Researchers have found that women with scoliosis, or abnormal curvature of the spine, who were exposed to multiple diagnostic X-rays during childhood and adolescence may be at increased risk of dying of breast cancer. The study appears in the 15 August 2000 issue of the journal Spine*. Authors included scientists from the National Cancer Institute (NCI) in
Bethesda, Md.; the Twin Cities Spine Center in Minneapolis, Minn.; the University of Texas
M.D. Anderson Cancer Center in Houston, Texas; Information Management Services in
Silver Spring, Md.; and the U.S. Scoliosis Cohort Study Collaborators, a group of physicians from 14 orthopedic medical centers across the country.
The 5,466 women in the study, who received an average of 24.7 X-rays, were found to have a 70 percent higher risk of breast cancer than women in the general population. There were 77 breast cancer deaths among the patients, compared to 46 expected deaths based on U.S. mortality rates. Patients were younger than 20 years old when they were diagnosed with scoliosis between 1912 and 1965. The mean age for scoliosis diagnosis in this study was 10.6 years, and the average length of follow-up was 40.1 years. Follow-up was complete for
89 percent of patients.
"These findings provide yet another indication that radiation exposure, especially in childhood, is associated with increased breast cancer risk later in life, and that the amount of risk is proportional to radiation dose," said Michele M. Doody, M.S., from NCI's Radiation Epidemiology Branch and the principal investigator of the study. Reported risks for exposures after age 40 are much lower.
Scoliosis occurs in approximately 2 percent of girls and 0.5 percent of boys. It is commonly diagnosed in early adolescence and may gradually progress as rapid growth occurs. Scoliosis patients typically undergo routine X-rays of the spine throughout their adolescent growth spurt to monitor curvature progression so that corrective action may be taken.
The researchers found that the risk of dying from breast cancer increased significantly with the number of X-rays. The vast majority (89 percent) of exams in this study involved definite or probable radiation exposure to the breast. Patients who had 50 or more exams had nearly four times the risk of dying from breast cancer as women in the general population. The number of exams per patient ranged from zero to 618. Six hundred forty-four patients had no recorded exams.
Similarly, the risk of dying of breast cancer increased with increasing estimated cumulative radiation dose to the breast. Patients who received doses of greater than 20 centigray (cGy) had more than three times the chance of dying from breast cancer than women in the general population. The estimated cumulative dose of radiation ranged from zero to 170 cGy; the average was 10.8 cGy**
This is by far the largest group of scoliosis patients followed to date. The number of
X-rays that each patient received was tabulated through detailed review of the medical records and films, and the breast doses were estimated using actual machine parameters derived from one medical center (University Hospital Rehabilitation Center, Hershey, Pa.). Information was available during most of the calendar time periods covered.
Part of the increased risk of dying from breast cancer may be due to other breast cancer risk factors, said Doody. Breast cancer risk in the general population tends to be higher for women who have not experienced a full-term pregnancy or whose first full-term pregnancy was at age 30 or older. Based on questionnaire responses by 3,100 women in the study who were alive at the end of the follow-up period, it appears that women with more severe scoliosis were less likely to have given birth than those with less severe disease. Since severity of scoliosis also correlates with number of X-rays and radiation dose to the breast, it is possible that some of the observed breast cancer excess could be related to reproductive history.
Almost all of the X-rays received in this study were taken before 1976, when the dose to patients was considerably higher than with current techniques. For example, the estimated breast dose from a full-spine anteroposterior view (facing the X-ray machine) in 1940 to 1959 was about six times higher than an anteroposterior view in 1976 to 1989 and 200 times higher than a posteroanterior (turned with back facing the X-ray machine) view in 1976 to 1989. Although radiation exposures to breast tissue are much lower today than during the time period covered by this study, they are not insignificant. The authors recommend that efforts to reduce exposures continue by having patients stand with their backs to the X-ray machine, carefully limiting the portion of the body exposed to the radiation beam, and shielding the breasts. Repeat exposures should also be minimized wherever possible.
* The study is entitled " Breast Cancer Mortality After Diagnostic Radiography: Findings from the U.S. Scoliosis Cohort Study." The authors are Michele Morin Doody, John E. Lonstein, Marilyn Stovall, David G. Hacker, Nickolas Luckyanov, and Charles E. Land. Spine,
Aug. 15, 2000, Vol. 25, No. 16.
** The amount of radiation energy absorbed by irradiated tissue is measured in centigray (cGy). The estimated breast dose for a single full spine X-ray with the patient facing the X-ray machine (anteroposterior view) during the 1940s was about 0.6 cGy. For comparison, the estimated breast dose today for a single full spine anteroposterior exam is on the order of 0.1 to 0.2 cGy, whereas the dose for the same examination with the patient's back facing the machine (posteroanterior view) is 0.02 cGy.