Reduced Toxicity Associated With MRI-Guided Adaptive Radiation for Prostate Cancer

Magnetic resonance imaging (MRI)-guided adaptive stereotactic body radiotherapy (MRg-A-SBRT) was found to significantly reduce short-term side effects associated with radiation therapy for prostate cancer, according to a systematic review and meta-analysis published in Cancer.

Integrating MRI with a radiation delivery system has demonstrated several advantages for limiting toxicity associated with SBRT, such as more accurate contouring of the prostate and other at-risk organs and real-time target tracking during beam therapy. Some drawbacks to MRI-guided SBRT include imaging artifacts and distortion, longer treatment times, and planning limitations due to limited facility capabilities. Therefore, according to the authors of this systematic review, “there is a critical need to assess the clinical value of this technology in comparison [with] more widely available and accessible technologies.”

The analysis included data from 29 clinical trials, comprising 2547 patients, which compared acute toxicity with MRg-A-SBRT with computed tomography-guided nonadaptive SBRT (CT-SBRT) for prostate cancer. Overall, 329 patients received MRg-A-SBRT and 2218 received CT-SBRT. Collection of acute toxicity data took place between 90 days to 3 months after SBRT.

MRI-Guided Radiotherapy Reduced Toxicity

Incidence of acute G2+ genitourinary (GU) toxicity ranged from 5.0% to 33.3% in studies of patients undergoing MRg-A-SBRT and 9.1% to 46.7% in studies of those undergoing CT-SBRT. Ranges of acute G2+ gastrointestinal (GI) toxicity were 0% to 8.3% with MRg-A-SBRT and 2.0% to 23.3% with CT-SBRT. Pooled estimates of acute G2+ GU and GI toxicities were 16% (95% CI, 10%-24%) and 4% (95% CI, 2%-7%) for MRg-A-SBRT and 28% (95% CI, 23%-33%) and 9% (95% CI, 6%-12%) for CT-SBRT. The authors noted significant heterogeneity between treatment groups when evaluating GU toxicity, as well as heterogeneity among CT-SBRT studies evaluating GI toxicity. Acute G3+ toxicity events were rare, with pooled estimates of 0% and 1% for MRg-A-SBRT and CT-SBRT, respectively.

The odds ratio (OR) for G2+ GU toxicity for MRg-A-SBRT (compared with CT-SBRT) was 0.5 (95% CI, 0.29-0.87; P=.01); for G2+ GI toxicity in the MRg-A-SBRT group, the OR was 0.4 (95% CI, 0.17-0.96; P=.04). Prescription radiation dose was associated with G2+ GU toxicity in this treatment group (OR, 1.04; P =.04).

“The potential reasons underlying reduced risk of acute toxicity with MRg-A-SBRT are numerous,” the authors wrote. “In comparison [with] CT-SBRT, MRg-A-SBRT makes use of daily online adaptive planning, MRI-based contouring that results in smaller treatment volumes, and MRI tracking, all of which may facilitate the precision and accuracy of treatment delivery.” Due to technical differences between these treatment modalities, the authors called for further prospective investigations with longer follow-up to pinpoint the underlying reasons for reduced toxicity.

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