SU‐E‐T‐649: Planning Margins Validation for Concurrent Treatment of Prostate and Pelvic Lymph Nodes Under Daily Image Guided Delivery

Abstract

Purpose: To validate prostate planning margins for concurrent treatment of the prostate and pelvic lymph nodes (PLN) under daily kilo‐voltage cone beam (KV‐CBCT) imaging guidance while applying MLC and iso‐center shifting methods to compensate for inter‐fractional prostate motion. Methods and Materials: Ninety‐four daily KV‐CBCTs from five patients, who received IMRT treatment with a clinical planning margin to the prostate of 8mm/6mm posterior (M(8,6)) and 5mm to the PLN, were selected. Three additional IMRT plans were created for each patient with the prostate planning margins of 6mm/4mm posterior (M(6,4)), 4mm/2mm posterior (M(4,2)) and 2mm uniform margin (M(2,2)). The PLN planning margin remained 5mm. Subsequently, each plan was applied to the daily KV‐CBCT using MLC and iso‐center shifting methods. Daily D95 of the prostate and D95 of the PLN, and D5 of the rectum and D5 of the bladder were evaluated for adequate planning margins. Results: For both the MLC and iso‐center shifting methods, D95 of the prostate was greater than or equal 97% of the prescription dose in 97.8% (100%), 98.9% (97.9%), 95.8% (97.9%), and 93.7%(96.8%) of 94 fractions, for M(8,6), M(6,4), M(4,2), and M(2,2) respectively. Accordingly, D95 of the PLN was greater than or equal 97% of the prescription dose in 98.9%(100%), 100%(98.9%), 98.9%(98.9%), and 100%(98.9%), for M(8,6), M(6,4), M(4,2), and M(2,2) respectively. For the rectum, D5 exceeded 105% of the original IMRT D5 for the MLC‐shift (and iso‐center shift) plans in 16%(14.9%), 14.9%(16.4%), 5.4%(12.2%) and 4.3%(12.2%) for these four planning margins respectively. For the bladder, D5 exceeded 105% of the original IMRT D5 for the MLC‐shift (and iso‐center shift) plans in 0%(1.1%), 6.4% 3.2%), 13.9%(4.3%), and 8.6%(4.3%), respectively. Conclusion: With 5 mm planning margin to the PLN, both MLC and iso‐center shifting methods can reduce the prostate planning margin to 4mm/2mm posterior while achieving adequate targets coverage. This research was supported by the United States Army Medical Research and Materiel Command (USAMRMC) (Grant No. W81XWH‐080‐0358)