Radiotherapy for Prostate Cancer: Study Says MRI Guidance Significantly Reduces GU and GI Toxicities

Emerging research suggests that magnetic resonance imaging (MRI) guidance is more advantageous than computed tomography (CT) guidance in reducing genitourinary (GU) and gastrointestinal (GI) side effects of stereotactic body radiotherapy (SBRT).

Reporting on findings from a phase III randomized trial (MIRAGE) at the recent American Society for Radiation Oncology (ASTRO) Annual Meeting, researchers reported a 43.4 percent rate of acute grade >2 GU toxicity for CT-guided SBRT in comparison to a 24.4 percent rate of acute grade >2 GU toxicity for MRI-guided SBRT with the MRIdian MRI system (ViewRay). The researchers added that a subsequent multivariable analysis, accounting for differences in prostate size, baseline urinary symptoms and use of a hydrogel spacer, revealed a 60 percent reduction in the risk of grade >2 GU toxicity with MRI guidance.

In the study, which involved 156 patients, the authors indicated that none of the patients in the MRI group had acute grade >2 GI toxicity in comparison to a 10.5 percent toxicity rate in the CT group.

In regard to patient-reported outcomes with prostate SBRT, the incidence of increased urinary symptoms measured by the International Prostate Symptom Score was 12.6 percent lower in the MRI guidance group, according to the study. The researchers also noted that half of the patients in the CT guidance group reported a decrease in bowel-related quality of life in comparison to 25 percent of those who had MRI-guided SBRT.

With these study results, Amar Kishan, M.D, a co-author of the study, said he and his colleagues at the University of California, Los Angeles (UCLA) have shifted to nearly “exclusive” use of MRI-guided prostate SBRT after offering CT-guided prostate SBRT since 2010.

“A major consideration with prostate SBRT is the margin of normal tissue around the target that is exposed to high-dose radiation. The highly positive final results of our phase III MIRAGE trial show that when MRI guidance is used to shrink that margin, there are significant improvements in both physician-scored and patient-reported toxicity in terms of urinary and bowel side effects,” noted Dr. Kishan, an associate professor and chief of the Genitourinary Oncology Service at UCLA.