Poster - 54: Development and Evaluation of Normal Tissue Objective Parameters and Avoidance Regions for Prostate Bed VMAT Treatments




A retrospective study was conducted to determine optimal NTO parameters on five patients that were treated with radical prostate doses of 66 Gy in 33 fractions to the prostate bed. The purpose of this work is to evaluate the effectiveness of various NTO settings and also provide insight into whether the use of multiple sparing techniques is beneficial or redundant for a given site.


The effect of varying the NTO parameters were systematically examined in conjunction with the option of an avoidance region: a “ring” structure around the planning target volume. Within each of these groups, four plans were made; three with varying NTO settings as well as a fourth control plan not using the NTO. The three NTO settings used were the Varian default values, the automatic setting (a selectable option in the NTO function), and a developed set of test parameters (Manual NTO).


The site-specific NTO plans showed the greatest improvement regardless of whether they were used in combination with the ring structure. The most striking improvement in OAR sparing occurred when using the Manual NTO settings without the ring structure. In this setting, the average reduction in mean bladder dose was 473 cGy. The automatic NTO and Varian default settings produced improvements of 146 cGy and 12 cGy, respectively.


The use of site-specific NTO parameters produced plans with marked improvement to normal tissue sparing in comparison to plans using the Varian default settings or the automatic NTO.