using EPID were within 2.9% of both IC and film measurements. In addition, calculated exit doses agreed with film values within ≤3.1%, for attenuating phantom thicknesses ≥15 mm. The agreement became worse with decreasing phantom thickness; for thicknesses of 10 and 5 mm, agreements were ≤5.7% and ≤6.9%, respectively. Compared to film, transit and exit profiles measured with the EPID showed average differences <2% for both homogeneous and inhomogeneous materials.
Conclusion: We developed and validated a novel 2D transit/exit dosimetry for a kV SA-IGRT system using an EPID. We verified the accuracy of our method to measure EPID transit and exit dose distributions for a range of dose rates, beam attenuation, and collimation. Our results indicate that the EPID can be used as a simple, convenient device for kV dose delivery verification in small animal radiotherapy.
Keywords: EPID; dosimetry; image-guided radiotherapy; kilovoltage; preclinical; small animal.
© 2018 American Association of Physicists in Medicine.