Online Adaptive Stereotactic Body Radiotherapy for Localized Prostate Cancer

Background: Stereotactic body radiotherapy (SBRT) for localized prostate cancer has demonstrated non-inferior oncological outcomes and toxicity profiles to conventionally or moderately hypofractioned radiotherapy regimens, while offering the advantage of shorter treatment durations. However, SBRT may not be suitable for all patients, particularly those with lower urogenital tract symptoms and/or prostatic hyperplasia. Methods: This study aims to evaluate the safety and efficacy of weekly computed tomography (CT) or magnetic resonance image-guided (MRI) online adaptive SBRT in patients with intermediate to (very) high-risk localized prostate cancer who present with lower urinary tract symptoms (International Prostate Symptom Score \[IPSS\] \> 12) and/or have prostate hyperplasia (prostate volume \>60 mL). The primary outcome measure is urogenital toxicity grade ≥3 within 3 months after completion of SBRT (according to CTCAE V5.0 and RTOG) or discontinuation of therapy. Our aim is to show that the event rate is at 3% below a clinically acceptable threshold, which is set at 20%. Under the null hypothesis, this design with an alpha of 0.05 and power of 80% results in an expected number of cases of 30. In our validation cohort we assume that around 3% of patients will have grade 3 long-term urogenital toxicity (i.e. 2 patients). The rate of treatment discontinuation within 3 months is considered negligible. Our aim is to show with high probability, that the event rate is below a clinically acceptable threshold, which is set at 12%. Under the null hypothesis, this design with an alpha of 0.05 and power of 80% results in an expected number of cases of 75.