Spatial Distribution Of Radiation Therapy (Rt)-Associated Lung Cancers Following Rt For Breast Cancer

The risk of lung cancer is increased following RT for breast cancer; with relative risks ≈ 10-30 in smokers. The location of RT-associated lung cancer relative to 3-dimensional (3D) doses is limited. We herein perform an in-depth analysis relating the location of “secondary” lung cancers to the prior RT beams/doses in patients irradiated for breast cancer. A retrospective review identified 29 patients who received RT for breast cancer, and developed subsequent lung cancers. The location of the lung cancer, relative to the RT beams/3D doses was defined. The RT dose received by the lung in the vicinity of the secondary tumor was estimated. 27/29 patients had a history of tobacco use. Of the 29, 10 (35%) of the lung cancers were ipsilateral, 18(62%) were contralateral, and 1 was central (relative to the breast RT). No primary lung tumors were in, or within 3 cm, of the tangent fields. Three lung tumors were in the apex, both in the vicinity of the supraclavicular field. Of the 26 others, 19 and 7 of the lung tumors were in areas that received <1 Gy and <8 Gy, respectively. The two non-smokers' lung cancers were in areas receiving <1 and <8 Gy. The majority of secondary lung cancers are not in the vicinity of the prior RT. Screening efforts therefore in patients deemed a high risk of RT-induced lung cancer following breast cancer RT (e.g. smokers) need to evaluate the entire lung, not just the lung tissue in the vicinity of the high dose exposure. Suspected nodules in all regions of the lung should be considered potentially-malignant. The observation that most of the “secondary” lung cancers are far from the RT beam suggests several possibilities: 1) there may not be a dose-response for RT-induced lung cancer, 2) that most of the secondary cancers are indeed due to the tobacco exposure, rather than the RT, and 3) the dose-response for lung cancer induction is difficult to appreciate since the volume of lung exposed to a low dose is far larger than the volume of lung exposed to a high dose. Additional study is underway to consider these issues.