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Purpose: Stereotactic magnetic resonance (MR)–guided adaptive radiation therapy (SMART) for prostate cancer allows for MR-based contouring, real-time MR motion management, and daily plan adaptation. The clinical and dosimetric benefits associated with prostate SMART remain largely unknown. Methods and Materials: A phase 1 trial of prostate SMART was conducted with primary endpoints of safety and feasibility. An additional cohort of patients similarly treated with prostate SMART were included in the analysis. SMART was delivered to 36.25 Gy in 5 fractions to the prostate ± seminal vesicles using the MRIdian linear accelerator system (ViewRay, Inc). Rates of urinary and gastrointestinal toxic effects and patient-reported outcome measures were assessed. Dosimetric analyses were conducted to evaluate the specific benefits of daily plan adaptation. Results: The cohort included 22 patients (n = 10 phase 1, n = 12 supplemental) treated in 110 fractions. Median follow-up was 7.9 months. Acute grade 2 urinary and gastrointestinal toxic effects were observed in 22.7% and 4.5%, respectively, and 4.5% and 0%, respectively, at last follow-up. No grade 3+ events were observed. Expanded Prostate Cancer Index-26 urinary obstructive scores decreased during SMART (mean, 9.3 points; P = .03) and returned to baseline by 3 months. No other significant changes in patient-reported outcome measures were observed. One-hundred percent of fractions required plan adaptation owing to exceeding organ-at-risk metrics (68%) or suboptimal target coverage (33%) resulting from anatomic changes. Minimum acceptable planning target volume, rectal, bladder, and urethra/bladder neck metrics were violated in 24%, 20%, 24%, and 33% of predicted plans, respectively; 0% of reoptimized plans violated metrics. Underlying causes for deficient dosimetry before reoptimization included changes in bladder filling, seminal vesicle position, prostate volume (median 4.7% increase by fraction 3; range, 0%-56%), and hotspots shifting into urethra/bladder neck. Conclusions: Prostate SMART results in low risk of acute toxic effects with improvements in target and organ-at-risk dosimetry. The clinical benefits resulting from daily plan adaptation, including urethra/bladder neck protection, warrant further investigation.

Purpose: The use of large-field external beam reirradiation (re-RT) after pelvic radiation therapy (RT) for genitourinary (GU) cancers has not been reported. We report the results of such treatment in patients with either symptomatic GU second malignant neoplasms or locally recurrent pelvic tumors after initial RT for whom surgery or further systemic therapy was not an option. Methods and materials: The records of 28 consecutive patients with advanced, bulky GU malignancies treated with high-dose, large-field re-RT with palliative intent between 2008 and 2014 were retrospectively reviewed. Descriptive outcome analyses focused on toxicities and symptom control, and responses were evaluated by 2 independent observers. Results: Twenty-seven male patients (96%) were included. Median initial external beam RT dose was 64 Gy (range, 30-75.6 Gy). The median time between initial RT and re-RT was 9.5 years (range, 0.2-32 years). At the time of re-RT, there were 16 local recurrences and 12 second malignant neoplasms together comprising 16 bladder, 10 prostate, 1 ureteral, and 1 penile cancer. Indications for re-RT were pain and bleeding/hemorrhage. The median equivalent sphere diameter planning target volume for re-RT was 8.6 cm (range, 4.7-16.3 cm). Given the severity of the symptoms and the bulk of the disease at the time of re-RT, a higher dose of RT was administered. The median re-RT dose was 50 Gy (range, 27.5-66 Gy). For patients who received