3171 - Matching Strategy on the Use of Image-Guided Hypo-Fractionated Radiotherapy for Localized Prostate Cancer

Y. Gao, X. Gao, M. Ma, X. Ren, and J. Chen; Department of Radiation Oncology, Peking University First Hospital, Beijing, China

Purpose/Objective(s): Image-guided radiotherapy (IGRT) is commonly recommended for prostate hypo-fractionated radiotherapy patients, but there is limited guidance on specific matching strategies. Moreover, there is a lack of consensus among healthcare institutions regarding the optimal frequency of IGRT image acquisition and verification protocols. This study aims to assess the effects of varying cone-beam computed tomography (CBCT) usage frequency and matching strategies on dose-volume parameters of both the target and organs at risk. Materials/

Methods: A total of 611 daily CBCT images were collected from 23 patients undergoing radical radiotherapy for prostate cancer. These patients were treated with moderate-fractionated volumetric modulated arc therapy (VMAT) at a prescription dose of 70Gy/2.8Gy/25f. Following the correction of setup errors using various matching strategies, the planning CT (pCT) scans were rigidly and then deformably registered with CBCT images. Subsequently, daily doses were mapped onto the pCT scan and the cumulative dose was assessed. A dosimetric comparison was then performed between daily CBCT and weekly CBCT, as well as between bone-matching and rectal anterior wall alignment methods (Wilcoxon rank test, p < 0.05).
Results: Compared to daily CBCT, weekly CBCT resulted in a significant reduction in the prescription dose to 95% of the planning target volume (PTV_D95%) (66.3 ± 1.6Gy vs. 60.6 ± 3.6Gy, mean ± SD) (P < 0.01). The actual cumulative dose of PTV_D95% based on bone-matching was 65.8 ± 2.0Gy, and the dose to 98% of the clinical target volume (CTV_98%) was 69.1± 1.6Gy, with no statistically significant difference in target coverage compared to the anterior rectal wall matching method. However, there was a significant increase in the high-dose volumes of the rectum, specifically V60Gy (2.0 ± 2.4% vs. 3.2 ± 3.1%) and V65Gy (0.3 ± 0.6% vs. 0.8 ± 1.2%) (P < 0.01).
Conclusion: In moderate-fractionated radiotherapy for prostate cancer, daily CBCT image guidance with sequential matching strategies involves an initial bone-matching phase, followed by soft-tissue matching, and importantly, adjustments based on the anterior wall of the rectum, improves target coverage and rectal protection compared to weekly IGRT or bone-matching IGRT.