Lobulitis is a frequent finding in prophylactically removed breast tissue from women at hereditary high risk of breast cancer.

The aim of this study was to investigate closely the nature of premalignant lesions that occur in prophylactically removed breast tissue from patients at hereditary high risk of breast cancer. Breast tissues obtained from 41 patients who underwent prophylactic mastectomy (pM) because of a hereditary high risk of breast cancer and control tissues from 82 age-matched healthy controls who underwent breast reduction surgery were screened for premalignant lesions. Premalignant and malignant lesions were more frequent (p = 0.0016) in pM samples (5/41) than in controls (1/82). Interestingly, lobulitis, defined as more than 100 lymphocytes and/or plasma cells per lobule in more than one section in morphologically normal lobules, was encountered in 21 of 41 (51%) pM patients, in contrast to only 8 of 82 (10%) controls (p < 0.0001). Preliminary observations indicate a predominance of T-cells in these infiltrates, in agreement with the already known frequent presence of lymphocytic infiltration in hereditary ductal in situ and infiltrating ductal/medullary carcinomas. This novel finding implies an immune reaction to an as yet unidentified antigen frequently present in women at hereditary high risk of breast cancer, possibly as part of an early carcinogenic event.