E. Chang1, S. Choi2, R. Z. Zacharia1, C. Tang2, C. J. Hassanzadeh3, O. Mohamad3, S. J. Frank4, S. J. Shah1, H. Mok2, R. Kudchadker1, W. Du5, J. Wang5, J. Yang5, L. L. Mayo6, G. M. Chronowski1, K. E. Hoffman1, S. E. McGuire2, D. A. Kuban1, and Q. N. Nguyen1; 1Department of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, 2Department of Genitourinary Radiation Oncology, Division of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, 3Department of Genitourinary Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX, 4The University of Texas MD Anderson Cancer Center, Houston, TX, 5Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, 6Department of Radiation Oncology, Division of Radiation Oncology, The University of Texas MD Anderson Cancer Center, Houston, TX
Purpose/Objective(s): Studies have demonstrated safety and efficacy utilizing stereotactic body radiotherapy (SBRT) for definitive treatment of localized prostate cancer. Magnetic resonance (MR) guided linear accelerators leverage advanced real-time imaging to improve visualization of tumors and organs at risk with potential to improve clinical outcomes. We report genitourinary (GU) and gastrointestinal (GI) toxicities and biochemical failure for patients treated with SBRT using MR or computed tomography (CT) guidance. Materials/
Methods: This is a pooled analysis of 201 patients with localized prostate cancer enrolled on prospective and retrospective registries at a single institution who received SBRT with CT (n=112) or MR guidance (n=89) from 2015-2024. Patients were treated with androgen deprivation therapy (ADT) per physician discretion. Initial patients received 37.5-40 Gy to the planning target volume (PTV) (n=104); subsequent patients received 36.25 Gy to the PTV with a simultaneous integrated boost (SIB) of 40 Gy to the clinical tumor volume (prostate +/- proximal seminal vesicles per physician discretion) (n=97), all in 5 fractions every other day. Acute (=90 days after SBRT) and late toxicities were assessed using modified RTOG criteria during treatment and at each follow-up visit. Patient characteristics were compared with Fisher’s exact test and chi-squared test. Biochemical failure (PSA nadir + 2 ng/mL) free survival was estimated by the Kaplan-Meier method. Results: Median age (range) was 65 (51-81) years vs 68 (52-94) in the CT vs MR cohort. In the CT cohort, 12% were low-risk and 88% were intermediate-risk. In the MR cohort, 3% were low-risk, 90% were intermediate-risk, and 7% were high-risk. A higher percentage of patients in the MR cohort had a PSA >10 ng/mL [17% vs 4% (p=0.003)], had a Gleason Grade Group 3+ tumor [41% vs 22% (p=0.004)], had a rectal hydrogel spacer placed [92% vs 19% (p<0.001)], and received 1-12 months of ADT [65% vs 39% (p<0.001)]. The incidence of acute grade 2+ GI toxicity was 1% vs 0% (p=1.0), acute grade 2+ GU toxicity was 7% vs 10% (p=0.559), late grade 2+ GI toxicity was 19% vs 0% (p<0.001), and late grade 2+ GU toxicity was 21% vs 13% (p=0.204) in the CT vs MR cohorts. All late grade 3+ toxicities (4% GI and 1% GU) were observed in patients treated with CT guidance to 40 Gy to the PTV without a rectal spacer. Estimated 5-year biochemical failure free survival was 100% for the CT cohort with median follow-up of 54 months (IQR 30-60). No biochemical failures were observed in the MR cohort with median follow-up of 6 months (IQR 3-60). Conclusion: SBRT with ADT for definitive treatment of localized prostate cancer is safe and effective with excellent biochemical control. Increased late GI toxicities were observed in patients treated to higher doses of 40 Gy to the PTV without an SIB approach and without a rectal spacer. Additional follow-up will be reported.
