Characterization of Copper and Thoraeus Filters for a Small Animal Irradiator Using a Water Phantom and Film Measurements
Abstract
Purpose
Small animal irradiator radiation measurements for kV x-rays typically use in-air absolute dose rate calibration and film in solid water for planar dose distributions. Using water as a medium provides a close representation of soft tissue and reduces uncertainties in material differences and increases accuracy in the dosimetry. In this work we take in-water depth dose and profiles using Thoraeus (Th) and copper (Cu) filters and compare with film measurements.
Methods
A small animal irradiator with an irradiation spectrum of 225 kVp, 20 mA with a 0.3 mm Cu filtration and large focal spot size was used. Radiochromic film measurements for Cu and Th filter were performed at isocenter with 4 depths and 3 field sizes using a beam on time of 75 seconds. Depth dose and planar profiles were acquired in water near isocenter and for extended distances. A container with dimensions, ~ 18 x 13 x 8.5 cm is used as our water tank and a cc04 ion chamber was utilized. A 3D printed chamber holder, made from polyethylene Terephthalate Glycol (PETG) was designed for proper chamber placement attached to the animal stage. Percent depth dose (PDD), field size, penumbra distance and other beam parameters were calculated and analyzed for comparison.
Results
A comparison between the treatment planning system and film measurements for Cu filters showed an average deviation of 0.13 Gy for mean point doses; 0.13 mm for field sizes and 0.72 mm for beam penumbras. In comparison to Th, Cu showed higher surface mean point dose, 0.28 Gy. For extended distances, PDDs at 50 mm depth were 55.5% and 63.4%Cu for the Cu and TH filters, respectively.
Conclusion
In this study we characterized and compared measured results during commissioning of a small animal irradiator for Copper and Thoraeus filters using water and radiochromic film data.