CT- vs 1.5T MR-guidance in salvage radiotherapy of locally recurrent prostate cancer after radical prostatectomy.

368 Background: This study compares the treatment outcomes of megavoltage CT-guided RT (CTgRT) and 1.5T MR-guided RT (MRgRT) in prostate cancer patients with gross local recurrence after radical prostatectomy. Methods: We reviewed patients who received salvage RT for locally recurrent prostate cancer between January 2009 and August 2024. All patients had a gross recurrence in the prostate bed, detected via diagnostic MRI and elevated prostate-specific antigen (PSA). Patients in the CTgRT group received conventionally fractionated (28–30 fractions) salvage RT with simultaneous integrated boost. A typical dose scheme is 2.5 Gy to the gross recurrence, 2.25 Gy to the prostate bed, 2.15 Gy to intermediate-risk planning target volume (PTV) with a 5-mm margin, and 1.8 Gy to low-risk PTV with additional 5-mm margin, delivered over 28 fractions. With the introduction of MRgRT in August 2021, patients received moderately hypofractionated RT (2.4 Gy x 24 fractions) to a PTV defined by 4-mm margin added to the prostate bed followed by cone down MRI/CT simulation and sequential boost (2.4 Gy x 3 fractions) to gross recurrence. Acute and late toxicities were evaluated using CTCAE 5.0. Results: A total of 38 patients, 20 in CTgRT and 18 in MRgRT groups, were included. The median recurrence interval and follow-up after radical prostatectomy were 44.0 and 28.5 months and 128.0 and 56.4 months for CTgRT and MRgRT, respectively. The median volume of the gross recurrence was 1.38 cc (range, 0.34–42.66 cc) and 1.30 cc (range, 0.16–27.89 cc) for CTgRT and MRgRT, respectively. Thirteen patients in CTgRT and 16 in MRgRT groups received a boost to the gross recurrence. The median total equivalent dose in 2-Gy fractions was 77.44 Gy (range, 64.18–85.71 Gy) for CTgRT and 72.21 Gy (range, 64.18–83.35 Gy) for MRgRT. Androgen deprivation therapy was administered at recurrence for 11 and 13 patients in CTgRT and MRgRT groups, respectively. Median prostate-specific antigen levels decreased from 0.4 ng/mL pre-RT to 0.11 ng/mL at three months post-RT in the CTgRT group, and from 0.31 ng/mL to 0.04 ng/mL in the MRgRT group. In the CTgRT group, 14 patients had imaging follow-up showing complete response (CR) in 11 patients. In the MRgRT group, 8 patients had imaging follow-up showing CR in 6 patients. No local failure was reported. No grade 3 or higher acute gastrointestinal or genitourinary toxicity was reported in either group. For late toxicity, grade 1 or 2 GU toxicities were reported in nine patients in the CTgRT group, while one patient experienced grade 3 GU toxicity. In the MRgRT group, one patient experienced grade 1 late GI toxicity, and four patients had grade 1 or 2 GU toxicities. Conclusions: MR-guided salvage RT for grossly recurrence prostate cancer allowed reduced target margin and total dose compared with CT-guided salvage RT, resulting in similar RT response and improved toxicity profile.