Radiation oncology saves lives, but at what cost?

Aug 15, 2006

Successful radiation treatment for cancer can prolong life by 15 to 40 years. However, with that reward still comes risk, and researchers are trying to determine to what degree radiation therapy may create new cancers later in a patient's life.

Dr. Herman Suit, chief of the department of radiation oncology at Massachusetts General Hospital in Boston, said there are three certainties about radiation therapy. One fact is that radiation is carcinogenic, even in a weakened state.

"Two, the effects of the radiation appear quite late, maybe 15 to 40 years after the treatment, and we have very little data on that," he added. "Three, there is a substantial number of patients who are being cured and can expect to live 30 to 40 years after treatment."

At the 2006 American Association of Physics in Medicine (AAPM) meeting in Orlando, Suit and colleagues presented their analysis of studies that examined the radiation effects on cancer patients, atom-bomb survivors, individual mammalian cells, and animals, such as mice and rhesus monkeys. The report found no uniform relationship between radiation exposure and secondary cancer incidence, but there were areas of concerns.

Patients who received relatively high radiation dose to the stomach, while being treated for another cancer, increased their chance of developing stomach cancer from approximately 1% to approximately 4% for the rest of their lives. The analysis, however, also found no significant increase in the risk of bladder or rectal cancer over the dose range of 1 to 60 Gy to those nearby organs.

While research has not conclusively found a concrete link between radiation dose and cancer risk, Suit said that one thing is clear -- radiation exposure, except at low dose levels (0.01 Gy for a routine diagnostic x-ray exam), is known to heighten cancer risk.

Rectal cancer

The debate over whether prostatic irradiation is a catalyst for rectal cancer recently rekindled. A 2005 University of Minnesota Medical School study reviewed approximately 85,000 U.S. men with prostate cancer but no previous history of colorectal cancer. The men received either surgery or radiation and lived at least five years.

The Minneapolis-based research found that colorectal cancers developed in 1,437 patients, 267 of which were in irradiated sites, 686 in potentially irradiated sites, and 484 in nonirradiated sites. In addition, radiation was independently associated with cancer development over time in irradiated sites, but not in the remainder of the colon. The conclusion was that the adjusted hazards ratio for developing rectal cancer was 1.7 for the radiation group, compared with the surgery-only group.

Thus, the study found what it described as "a significant increase in development of rectal cancer after radiation for prostate cancer." "Radiation had no effect on development of cancer in the remainder of the colon, indicating that the effect is specific to directly irradiated tissue," the authors wrote (Gastroenterology, April 2005, Vol. 128:4, pp. 819-824).

Upon further review

Dr. Wayne Kendal, Ph.D, a radiation oncologist at Ottawa Hospital Regional Cancer Center in Ontario, Canada, recently took the Minnesota study one step further with a sample of 285,000 men, including the 85,000 men in the Minnesota report. Both research ventures used information from the Surveillance, Epidemiology, and End Results (SEER) database at the U.S. National Cancer Institute.

Of the men who had radiation for prostate cancer (33,831 in Kendal's sample), 0.7% developed rectal cancer, while those who did not have radiation or surgery (36,335) had a 0.8% rectal cancer rate, according to the results published in the International Journal of Radiation Oncology, Biology, Physics. Men who underwent a prostectomy (167,607) had a rectal cancer rate of 0.3%. The numbers led Kendal to conclude that the effect of radiation is minimal and that age may have played a role in the results (IJROBP, July 1, 2006, Vol. 65:3, pp. 661-668).

"We know that the incidence of rectal cancer on its own will increase with age," Kendal said in an interview with AuntMinnie.com. "When we did our analysis, using these types of methods and these three groups, there was no increased rate of rectal cancer in those who had radiation. We thought this was an important result, because this would tend to put to rest many men's fears" of developing radiation-induced rectal cancer after prostatic radiotherapy.

Kendal currently has a second paper in peer review, which takes into consideration potential confounding factors and may produce more evidence for his conclusions.

Beware the scatter

In radiation therapy, the basic principle is to direct as much dose as possible into the target organ. Typically, oncologists prescribe 76 Gy for conformal treatment, though Kendal said the amount "does not control all the disease all the time." "The main limitation is the tolerance of the surrounding tissue. You don't want to cause a complication, and that's why we use 76 (Gy) with our conformal treatment," he explained.

While Suit readily noted that the benefit of radiation therapy outweighs the risks, oncologists must take every precaution to concentrate dose on the targeted cancer tissues. "When you do radiation treatment, you have low doses scattered and you radiate (other areas)," he said. "It is extremely low, but it isn't zero."

Suit cited several improved techniques and technologies that are having a positive effect to minimize secondary cancer risks. Specifically, he mentioned intensity-modulated radiotherapy (IMRT) -- which, he said, "clearly reduces scattered dose" -- as well as intraoperative electron beam therapy, image-guided brachytherapy, and 4D treatment planning and delivery, which adjusts radiation therapy to the motion of the cancerous organ.

By Wayne Forrest
AuntMinnie.com staff writer
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