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Title: TARGETED RADIATION MAY CAUSE POST-RT impotence ,  By: Bankhead, Charles, Urology Times, 00939722, Sep2000, Vol. 28, Issue 9

Atlanta–The base of the penis receives substantial exposure to radiotherapy targeted against prostate cancer even with the best conformal radiation techniques, suggesting an explanation for the significant rates of post-radiotherapy impotence .

“Our analysis revealed that the proximal-most 1-cm segment of the corpora bodies received almost 96% of the intended treatment dose that was supposed to land on the prostate,” explained Paul Yonover, MD, during a presentation at the annual AUA meeting. “The second and third centimeters received 68% and 20% of the intended therapeutic dose. The three segments combined received 65% of the intended dose of radiation therapy.”

Despite advances in targeting external beam radiation to the prostate, the literature reveals post-irradiation erectile dysfunction rates ranging between 25% and 62%, said Dr. Yonover, a surgery resident at Loyola University, Maywood, IL, working with John Mulhall, MD, director of the male center for sexual dysfunction.

The mechanism of radiation-induced impotence remains undetermined, but several hypotheses exist, Dr. Yonover said.

“Although the nerves are thought to be relatively radiotherapy-insensitive, there is growing concern that nerve injury is occurring with radiation exposure,” he said.

Radiation-induced fibrosis

Preclinical studies have demonstrated a dose-dependent decrease in nitric oxide-containing nerves in the cavernous smooth muscle of rats after exposure of the prostate to external beam radiation (Radiology 1995; 195:95-9). Acute smooth muscle fibrosis also may occur after radiation exposure, which might induce venous leak and possibly play a role in post-radiation impotence , said Dr. Yonover.

Loyola investigators retrospectively evaluated radiation exposure to the cavernosum in 13 prostate cancer patients. All patients had T1-T2 disease and received standard 3-D conformal radiation therapy. In all cases, the clinical target volume (CTV) was defined as the prostate and seminal vesicles, and the planning target volume was the CTV surrounded by a 5- to 10-ram margin. The mean radiation dose to the prostate was 7,400 Gy.

Computer analysis of the 3-D images generated for conformal radiotherapy revealed that the most proximal centimeter of the corpora bodies received an average radiation dose of 7,129 Gy, Dr. Yonover reported. At the second most proximal centimeter, the dose remained high at 5,083 Gy. The third most proximal centimeter of the corpora bodies received an average radiation dose of 4,683 Gy.

“These data indicate that large doses of radiation therapy are being administered to the proximal regions of the corpora bodies despite using conformal techniques,” said Dr. Yonover. “The doses of radiation to the base of the penis may explain the significant rates of post-radiation therapy impotence .

“This model might also be used to analyze other modalities of radiation therapy, including brachytherapy and [intensity modulated radiotherapy],” he said. “Such data may also lead to greater efforts to exclude the penis from the planning target volumes associated with conformal radiation therapy.”

Post-RT impotence rates may increase

Dr. Yonover noted that the issue of postradiation impotence will likely grow in significance as more prostate cancers are detected each year by PSA screening. Additionally, more cases of prostate cancer are being detected in younger men, in whom post-treatment sexual potential will be an even greater issue.

“There is increasing pressure not only for more effective treatment of prostate cancer but also less morbid treatment modalities, with special emphasis on sexual dysfunction,” he said. “We have already observed a trend among urologic oncologists and radiation oncologists to refine their respective therapies for localized prostate cancer. However, the available data suggest that radiation-associated erectile dysfunction still occurs in a substantial number of prostate cancer patients, despite refinement in the techniques.”

At a Glance

Targeted RT off the mark >> Even with the best conformal radiation techniques, high doses may still reach the base of the penis, perhaps explaining the significant rates of post-RT impotence .

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By Charles Bankhead, UT Contributing Editor

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Source: Urology Times, Sep2000, Vol. 28 Issue 9, p18, 1p

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