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Stereotactic prostate radiotherapy is emerging as an effective approach for localised prostate cancer.  The most common site for local recurrence after prostate radiotherapy is the dominant intraprostatic tumour lesion (DIL), suggesting that focal radiation boosts to the DIL can improve the therapeutic ratio. Therefore, we investigated the toxicity associated with focal boost SBRT delivered alternate days to the DIL identified on MRI.

Patients with intermediate or high-risk prostate cancer were recruited in a single-institution, prospective phase II trial. Patients treated within Cohort E of the DELINEATE trial (ISRCTN04483921) received 36.25Gy to the whole prostate, with a simultaneous integrated boost (SIB) to the dominant nodules planned to a total dose of 45Gy in 5 fractions. Boost was permitted for no more than 3 separate nodules, and no maximum boost volume was stipulated. Mandatory dose constraints were defined for both target coverage and OARs including rectum, bowel, bladder and urethra. Clinician-reported outcomes (CRO) were assessed using CTCAE v4 and RTOG, with patient-reported outcomes (PRO) including EPIC-26 and IPSS. The primary endpoint was cumulative RTOG late rectal toxicity of grade 2+ at 1 year.

We present the acute and initial late toxicity, including the primary endpoint.

Between July 2018 and July 2020, 51 patients were recruited, with 2 patients withdrawing prior to treatment. Median follow-up at time of analysis was 26 months (minimum 13 months). At time of recruitment, mean age was 70.6(sd: 5.2) years, median PSA of 9.0(IQR: 6.1-12.0) ng/ml and 63.3% had NCCN intermediate risk disease, with 30% of patients having radiological T3a disease. Median MRI-segmented DIL volume was 1.28(IQR: 0.77-1.93) cm³. Distance to urethra was <5mm in 17/51 patients. All patients received hormone therapy, with the majority of patients (87.8%) prescribed six months LHRHa therapy. The highest proportion of patients with worst acute RTOG GI and GU of grade > 2 was seen at week 3 and 4 respectively, with this settling back to baseline by week 18 (Figure 1). The cumulative 1-year incidence of RTOG grade > 2 rectal toxicity was 6.2% (1-sided 95% CI: 15.2%). There was no reported late grade 3 or worse rectal toxicity at 1 year using either RTOG or CTCAEv4 scales. At 1 year after the start of radiotherapy, there was a peak in RTOG and CTCAE urinary toxicity. The 2 year late cumulative RTOG grade 2+ GI and GU toxicity rates were 11.1% and 17.7% respectively. Figure 2 displays the grade distribution (%) of rectal and urinary adverse events measured with RTOG and CTCAE and the cumulative incidences. There were no recorded events of progression.

Delivery of a focal boost with SBRT is feasible, with the trend in urinary toxicity comparable to contemporary SBRT series without a boost. The incidence of GU toxicity has predominance over GI toxicity and further dosimetric analysis is planned to help identify the determinants of urinary toxicity in prostate SBRT.