The enhanced soft tissue visualization versus CT has made contouring much simpler, Dr. Champion adds. “MRI offers much better discrimination of the anatomical cleavage points, which improves contouring accuracy and reproducibility,” he says. “For example, the erectile plexus and mesorectum are much better visualized on MRI than on CT. The same applies to the cranial limit of penile bulb, where we can more confidently spare this sensitive structure, thus minimizing the risk of radiation-induced erectile dysfunction. And, although the rectal and bladder walls can be well-visualized on CT, MRI provides finer definition.”
In addition to allowing easier contouring, MR-only radiotherapy potentially improves the long-term results of specific toxicities of radiation therapy, Dr. Champion adds.
“Moreover, it has direct consequences on the margins we apply to the prostate clinical target volume (CTV),” he says. “Teams working exclusively on CT scans usually use isotropic margins of seven to five millimeters, except for the back where the margin is five to four millimeters. With MRI support, we are comfortable using four-millimeter isotropic margins.”