SU‐E‐T‐448: A Dual‐Plan Superposition and Adaptive Setup Strategy for the Reduction of Planning Margins in Whole‐Pelvis and Prostate IMRT

Purpose: Increased global setup margins are needed to account for prostate motion within the pelvis, for alignment of the prostate and associated pelvic lymph node (PLN) volumes. This leads to increased rectum and bladder doses. We present a combined treatment planning and image guidance strategy wherein both the prostate and PLNs can be independently targeted, using a dual treatment plan (DP) that superimposes prostate and PLN dose prescriptions. Methods: Two IMRT plans of prescription dose 45 Gy, a conventional whole‐pelvis and prostate plan (WP) and a DP are developed. The WP possesses a 1‐cm setup margin for the prostate and PLN. It is assumed that pelvic bony anatomy will be used for image guidance during treatment. In the DP, a first prescription assigns 100% of prescription dose to the PLN volume and 80% to the prostate. Setup margins remain same as that of WP. In a second prescription, the additional 20% is assigned to the prostate volume only. Setup margin for the prostate is 3 mm. Here it assumed that bony‐anatomy image guidance is used for prescription 1 and that soft‐tissue (prostate) image guidance is used for prescription 2. Isocenter‐shift plans (Iso_shift DP) are developed in which the prostate‐only isocenters are moved 5mm in all directions: the Anterior, Posterior, Superior, Inferior and Lateral. Results: Rectum and bladder doses are significantly reduced in the DP relative to the WP. The rectum V35 is reduced to 21% from 72%; the bladder V35 is reduced to 15% from 26%. PTV coverage is essentially unchanged. The Iso_shift DP shows only very small differences from the DP. Conclusions: Significant reductions in rectum and bladder doses are achievable when this dual‐plan (DP) and dual image‐guidance strategy is employed. Even under significant prostate‐ motion conditions, PTV coverage is maintained with better critical structure sparing.