MR Guided SBRT for Early Prostate Cancer: Initial Experience with 270 Consecutive Patients

Abstract

<h3>Purpose/Objective(s)</h3> Stereotactic body radiotherapy (SBRT) is a recommended treatment for localized prostate cancer. We present our experience with the first consecutive 270 patients (8.2019-11.2022) treated with MR-guided SBRT with minimum 3 months follow-up and acute side effects. <h3>Materials/Methods</h3> This IRB approved series includes histologically confirmed prostate cancer patients, consecutively treated with MR-guided SBRT. Staging included diagnostic MRI (86%) and PET-PSMA (78.5%) for patients with grade >1 disease. No fiducials or rectal spacers were used. Treatment planning included fusion with pre-treatment imaging and contour review with an expert MRI radiologist. Prescription dose was 36.25Gy in 5fx delivered over 2 weeks to the entire prostate with 3mm margins. High-risk area identified on MRI (PIRADS4-5) or PET received 40Gy. On each fraction, MRI was performed and plan adapted if tumor and OAR doses differed significantly from the simulation-based plan. During radiation, the prostate was monitored and treatment automatically interrupted if the target volume moved out of the PTV range. <h3>Results</h3> Mean age was 71.2(35-90) years. Clinical stage was I-85.5%, II-13%, III-1.5%. MRI showed suspected ECE- 12.5%, NVB involvement-9.5%, SV involvement-1.5%. Group grade was 1-20.5%, 2-52.0%, 3-13.5%,4-13.5%, 5-0.5%. 77 (38%) patients had involvement of ≥50% cores. Pre-treatment PSA ranged from 1-to 77, PSA was>10 in 53 patients (26.5%). Median prostate volume-57cc (16-171). Sixteen (36%) received adjuvant hormone therapy. 739/1000 (73.9%) daily treatments required plan adaptation. Significant prostate movement during treatment noted in 7 patients. Acute GU toxicity was Grade I-21%, II-14% (4 patients required temporary catheter). Acute GI toxicity was grade I-7.5%, II-1.5%. All grade II and 95% of grade I toxicities resolved by 3 months follow-up. No grade III toxicity reported. Three-month post-treatment PSA decreased in 99% of patients, range 0.14-23 (median 2.87). 99.5% are disease free with median follow up of 14 months. Four patients required a temporary catheter during treatment, one following a previous event unrelated to radiation and the other three following radiation. Two of them after three fractions and one after two fractions of radiation. The catheter was removed after 5-7 days except in one patient who was weaned after 3 weeks. Age was significantly associated with acute nocturia, (p=0.005). Number of adapted fractions, catheter insertion was not associated with age, PSA at dx, prostate volume and BMI. Prostate size was not associated with either ISUP or Gleason. <h3>Conclusion</h3> MR guided prostate SBRT shows very low risk of transient's acute toxicity, little long-term toxicity and excellent response (median 14 months). Real-time MR imaging and plan adaptation when required is important for ensuring accurate positioning and SBRT delivery, and prevents geographical miss and inadvertent dose delivery to OAR.