Radiation therapy is effective for prostate cancer, but up to 90 percent of men will develop impotence after the treatment. New research indicates that this frustrating side effect may be avoided if both MRI and CT, rather than just CT, are used to plan exactly how and where radiation will be delivered.

Previous reports have suggested that prostate radiation causes impotence by damaging the structures that control blood supply in the penis. Therefore, radiotherapy that avoids these structures could potentially preserve sexual function. This is where treatment planning comes in.

With standard CT planning, it is often assumed that the distance between the prostate and these key structures is 1.5 cm. However, applying this estimate to all patients, “you’re going to treat way more (tissue) than you need to or you’re going to miss the prostate,” Dr. Patrick W. McLaughlin, from the University of Michigan in Ann Arbor, said in a statement.

McLaughlin and his colleagues used a combination of MRI and CT scans to determine the exact distance from the prostate to the blood-containing structures in 25 men with prostate cancer. Although the average distance - 1.45 cm - was close to that used with CT-based planning, the values varied widely from 0.7 to 2.1 cm.

The exact determination of the distance allowed the team to target radiotherapy more precisely, and dramatically reduce the dose to the critical erection structures.

The new findings appear in the International Journal of Radiation Oncology Biology Physics.

Further studies are needed to determine if MRI+CT planning of radiation therapy actually prevents impotence, the authors note.

SOURCE: International Journal of Radiation Oncology Biology Physics, January 2005.