3312 - Preliminary Acute Toxicity Results of Locally Advanced Prostate Cancer Treated with MR-guided Radiotherapy

Purpose/Objective(s): MRLINAC systems are usually preferred for short course stereotactic radiotherapy (RT) to small fields due to substantial time and dedicated staff required for each fraction. In prostate cancer, it is mostly used for localized disease with excellent results. This study aims to investigate acute toxicity of daily adaptive, moderate hypofractionated MR-guided RT to extended field including pelvic lymphatics for high risk prostate cancer (HRPC) (T2a-4,N0-1,M0-1), and discuss the safety and feasibility of MRLINAC system.

Materials/Methods: Between August 2022 - October 2024, 30 HRPC patients started on androgen deprivation therapy (ADT) and treated at the 13th week of ADT on our MRLINAC device were included in the study. With daily adaptive moderate hypofractionated scheme, 44 Gy to pelvic lymphatics, 60 Gy to prostate and seminal vesicles and 66 Gy to the intraprostatic lesion were prescribed in 20 fractions with simultaneous integrated boost technique. Genitourinary (GU) and gastrointestinal (GI) complaints were noted before RT, once a week during RT and at 6 weeks after RT. IPSS was calculated, toxicity grades were measured according to CTCAE V5. Highest IPSS value and toxicity grade in the first 3 months were taken into consideration. Correlation between urethra Dmax, rectum-bladder D1, D0.1cc and toxicities were analyzed.

Results: Mean age was 70 (52-84) years, and mean vPSA was 11.2 (1,77-58,8) ng/mL. Mean follow-up was 12.43 (1.47-22.35) months after RT. 3 patients had TUR-P before RT. Mean preRT IPSS was 4.3 (1-12). 16 patients started RT with grade 1, 10 with grade 2 GU complaints. Dosimetric plans ensured protecting the urethra from hot doses without creating cold spots within GTV60 (Table). Mean IPSS during RT or within the first 3 months was 3.97 (1-9). After RT, 21 patients had grade 1, 9 had grade 2 GU toxicity. Most common GU toxicity before and after RT was nocturia. 2 patients had grade 1, and 2 had grade 2 GI toxicity (proctitis), all completely alleviated right after RT. RT increased grade 1 GU toxicity from 33% to 70%, while grade 2 decreased from 54% to 30%. No patient had grade =3 toxicity. No correlation was found between urethra, rectum or bladder doses and toxicities, most probably due to few number of events.

Conclusion: MR-guided daily adaptive, moderately hypofractionated RT for HRPC yielded mild GU toxicity without worsening IPSS of patients, and very low GI toxicity which dissipated after RT. Although long-term follow-up is required for late toxicity and treatment outcomes, preliminary results of our study indicate that MRLINAC systems could be considered a safe choice for wide-field and hypofractionated treatments in HRPC patients, and feasible for centers with dedicated staff.