CBCT-based Online Adaptive Radiotherapy for Prostate Cancer: Dosimetrical Aspects and Comparison to Non-Adaptive Conventional IGRT

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Introduction Daily anatomical variations in prostate cancer radiotherapy, particularly due to pelvic organ motion and filling, can compromise target coverage and increase exposure to organs at risk (OARs). Conventional image-guided radiotherapy (IGRT) uses fixed safety margins and daily couch corrections to account for these variations, potentially leading to overtreatment of healthy tissue or insufficient tumor coverage. Online adaptive radiotherapy (oART), based on cone-beam computed tomography (CBCT), enables daily plan adaptation to the patient's anatomy, offering improved precision, enhanced target coverage, and better OAR sparing. This retrospective study compares oART to conventional IGRT in prostate cancer treatment. Methods A total of 153 treatment fractions from six consecutive prostate cancer patients treated with oART on a Varian Ethos system were analyzed. For each fraction, three plans were evaluated: the scheduled plan (initial plan recalculated on daily CBCT), the adapted plan (reoptimized based on daily anatomy), and the verification plan (applied dose recalculated on a post-adaptation CBCT). Dose–volume metrics for target volumes and OARs were assessed, and clinical acceptability was evaluated. Interfractional prostate volume changes and treatment times were examined. Results CTV D 98% improved significantly with adaptation (median 97.85% to 98.55%; p < 0.01) and further increased in the verification plan (98.8%; p < 0.01), alongside reduced interquartile ranges. PTV D 98% rose from 90.1% to 97.1% with adaptation and to 96.9% after verification (p < 0.01). Bowel and bladder doses showed dosimetrical advantage. Clinically acceptable plans increased from 24.8% (scheduled) to 98% (adapted) and 85.6% (verification). Scheduled plans were not used clinically. Median prostate volume remained stable despite inter-individual variation. oART required about twice the treatment time of IGRT. Conclusion Although more time-consuming, oART improved target dose coverage and optimized OAR sparing, while simultaneously reducing dose variability for both the target and some OARs compared to IGRT. The plan acceptability improved significantly.

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