Radiotherapy bears no increased contralateral breast cancer risk among smokers

For young women with first primary breast cancer who currently smoked or who are former smokers, radiation therapy (RT) confers no further increase in the risk of contralateral breast cancer (CBC), a study has found.

Cigarette smoking has been reported to contribute to second primary CBC risk, and the current study examined whether RT interacted with smoking to modify this risk. The analysis included 1,521 CBC patients and 2,212 controls with unilateral breast cancer, all diagnosed with first invasive breast cancer at age <55 years.

All participants were interviewed regarding their smoking history. Thermoluminescent dosimeters in tissue-equivalent anthropomorphic phantoms were used to estimate absorbed radiation doses to contralateral breast regions.

Multivariable conditional logistic regression revealed that RT dose did not interact with any measure of smoking to increase CBC risk. The interaction estimates of continuous RT dose were all null (smoking at first breast cancer diagnosis [ever/never]: rate ratio [RR], 1.00, 95 percent confidence interval [CI], 0.89–1.14; years smoked: RR, 1.00, 95 percent CI, 0.99–1.01; lifetime pack-years: RR, 1.00, 95 percent CI, 0.99–1.01).

While smoking history is important for assessing subsequent mortality risk, the present data indicate that smoking history should not influence the development of RT plans for young women with primary breast cancer.

The results are generalizable to younger breast cancer survivors. However, true smoking behaviour may have been underestimated as smoking history was self-reported.