(ChemotherapyAdvisor) – Women in the United States who received breast cancer radiotherapy prior to 1993 face increased risks of lung cancer more than two decades later, concluded authors of a retrospective analysis of data from the National Cancer Institute Surveillance, Epidemiology and End Results (SEER) database, published online ahead of print in the British Journal of Cancer.

“In this population, the radiation-related risks were larger in the third decade after exposure than during the first two decades,” reported lead author Katherine E. Henson and coauthors of the Clinical Trial Service Unit at the University of Oxford in England.

While prior studies have suggested that radiation-related heart disease and lung cancer mortality is a persisting issue many years after radiotherapy, “no study has demonstrated so clearly a progressive increase in risk with time since exposure lasting into the third decade,” they wrote. “In this population, the radiation-related increases in mortality are very clear for women irradiated during 1973-1982.”

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“For women irradiated during 1983-92 there was evidence of radiation-related mortality for lung cancer, but not for heart disease,” they reported. “For women irradiated since 1993 there is, as yet, little evidence of radiation-related mortality.”

The authors studied mortality risks for 558,871 US women diagnosed with breast cancer and treated with radiotherapy during 1973-2008, and followed through December 2008.

“For women diagnosed with breast cancer during 1973-1982 and given radiotherapy shortly afterwards, the cardiac mortality ratios, left-sided versus right-sided, were 1.19 (1.03-1.38), 1.35 (1.05-1.73), 1.64 (1.26-2.14), and 1.90 (1.52-2.37) at <10, 10-14, 15-19 and 20+ years since diagnosis” (2P for trend<0.001), the authors reported. Lung cancer mortality ratios comparing ipsilateral and contralateral rates were 1.05 (0.57-1.94), 2.04 (1.28-3.23), and 3.87 (2.19-6.82) at <10 years, 10-19 years, and 20+ years, respectively (2P for trend=0.002), they reported.

Higher risks for women treated longer ago may, in part, reflect evolving radiotherapy techniques and technologies. Improvements in radiotherapy targeting should reduce incidental radiation doses to healthy nontarget tissues, they noted.

“The current average mean heart dose is likely to be around 2-7 Gy for left-sidedl, and around 1.5 Gy for right-sided breast radiotherapy,” the authors noted. “Therefore, the risks for women irradiated today are likely to be lower.”