MRI-Guided Radiotherapy Reduces Adverse Effects in Prostate Cancer

Why a New Approach to Prostate Cancer Is Needed

Prostate cancer is among the most common cancers in men, and its treatments need continual improvement. Stereotactic body radiotherapy (SBRT) is increasingly used in prostate cancer management, but reducing its adverse effects has become a major priority. In this context, magnetic resonance (MR) imaging guidance offers several advantages that may help reduce prostate SBRT adverse effects. This article presents a systematic review and meta-analysis comparing acute urinary and bowel toxicity rates between MR-guided adaptive SBRT (MRg-A-SBRT) and CT-guided SBRT (CT-SBRT).

According to a study led by Dr. Jonathan Leeman of Dana-Farber Cancer Institute and published on 24 July in Cancer, MR-guided daily adaptive SBRT for prostate cancer reduces grade 2 or higher acute urinary adverse effects by 44% and grade 2 or higher acute bowel adverse effects by 60% compared with standard CT-SBRT.

What Is MR-Guided Radiotherapy?

MR-guided radiotherapy integrates MR imaging with radiation treatment to target cancer cells more precisely. Imaging allows real-time monitoring of changes in the body and adjustment of the radiation dose and direction at each session. This improves targeting while minimizing healthy-tissue exposure.

What Is SBRT?

Stereotactic body radiotherapy (SBRT) precisely directs high-dose radiation into a tumor, delivering the maximum tumor dose while minimizing harm to surrounding healthy tissue. It is usually delivered over a few sessions—typically four to five—and is particularly effective for small, well-defined tumors. SBRT is used for prostate cancer, lung cancer, brain tumors, and some other cancers.

Advantages and Limitations of MR-Guided SBRT

MR-guided radiotherapy uses imaging for more accurate delineation of the prostate and organs at risk, to account for anatomical changes (tumor shrinkage) between fractions, and to monitor the target in real time during radiation delivery. However, its clinical value must be critically compared with more widely available, accessible technologies such as CT-SBRT.

The MIRAGE Trial and Its Results

The single-center MIRAGE trial randomized 156 patients to MR-guided SBRT or CT-SBRT. MRI-Linac treatment significantly reduced acute genitourinary (GU) and gastrointestinal (GI) toxicity and improved short-term patient-reported outcomes. Its single-center design and sample size may limit generalizability; importantly, daily online adaptive radiotherapy was not used.

Methods: Comparing MR-SBRT and CT-SBRT

To assess whether the approach reduces adverse effects, researchers pooled 29 studies involving 2547 patients, comparing short-term physician-assessed bowel and genitourinary toxicity with MR-guided versus standard CT-SBRT. The mean window for collecting acute toxicity data was 70 days in MR-guided studies and 94 days in CT-SBRT studies.

Results: Acute Toxicity Rates

• With MR-guided radiotherapy (MR-SBRT), the risk of serious urinary adverse effects (grade 2 or higher) is 16%, rising to 28% with standard CT-guided SBRT.
• Similarly, the risk of serious bowel adverse effects (grade 2 or higher) is 4% with MR guidance, rising to 9% with standard CT-guided SBRT.
• There is no difference between the treatments in very rare serious adverse effects of grade 3 or higher.
• With CT-guided SBRT, using or not using fiducial markers—small metal pieces that help locate the tumor during treatment—makes no difference to adverse effects.

In Practice: Clinical Benefits of MR-SBRT

The researchers state that MR guidance enables more accurate and effective radiotherapy, providing a measurable clinical benefit that may improve outcomes. It plays an important role in reducing adverse effects through daily online adaptive planning, more precise tumor identification using MR imaging, and MR monitoring throughout treatment. These features make delivery more accurate and precise.

Study Limitations

The analysis did not account for differences between studies in dosimetry, radiation planning, or toxicity management and assessment. Late toxicity and cancer control were not monitored and may differ between approaches.