2252 - Evaluation of Online Adaptive Radiation Therapy for PSA Recurrence in Prostate Cancer: Impact on PTV Coverage and Dose Reduction in Organs at Risk in Clinical Practice

Purpose/Objective(s): In salvage radiation therapy for prostate cancer patients with PSA recurrence following prostatectomy, the CTV includes the prostate bed. However, substantial interfractional changes in the prostate bed can occur due to variations in bladder and bowel control. Additionally, in some cases, organs at risk (OARs), such as the small intestine, are located near the CTV. Since 2022, our institution has been using ETHOS™ therapy to enable online adaptive radiation therapy (ART). This study evaluates the dose distribution to the planning target volume (PTV) and OARs in ART for PSA-recurrent prostate cancer and assesses its clinical utility.

Materials/Methods: Four patients with PSA recurrence after prostatectomy were included in this study. Salvage radiation therapy to the prostate bed was delivered with 66 Gy in 33 fractions using ETHOS therapy between July and October 2024. We compared the pretreatment scheduled plan (SCH), created based on the initial planning CT, with the adapted plan (ADP), generated using daily cone-beam CT (CBCT). PTV coverage (D98%, D95%, D50%, D2%) and OAR doses (rectum, bladder, bowel) were evaluated using the Wilcoxon signed-rank test. The correlation between bladder/rectal volume and PTV D98% was analyzed using Pearson’s correlation coefficient. Acute radiation toxicity was assessed based on CTCAE v5.0.

Results: PTV D98%, D95%, and D50% were statistically significantly higher in ADP than in SCH (SCH: 79.1%, 88.3%, and 99.6%; ADP: 90.8%, 94.2%, and 100.3%; p < 0.001), indicating improved PTV coverage (Table 1). The median PTV D2% was slightly higher in ADP than in SCH (SCH: 103.8% vs. ADP: 104.3%, p < 0.001); however, although statistically significant, this difference is likely of minimal clinical relevance (Table 1). Rectal V65Gy was significantly lower in ADP than in SCH (0.66% vs. 0.09%, p < 0.001), and bowel dose (0.1 cc) was also significantly lower in ADP (36.6 Gy vs. 30.8 Gy, p < 0.001) (Table 1). A negative correlation was observed between bladder volume and PTV D98% in SCH, whereas in ADP, PTV D98% remained stable regardless of bladder and rectal volume variations. No grade 2 or higher acute radiation toxicity was observed.

Conclusion: Online ART improved PTV coverage and reduced the influence of bladder and rectal volume variations. Additionally, ART showed favorable safety, as acute radiation toxicity was minimal.