Sci—Sat AM(1): Planning — 07: A Novel and Efficient Offline Approach to Replanning Using Daily CBCT Imaging

Purpose: To present an original adaptive radiation therapy technique using daily CBCTimages.Method and Materials: For each fraction (fx) of a prostate treatment of 76Gy (n=38 fx), daily kV CBCT dataset was used for: (1) patient repositioning based on prostate soft tissue manual match; (2) generating an optimized 7‐beam IMRT plan which was also calculated on every subsequent fx. This allows building a pool of plans containing numerous anatomical configurations considering bladder and rectum fillings and relative positions. Three strategies were elaborated: (1) “classical strategy”, a non‐correction strategy where the patient is entirely treated with the initial CT‐based plan; (2) “optimal strategy”, an online strategy where the patient is treated with the daily optimized CBCT‐based plan; (3) “Continuous Offline Replanning” (COR) strategy, uses a pool of calculated offline plans from previous fx CBCT sets. The patient is treated with the “best” available plan satisfying volumetric and dosimetric indices. Results: A total of 741, n(n‐1)/2, plans were calculated. With the classical strategy, for 14/38 fractions, PTV dose coverage V(95%) ranged from 94.83% to 100%. Unsurprisingly, dose coverage was excellent in the “optimal strategy” with an average of 99.95% for the 38 fractions. In the COR strategy, dose coverage ranged from 95.31% to 100% for 35/38 fractions which represents a significant treatment enhancement compared with the classical strategy. A subset of only 8 plans was enough to achieve this result with COR. Conclusion: This study opens a novel and efficient offline approach to replanning using daily CBCTimaging.