Radiation planning for prostate cancer that combines magnetic resonance imaging and computed tomography helps target tumor more precisely, so nearby vessels are more likely to be spared and erectile dysfunction is prevented, according to an article in the January issue of the International Journal of Radiation Oncology, Biology, Physics. The results come from an initial American study with 25 patients.

Roughly 230,000 American men were diagnosed with prostate cancer in 2004. Although it is more common in older men, a growing number of men are being diagnosed in their 50s.

“As we treat younger men, erectile function is an important concern. We’re often treating men in their 50s, and this is a very important issue for them. Most of the men I see are going to be cured. Once you start curing cancers at an extremely high rate, the focus moves to quality of life,” said Patrick W. McLaughlin, MD, lead author of the article.

During surgery, the nerves that control erectile function may be severed - which has led to new surgical techniques to avoid cutting those nerves. But doctors are less sure what causes erectile dysfunction after radiation therapy. Erectile dysfunction among men without prostate cancer is most commonly caused by vascular disease, and doctors do know that radiation causes obstruction of the vessels that fall within the treatment area. Using that as a starting point, the team involved with the current study began investigating radiation-related erectile dysfunction as a vascular problem.

Typically, radiation oncologists rely on a computed tomography scan to identify the prostate and plan treatment. But because of limitations in such scanning, the images do not show the bottom of the prostate. Doctors instead estimate where the prostate ends, based on average distance from identifiable structures. When the researchers used magnetic resonance imaging in addition to computed tomography to get a better picture of the whole prostate, they found the distance between the prostate and penile bulb ranged from 0.5 cm to 2.0 cm.

“We condemned one of the common tricks people try to use. By assuming an average distance of 1.5 cm between the prostate and the penile bulb, either you’re going to treat way more than you need to or you’re going to miss the prostate,” McLaughlin said.

With use of the additional imaging, the investigators were able to plan treatment to include the entire prostate but avoid the critical blood vessels below the gland. Preliminary results suggest that avoiding the vessels prevents erectile dysfunction.

“Because we can’t see any detail of this area on computed tomography scans, we just assume if we treat below the prostate it’s no big deal. But it is a big deal. There is no cancer below the prostate, but there are critical structures related to erectile function as well as urine sphincter function. Treating below the prostate may result in needless problems,” McLaughlin said. “I don’t have much doubt from what I’ve seen that this approach is likely to have huge impact.”

About one in two men who undergoes radiation therapy for prostate cancer is unable to have sex five years later unless sildenafil or similar medications are used.

In addition, the vessels involved in erectile function also play a role in bowel and bladder control. The researchers suspect avoiding radiation to these areas will improve other quality of life issues, such as urinary leakage and bowel problems.