Purpose: To provide small-animal percentage depth-dose (PDD) data as a function of source-to-skin distance (SSD), field size (FS) and animal size (AS) in order to translate experimentally-measured TG-319 skin dose to the dose at target depth.
Methods: The x-ray tube designs for two biological irradiators, the RT250 and the PXi XRAD-320, were simulated using the BEAMnrc Monte Carlo (MC) code to generate 200 and 320kVp photon beams, respectively. The 320 kV beam was simulated with two filtrations: a soft F1 aluminium filter and a hard F2 filter made of aluminium, tin and copper. Beams were collimated into circular fields of 0.5–10cm in diameter at SSDs of 10–60cm. MC dose calculations in 1–5cm diameter homogeneous small-animal phantoms as well as in heterogeneous phantoms with 3-mm diameter cylindrical bone inserts were performed using DOSXYZnrc. The depth doses in three simulated test-cases were estimated by applying SSD, FS and AS correction factors to a reference case (40cm SSD, 1cm FS and 5cm AS) and comparing the results with the simulated doses to assess the accuracy of dose estimation.
Results: The depth doses for three test-cases calculated at 200, 320 F1 and 320 F2 kVp agreed well with simulated doses, yielding dose differences of 1.4, 0.1 and 1.0%, respectively. Dose enhancement on the bone-tissue proximal interface ranged from 1.2 to 2.5 times the dose in the absence of bone for rib and 1.3 to 3.7 times for cortical bone, whilst the dose drop-off at 1cm depth behind the bone ranged from 1% to 6% for rib and 3% to 12% for cortical bone.
Conclusion: The developed dose estimation method can be used to translate TG-319 measured skin dose to dose at any depth.
Funding Support, Disclosures, and Conflict of Interest: This research was funded by the Natural Sciences and Engineering Research Council (NSERC) and the Canada Research Chair program.
Not Applicable / None Entered.