SU‐E‐T‐741: The Effect of Grid Size on Dose Calculation Accuracy in Head & Neck and Prostate Cancers

Purpose: In this study, we evaluated the effect of grid size on dose calculation accuracy using 2 head & neck and 2 prostate IMRT cases. Methods: Dose distribution of four IMRT plan data were calculated at four calculation grid sizes (1.25, 2.5, 5, and 10 mm) and the calculated dose distributions were compared with measured dose distribution using 2D diode array detector. Results: there was no obvious difference in pass rate of gamma analysis with 3 mm/3% acceptance criteria except 10 mm grid size, we found that the pass rates of 2.5, 5 and 10 mm grid size were decreased 5%, 20% and 31.53% respectively. The calculation time were about 11.5 min, 4.77 min, 2.95 min, and 11.5 min at 1.25, 2.5, 5, and 10 mm, respectively and as the grid size increased to double, the calculation time decreased about one‐half. The grid size effect was observed more clearly in the high gradient area than the low gradient area. Conclusions: In conclusion, 2.5 mm grid size is considered acceptable for most IMRT plans but at least in the high gradient area, 1.25 mm grid size is required to accurately predict the dose distribution.