Prostate Brachytherapy Seeds Do Not Expose Family Members to High Levels of Radiation

CHICAGO—The permanent placement of radioactive seeds in the prostate gland is proving to be equivalent to radical prostatectomy in the treatment of men with prostate cancer, according to some recently published data. However, some men have been reluctant to choose this treatment option because of the chance that the seeds may expose family members to excessively high radiation doses.

A new study has found that the seeds produce radiation levels less than one tenth of Nuclear Regulatory Commission (NRC) limits, said Jeff M. Michalski, MD, assistant professor of radiology, Washington University School of Medicine, St. Louis. He presented the data at the 86^th Annual Meeting of the Radiological Society of North America (RSNA).

59 Prostate Cancer Patients

The study of 59 prostate cancer patients with radioactive brachytherapy seeds compared the direct exposure of family members to radiation from the seeds with the amount of radiation in the everyday environment.

The NRC sets 500 millirem per year as the boundary of safety for adults who are exposed to radiation in the workplace. For people who are not in the radiation industry or come into contact with radiation in other occupational ways, such as radiologists or radiation oncologists, the safe limit is 100 millirem per year.

The results showed that the average amount of exposure to radiation for a spouse was only 10 millirem per year, or only one tenth the NRC limit. At worst, one spouse had a total annual exposure of 55 millirem, which is still only half the NRC limit.

Exposure to radiation from brachytherapy seeds also is in keeping with overall background exposure to cosmic and gamma rays, Dr. Michalski said.

He explained that total annual background radiation exposure from terrestrial gamma rays in rocks and earth varies from 180 millirem for an individual who lives in St. Louis to 400 millirem for one who resides in Denver. The average annual background exposure to radiation in the United States is approximately 300 millirem.

"We can now tell a woman living in St. Louis that the amount of radiation she will get from her husband in 1 year is less than she would get from living in Denver for 3 or 4 months," he said.

High-altitude travel is another source of exposure to radiation from cosmic rays. A round-trip flight from New York to Tokyo, for example, adds 20 millirem of exposure. When Dr. Michalski compiled data from patients and their families who had the most exposure to radiation, he calculated that the average annual exposure was 21 millirem.

"So a wife who spends a lot of time with her husband would be getting no more radiation exposure in 1 year than from an act [a long roundtrip flight] we accept as being appropriate," he said.

Dr. Michalski recruited 59 patients who consented to wear two radiation film badges, one on the collar and one at the waist. A badge also was given to each spouse and every other member of the family or household who would be in contact with the patient, and a badge was placed in each of four rooms in the house—bedroom, kitchen, bathroom, and family room. Patients were told to keep the room badges close to the space they normally occupy, for example, next to the nightstand at the edge of the bed.

The badges, which are sensitive down to 1 millirem, were assessed 3 weeks after the seeds where implanted when patients returned for a routine follow-up CT scan of the pelvis to verify that the seeds had been placed appropriately.

Although the study accumulated data only on 3 weeks of radiation exposure. the researchers used information on the half-life and decay pattern of the radioactive isotopes to extrapolate or amortize the exposure to which an individual would be subjected over a designated period of time, assuming the behavior of the patient and the lifestyle of the household would not change.

"We could assume that an iodine implant would deliver 98% of its lifetime dose in a year, and the palladium implant, which has a shorter half-life, would deliver nearly 100% of the radiation dose in the first year," he said.

Based on this information, Dr. Michalski concluded that radiation exposure to members of the household of a brachytherapy patient are well within NRC limits. "We found that it is extremely difficult for anyone to expose a member of the family to amounts of radiation that we can even detect, and the amounts we can detect are very low," he said.

He noted that radiation oncologists at Washington University nevertheless caution brachytherapy patients to minimize direct contact with family members. "The greatest exposure is in the lap; so we recommend that patients limit direct exposure for their grandchildren, for example, by not holding them in their lap for long periods of time, because it is a simple measure they can do," he said.