Kilovoltage transit and exit dosimetry for a small animal image-guided radiotherapy system using built-in EPID

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.

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.

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.

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.