Only a relatively small proportion of second cancers in adults is related to routine radiotherapy administered for initial disease, according to the findings of a recent study.
Amy Berrington deGonzalez, DPhil, from the National Cancer Institute (NCI) in Bethesda, Maryland, and colleagues analyzed data from nine US Surveillance, Epidemiology and End Results (SEER) cancer registries. They focused on 15 cancer sites that are routinely treated with radiotherapy: oral and pharynx, salivary gland, rectum, anus, larynx, lung, soft-tissue, female breast, cervix, endometrium, prostate, testes, eye and orbit, brain and CNS, and thyroid. The subjects were 20 years and older, and a first primary, invasive, solid cancer was diagnosed between 1973 and 2002.
The researchers determined the relative risks for second cancer in patients treated with radiotherapy compared with patients not treated with radiotherapy. Among 647,672 5-year survivors who had been followed for an average of 12 years, 60,271 (9%) developed a second solid cancer.
“We estimated a total of 3,266 … excess second solid cancers that could be related to radiotherapy, that is 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,” wrote Dr. Berrington de Gonzalez’s group in their report for The Lancet Oncology.
For each of the sites of first cancer, the relative risk of developing a second cancer associated with radiotherapy exceeded 1, and varied from 1.08 after cancers of the eye and orbit to 1.43 after cancer of the testes. In general, relative risk was highest for organs that typically received radiation doses greater than 5 Gy, decreased with increasing age at diagnosis, and increased with time since diagnosis.
Clinicians and patients can use these findings to put the relatively low risk of radiation-related cancer into perspective when compared with the probable benefits of treatment, suggested the authors. They cautioned, however, that studies of second-cancer risks from newer radiotherapy treatments such as intensity-modulated radiation therapy (IMRT) are still needed.