MRI- vs CT- guided radiotherapy for localized prostate cancer linked to lower toxicity, symptom burden

Magnetic resonance imaging (MRI) guidance in stereotactic body radiotherapy (SBRT) for prostate cancer offers significant advantage over computed tomography (CT) guidance, with lower physician-scored toxic effects and patient-reported symptom burden, according to the results of the phase III MIRAGE* study.

Acute grade ≥2 genitourinary toxic effects occurred less frequently with MRI vs CT guidance (24.4 percent, 95 percent confidence interval [CI], 15.4–35.4 vs 43.4 percent, 95 percent CI, 32.1–55.3; p=0.01), as did acute grade ≥2 gastrointestinal toxic effects (0.0 percent, 95 percent CI, 0.0–4.6 vs 10.5 percent, 95 percent CI, 4.7–19.7; p=0.003). [JAMA Oncol 2023;doi:10.1001/jamaoncol.2022.6558]

MRI guidance also led to a smaller proportion of patients experiencing a ≥15-point increase in International Prostate Symptom Score (IPSS; 6.8 percent vs 19.4 percent; p=0.01) or a clinically significant decrease (≥12-point) in Expanded Prostate Cancer Index Composite-26 (EPIC-26) bowel scores (25.0 percent vs 50.0 percent; p=0.001) at 1 month.

“The toxicity benefits were likely directly attributable to the significantly reduced planning target volume (PTV) margins, which in turn were achievable due to the ability of MRI guidance to frequently monitor intrafraction motion and initiate an automatic beam-hold, as well as to reduce uncertainty in contouring due to a direct MRI-MRI fusion,” the investigators explained.

MIRAGE, they added, was unique in that the study included outcome measures assessed by patients as well as physicians. From both perspectives, MRI guidance was more beneficial than CT guidance in the context of SBRT for localized prostate cancer.

MIRAGE included 156 patients (median age 71 years) who were receiving SBRT for clinically localized prostate adenocarcinoma. Of these, 77 were assigned to the CT arm and 79 to the MRI arm. All but two patients (one in each group) were included in the analysis.

Narrow margins

Planning margins of 4 mm in the CT arm and 2 mm in the MRI arm were used to deliver 40 Gy in five fractions. The 2-mm margin used with MRI-guidance in the trial was narrower than what had been used in any previous large study.

The investigators pointed out several attributes of the MRI-guided system that allowed them to confidently reduce the margin to 2 mm, including improved soft tissue contrast, the ability to directly monitor prostate motion, and more accurate alignment prior to radiation.

"MRI guidance offers several advantages over standard CT guidance, most notably the ability to dramatically reduce planning margins, providing more focused treatment with less injury to nearby normal tissues and organs,” said lead study investigator Dr Amar Kishan, a radiation oncologist at the David Geffen School of Medicine at University of California, Los Angeles (UCLA) and the UCLA Jonsson Comprehensive Cancer Center.

“MRI technology is more costly than CT, both in terms of upfront equipment expenses and longer treatment times, which is one reason our study set out to determine if MRI-guided technology offers tangible benefits for patients,” Kishan added.

“In this trial, we demonstrated that the reduction in treatment volumes facilitated by MRI guidance leads to a significant reduction in moderate physician-scored toxicity and to a reduction in the proportion of patients noting significant decrements in patient-reported outcome metrics in the near term,” he said.

Kishan and colleagues are hopeful that the data from MIRAGE, despite the need for more studies to confirm the benefits of MRI guidance over time, will translate to better outcomes for men with prostate cancer.

*MRI-Guided Stereotactic Body Radiotherapy for Prostate Cancer