Small number of second cancers attributable to radiotherapy

In adults receiving radiotherapy for cancer, a relatively small proportion of second cancers are related to the treatment itself, suggesting that most are due to other factors, such as lifestyle or genetics, according to a recent study.

Study co-author Amy Berrington de González, DPhil, of the National Cancer Institute, and colleagues used U.S. Surveillance, Epidemiology and End Results (SEER) registries to analyze 15 cancer sites that are routinely treated with radiotherapy, including the prostate and testes. The cohort was composed of patients aged 20 years or older who were diagnosed with a first primary invasive solid cancer between Jan. 1, 1973, and Dec. 31, 2002. Relative risks for second cancer in patients treated with radiotherapy versus patients not treated with radiotherapy were estimated with Poisson regression adjusted for age, stage, and other potential confounding factors.

Of the patients, 647,672 who were 5-year survivors were followed for a mean of 12 years, and 60,271 (9%) developed a second solid cancer. Researchers say the study described for the first time the overall absolute risk of second cancers related to radiation therapy: an estimated five of every 1,000 patients treated with radiation therapy who survive for 15 years would be anticipated to develop a radiation-related cancer.

"That number can be used by doctors to convey the message to patients that the absolute risk of developing a second cancer related to radiotherapy is quite small," Dr. Berrington de González explained.

In general, the relative risk was highest for organs that typically received greater than 5 Gy, decreased with increasing age at diagnosis, and increased with time since diagnosis. The researchers estimated a total of 3,266 excess second solid cancers that could be related to radiotherapy, or 8% of the total in all radiotherapy patients (≥1 year survivors) and five excess cancers per 1,000 patients treated with radiotherapy by 15 years after diagnosis.

Results from the study were published in The Lancet Oncology (2011; 12:353-60).