Preferred treatment position between supine and prone for pelvic radiation therapy; quantification of the intrafractional body motion component by 3D surface imaging system

Purpose: We investigated the preferred treatment position between supine and prone during pelvic radiation treatment using real time tracking data from AlignRT. Our findings will provide valuable information regarding the role of intrafractional body motion in answering the question of prone versus supine position for pelvis radiation. Methods: Ten patients receiving pelvic radiation were enrolled in this study. For each patient, two simulation helical CT scans were performed, one in supine and one in prone position. Body surface contours were automatically generated and then exported to the AlignRT system as reference images. AlignRT continuous patient body motion tracking (1.5 to 2 minutes) was performed for both positions for each patient once per week for five weeks. The equivalent patient body motion along three principle directions was calculated from the six degree of freedom real time patient displacements data. The maximum and the standard deviation (STD) of equivalent patient body motion were calculated, so as the average of maximum and STD of equivalent patient motion over five fractions. These were then compared between supine and prone orientations. Results: A correlation was observed between the intrafractional body motion and large BMI. For overweight/obese patients, the intrafractional body motion was smaller for the supine position in both vertical and longitudinal directions. For normal range BMI patients, we observed no clear advantage for either supine or prone position in both vertical and longitudinal directions. In lateral direction, the intrafractional motion did not have statistically difference between two positions. Conclusion: Our study shows that the amount of intrafractional body motion between supine and prone orientation is correlated with patient BMI. Overweight/obese patients experienced significantly less overall body motion in supine orientation. The preferred treatment position for normal BMI patients was seen to be individually variable.