Genetic factors and breast cancer laterality

Women are more likely to develop cancer in the left breast than the right. Such laterality may influence subsequent management, especially in elderly patients with heart disease who may require radiation therapy. The purpose of this study was to explore possible factors for such cancer laterality.

In this work, clinical data for consecutive patients with histologically confirmed breast cancer were reviewed, with emphasis on clinical presentation and family history.

Between 2005 and 2012, 687 patients with breast cancer were seen. Two women with incomplete data and eleven men were excluded. In total, 343 (50.9%) patients presented with left breast cancer, 311 (46.1%) with right breast cancer, and 20 (3.0%) with simultaneous bilateral malignancy. There were no significant differences between the three groups, especially in regards to clinical presentation and tumor characteristics. A total of 622 (92.3%) patients had unilateral primary, 20 (3.0%) had simultaneous bilateral, and 32 (4.7%) had metachronous primary breast cancer with subsequent contralateral breast cancer after 7.5-236 months. The worst 10-year survival was for bilateral simultaneous (18%) compared with unilateral (28%) and metachronous primaries (90%). There were no differences in survival in relation to breast cancer laterality, handedness, and presence or absence of a family history of cancer. There were significant similarities between patients and first-degree relatives in regards to breast cancer laterality, namely same breast (30/66, 45.5%), opposite breast (9/66, 13.6%), and bilateral cancer (27/66, 40.9, P=0.01163). This was more evident among patients and their sisters (17/32, 53.1%) or mothers (11/27, 40.7%, P=0.0689). There were also close similarities in relation to age at initial diagnosis of cancer for patients and their first-degree relatives for age differences of ≤5 years (48/166, 28.9%), 6-10 years (34/166, 20.5%), and >11 years (84/166, 50.6%, P=0.12065).

High similarities between patients and their first-degree relatives in regards to cancer laterality and possibly age at initial diagnosis of cancer may suggest an underlying inherited genetic predisposition.