Room: ePoster Forums
Purpose: Proton range is indirectly calculated in Monte Carlo Dose Calculation (MCDC) by Hounsfield unit (HU) to mass density for human tissues with their scattering properties. However, stoichiometric calibration of HU is inaccurate when body size or elemental composition deviates from its assumed tissue conditions. Therefore, site-specific validation of anthropomorphic and biological tissue phantoms must be done to assess the magnitude of associated dosimetric uncertainties.
Methods: Pseudo proton radiography technique is selected to penetrate anthropomorphic and animal tissue phantoms and two-dimensional ionization chamber array MatriXX PT was positioned directly under the treatment couch to detect exit dose. The same stoichiometric calibration of HU versus mass density was used in RayStation 8A and Eclipse AcurosPT 13.7.20. Anterior pencil beam scanning beams of various energies were used to achieve different percentage of penetration for the brain, head/neck, lung and pelvic parts of anthropomorphic and a 20x20x20 cmÂ³ biological phantom made of pig tissues. Measurements were compared to three-dimensional dose cubes calculated with MCDC from both TPS using gamma criteria from 2 mm/2% to 4 mm/4% to achieve >90% passing rate.
Results: Based on the best three-dimensional match of dose distributions, range disagreements for the were up to 4 mm between both TPS and measurements for anthropomorphic phantom but within 2 mm for the biological phantom. 2 mm/2% can be achieved for most brain and biological phantom measurements but 3 mm/3% for most head neck, lung and thoracic measurements. The worst disagreement of 4 mm/4% typically happens where primary radiation is stopped but there are significant scattered doses from adjacent regions without as many bony structures.
Conclusion: Stoichiometric calibration of HU is more accurate in biological than anthropomorphic phantom. Pseudo proton radiography measurements can assess MCDCâ€™s dosimetric accuracies and potentially provide patient specific HU calibration to achieve better range uncertainties.