MRI Far Safer Than CT for Guiding Radiotherapy in Prostate Cancer

M. Alexander Otto

February 25, 2022

There was a remarkable reduction in bowel and urinary side effects when MRI was used instead of CT to guide stereotactic body radiotherapy (SBRT) for localized prostate cancer, shows a study from the University of California, Los Angeles.

Among the first 100 men in the phase 3 MIRAGE trial (Magnetic Resonance Imaging–Guided Versus Computed Tomography–Guided Stereotactic Body Radiotherapy for Prostate Cancer), MRI guidance more than halved the incidence of grade 2 or higher physician-reported genitourinary toxicity within 90 days of the procedure, which fell from 47.1% with CT to 22.4% with MRI.

While 13.7% of men had gastrointestinal complications with CT guidance, there wasn't a single case in the MRI arm. The findings were presented Feb. 17 at the American Society of Clinical Oncology Genitourinary Cancers Symposium.

The investigators thought they'd need 300 men to detect a safety difference, but the results are so strong that they've scaled back enrollment to 154. In the meantime, MRI-guided SBRT is now offered routinely to men with localized prostate cancer at UCLA.

"Our final results are expected later this year, but we are extremely optimistic by what we're seeing, and hope this technology will soon begin to offer men undergoing radiotherapy for prostate cancer better outcomes," said lead investigator Amar Upadhyaya Kishan, MD, a genitourinary oncology radiologist, in a UCLA press release.

The better outcomes are caused by the enhanced imaging capabilities of MRI, including real time tracking and automatic beam shutoff when the prostate moves too far outside of the treatment boundary, Kishan explained on Twitter.

Because of the extra precision, "we felt we could safely reduce the planning margins to only 2 mm" with MRI, down from 4 mm with CT. It translated to smaller treatment volumes and less collateral tissue damage, he said.

Across the first 100 subjects, 49 men were randomized to MRI-guided SBRT and 51 to SBRT with CT guidance. Their prostates and proximal seminal vesicles were dosed with 40 Gy of radiation in five fractions. Rectal spacing and nodal irradiation were at physician discretion.

Patients in the MRI arm also reported significantly fewer urinary symptoms, including urgency, incontinence, burning sensations, and bowel dysfunction, such as pain, diarrhea, and obstruction, among others, at 1 month with MRI guidance. The differences diminished at 3 months with adverse event management in the CT arm.

Lymph nodes were irradiated in 29% of men in the CT group versus 20% in the MRI arm, and 37% of the CT group versus 27% with MRI had rectal spacing.

Baseline gland size was a median of 39 mL in both groups. Baseline International Prostate Symptom Scores were a median of 8 points in the MRI group but 5 points in the CT arm.

The work was funded by UCLA, among others. Kishan has ownership interests in ViewRay, the company that makes the MRI-guiding technology used in the trial, and reported honoraria and research funding from the company.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

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