Radiotherapy used to treat solid cancers increases the risk of developing mesothelioma as a secondary cancer, according to a study published in the journal Cancer Medicine. The risk, however, is of little clinical relevance, since the total number of patients who developed a secondary mesothelioma after four decades is infinitesimally small.
In the absence of asbestos exposure, the risk of developing mesothelioma has been estimated at 3 per 10,000 individuals. Radiation therapy has been suggested to increase the risk of developing secondary mesothelioma.
A research team at the University of Bologna, Italy, studied the link between external beam radiotherapy and secondary mesothelioma in the U.S. National Cancer Institute’s Surveillance, Epidemiology, and End Results (SEER) database, which holds information about cancers and treatments from a large number of countries.
The study, “Radiation-induced mesothelioma among long-term solid cancer survivors: a longitudinal analysis of SEER database,” first identified all patients with a solid cancer reported to the SEER registry between Jan. 1, 1973, and Dec. 31, 2012. Cancers that might have been a misdiagnosed mesothelioma were excluded.
Given that radiation induces changes that may lead to cancer, but only in the long term, the team studied patients who survived more than five years after initial treatment, and compared patients receiving external beam radiotherapy with patients not treated with radiotherapy.
In total, the team analyzed 935,637 individuals with primary solid cancers, of which 301 developed mesothelioma. The site of the secondary mesothelioma was mostly pleural, accounting for 265 patients, with only 32 patients developing peritoneal disease, and 4 cases had mesothelioma in another or unknown site.
Researchers noted that external beam radiotherapy was associated with an increased risk of developing mesothelioma at any site. The risk was also increased when considering only patients with pleural mesothelioma, but the team noted no relationship between irradiation dose and cancer risk.
The team also observed that only patients who had received direct irradiation to the peritoneal region had an increased risk of peritoneal mesothelioma. The authors, however, cautioned that this might be a chance finding given the small number of patients.
In all the analyses, the risk was higher in individuals living more than 10 years after radiotherapy. However, when calculating the accumulated risk of a secondary mesothelioma 40 years after the first cancer diagnosis, the risk was still extremely low, both in patients who received radiation and in those who did not.
The SEER database does not hold information about asbestos exposure, so to evaluate if exposure to asbestos could have influenced the findings, the research team performed an analysis based on country-specific exposure data. Studies have found that asbestos exposure at the population level can be used as a proxy for individual exposure, and the team noted that patients coming from high-risk countries had a much higher risk of developing a secondary mesothelioma compared to patients from low-risk countries. In fact, mesothelioma risk increased as the country-specific exposure got higher.
Researchers concluded that radiotherapy might increase the risk of developing mesothelioma, but added, “The clinical impact of mesothelioma after EBRT [external beam radiotherapy] for primary solid cancers is limited.”