Room: AAPM ePoster Library
Purpose: To evaluate the difference between the dose to water (D?w?) calculated by Monte Carlo (MC) and the D?w? calculated from the dose to medium (D?m?) using three different conversion factors.
Methods: The MC model of a linear accelerator of 6 MV photon was created and validated using BEAMnrc MC code. Synthetic water phantoms were created using a middle layer of water, bone, lung or air. PPDs of D?m? and D?w? were calculated for 1x1 and 10x10 cm² fields using MC. The MC uncertainty for all PDDS points is < 1%. The voxel's size of water phantoms is 2 mm. D?w?s were calculated from D?m? of MC using 3 conversion factors: 1) averaged mass stopping power ratio (AMSP), 2) mass-energy absorption coefficient ratios (MEA) and 3) relative electron density ratio (RED).
Results: D?m? and D?w? of MC showed agreement within <2 % for the studied materials except in the bone. The absolute difference between D?m? and D?w? in the bone layer is not fixed which is higher near the interface regions. Using the three conversion factors, the maximum absolute differences of D?w? are 10.8%, 5%, and 5% for AMSP, MEA, and RED respectively comparing to D?w? of MC for 1x1 cm² field. This was less pronounced in the deeper part of the phantom.
Conclusion: D?w? is overestimated using the AMSP conversion factor compering to D?w? of MC. RED and MEA conversion factors are better for D?w? calculation of radiotherapy's dosimetry. The most accurate methodology to calculate D?w? is by using MC with various densities of water. The dose difference in the interface regions is due to electronic disequilibrium. For patient dosimetry where dose in bone clinically relevant like in bone metastasis, only D?m? should be used to have precise dosimetry in planning target volume (PTV).