1053 - Phase II Study of Stereotactic Body Radiation Therapy with Simultaneous Integrated Boost (SBRT-SIB) Delivered on an MR-LINAC for Intermediate and High Risk Prostate Cancer: 2 Year Outcomes

03:25pm - 03:30pm ET

Room 253

Presenter(s)

Emily Weg, MD - Weill Cornell Medicine, New York, NY

E. S. Weg¹, F. Gregucci², X. K. Zhou¹, S. T. Tagawa³, H. Nagar⁴, A. E. Marciscano⁵, and S. C. Formenti²; ¹Weill Cornell Medical College, New York, NY, ²Department of Radiation Oncology, NewYork-Presbyterian/Weill Cornell Medicine, New York, NY, ³Division of Hematology & Medical Oncology, Weill Cornell Medical College, New York, NY, ⁴Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, ⁵Department of Radiation Oncology, Mass General Brigham Cancer Institute, Boston, MA

Purpose/Objective(s): This prospective phase II study aimed to assess 2-year recurrence free survival rates along with toxicity and quality of life (QOL) outcomes in men with NCCN unfavorable intermediate and high risk prostate cancer treated with PSMA PET-informed MR-guided SBRT-SIB with six months of androgen deprivation therapy (ADT).

Materials/Methods: 37 patients were treated on study on a 0.35-Tesla magnetic resonance imaging linear accelerator (MR-LINAC). Patients received a dose of 36.25Gy in 5 fractions to the prostate and seminal vesicles with a SIB up to 45Gy to the dominant intraprostatic lesion (DIL) based on diagnostic PSMA PET/MR, along with ADT for 6 months. Elective pelvic nodal RT was not permitted. Organ-at-risk constraints were prioritized over GTV coverage following an isotoxic approach. A post-SBRT surveillance PSMA PET/MR was obtained at 1 year after treatment. Patients were followed with toxicity assessments and QOL questionnaires along with PSA for 2 years and with standard of care surveillance thereafter.

Results: 29 (78%) had unfavorable intermediate and 8 (22%) had high risk disease. Radiological T stage was T2a/b/c in 32 (86%) patients and T3a in 5 (14%) patients. Median PSA at diagnosis was 8.5ng/dl (interquartile range (IQR) 5.23-12.29ng/dl), with Gleason grade group 2, 3, 4/5 seen in 16 (43%), 17 (46%), and 4 (11%) patients, respectively. 34 (92%) patients had a perirectal hydrogel spacer placed prior to SBRT. A median dose of 45Gy was prescribed to the GTV, with a median minimum dose to GTV of 42Gy (IQR 39.4 – 44.2Gy). Only 6 of 185 total fractions were adapted on the MR-LINAC. Median follow up was 3.6 years, and median PSA at last follow up was 0.14ng/dl (IQR 0.06 – 0.39ng/dl). Post-treatment PSMA PET/MR was obtained for 30 patients at median 1.1 years after SBRT. No scan showed suspicious activity in the prostate, and only 1 scan demonstrated clinically significant findings with metastatic abdominopelvic adenopathy; this was the only patient who developed metastatic disease, and this scan was acquired 2.7 years after RT. The 2-year recurrence free survival rate was 100%.

Rate of acute grade 2 genitourinary (GU) and gastrointestinal (GI) events was 10% (3/30) and 3.3% (1/30), respectively. Rate of late grade 2 GU and GI events was 6.5% (2/31) and 0% (0/31). No patients developed grade 3 or higher GU or GI toxicity. 10% of patients reported a clinically relevant deterioration in urinary function based on American Urological Association score at any point between treatment and 2 years.

Conclusion: SBRT-SIB delivered on an MR-LINAC for unfavorable intermediate and high risk prostate cancer achieved high doses to the DIL with excellent disease control, low incidence of GU and GI toxicity, no severe adverse effects, with minimal negative impact on urinary quality of life. Correlative analyses of blood specimens are forthcoming.