Room: Exhibit Hall
Purpose: External beam plan QA is an important component of patient radiotherapy safety. Plan QA such as independent MU calculation has been well established. As more clinical plan data are available, statistical analysis of these data may provide a helpful tool for quality control. This study is to investigate the statistical pattern of modulation factor (MF) associated with treatment depth, delivery technique, and treatment site (particularly for breast and prostate). MF was defined as the ratio of total plan MUs and the prescription dose.
Methods: Clinical photon beam plan data were retrieved which include 84 breast patients and 69 prostate patients. For breast RT, the commonly used tangent plans were considered in this study. Prostate plans include VMAT and IMRT. The MF value was calculated for each plan. The average treatment depth was also calculated which was defined as source-axis-distance (SAD=100) minus the average SSD of all treatment fields. MF was analyzed separately according to delivery techniques, i.e. 3D-CRT, VMAT and IMRT. Plot of MF versus treatment depth was analyzed.
Results: For tangent breast plan, MF values distributed within a narrow range (MF = 1.1-1.31; mean MF = 1.2076+/-0.0488). Plot of MF vs. average treatment depth shows a linear correlation with a linear regression of MF = 1.0708 + 0.0124 x (treatment depth), R2=0.2303. Prostate VMAT plans show a mean MF = 3.2667+/-0.4423 vs. mean MF = 5.3375+/- 0.9230 for IMRT plans. Its correlation with average treatment depth was not clearly demonstrated in the plot of MF vs. treatment depth. However, the plot show where most likely a plan MF value will be. Any point far from the majority may provide a warning flag.
Conclusion: MF may provide an interesting index for plan quality control. Plot of MF vs. treatment depth may provide a visual tool for such purpose.
Not Applicable / None Entered.