Stereotactic Body Radiation Therapy to the Prostate With Focal Boost: Analysis of the Primary Endpoint in the DELINEATE Trial Cohort E

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Background Prostate stereotactic body radiotherapy (SBRT) is an effective approach for localised prostate cancer. The most common location for local recurrence after prostate radiotherapy is at the site of MRI-visible tumour at diagnosis. Therefore, we investigated the toxicity associated with focal boost SBRT. Methods Patients with intermediate or high-risk prostate cancer were recruited in a single-institution, prospective phase II trial, XXXXXX Cohort E. SBRT to a dose of 36.25Gy, with a simultaneous integrated boost to the tumour(s) up to 45Gy in 5 fractions, was delivered to 49 men. The primary endpoint assessed late grade 2+ and late grade 3+ Radiation Therapy Oncology Group bowel toxicity at 1 year. Secondary endpoints included acute and late toxicity measured with clinician- and patient-reported outcomes and freedom from biochemical or clinical failure. Results Median follow-up was 48.8 months. Most (63%) were classified as having NCCN intermediate-risk disease, the remainder high risk. The median (IQR) boost volume was 1.9 (IQR: 1.4-2.81) cm 3 and all patients received at least short-course hormone therapy. The highest rate of acute RTOG GI and GU of grade 2+was seen at week 3 and 4 respectively, settling to baseline levels by week 12. Cumulative 1-year incidence of RTOG grade ≥2 GI late toxicity was 6.1% (90% CI: 2.4%, 15.1%). There was no reported late grade 3 or worse bowel toxicity using RTOG or CTCAE scales. There was no late grade 3+ GU CTCAE GU toxicity but 4.1% recorded RTOG Grade 3 GU toxicity by 1 year. Cumulative 1-year incidence of RTOG grade ≥2 GU toxicity was 10.2%. Conclusion Delivery of a focal boost with SBRT is feasible, with point estimates of toxicity rates below the pre-specified safety levels for this pilot study. The incidence of GU toxicity has predominance over GI toxicity and further analysis is ongoing to help identify the determinants of urinary toxicity after prostate SBRT.

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