A prospective single-arm phase II study of stereotactic magnetic-resonance-guided adaptive radiotherapy for prostate cancer: Early toxicity results

Abstract Purpose Use of stereotactic body radiation therapy (SBRT) is increasing in patients with localized prostate cancer, but concerns about early and late gastrointestinal (GI) and genitourinary (GU) toxicity exist after moderately- or extremely hypofractionated radiotherapy schemes. Magnetic resonance-guided radiation therapy (MRgRT) was clinically introduced in 2014, allowing for SBRT delivery with smaller uncertainty margins and permitting daily adaptive planning. A phase II study in patients with localized prostate cancer was performed to study early GI and GU toxicity following SBRT using MRgRT. Methods /Materials: 101 patients with clinical stage T1-3bN0M0 prostate cancer were enrolled in this prospective phase II study. All but four patients had intermediate- or high risk prostate cancer and 82.1% received adjuvant hormonal treatment. MRgRT was delivered in 5 fractions of 7.25 Gy to the target volume using daily plan adaptation with simultaneous relative sparing of the urethra to a dose of 6.5 Gy/fraction. Early toxicity was studied using both clinician- (CTCAE and RTOG) and patient-reported outcome measurements (EORTC QLQ- C30, QLQ PR25 and International Prostate Symptom Scoring). Results The maximum cumulative grade ≥2 early GU and GI toxicity measured by any symptom at any study time point was 23.8% and 5.0%, respectively. No early grade 3 GI toxicity was observed. Early grade 3 GU toxicity was 0% and 5.9% according to the CTCAE and RTOG and scoring systems, respectively, as a result of different grading of radiation-cystitis. The low incidence of early GI toxicity was confirmed by patient-reported outcome data. GU grade ≥2 toxicity peaked to 19.8% at the end of MRgRT, followed by a return to the baseline average score at three months follow-up. Conclusion This prospective study of MRgRT in patients with localized prostate cancer observed a low incidence of early GI and GU toxicity, both in clinician- and patient reported outcome measurements.