Idiopathic Granulomatous Mastitis with Incidental Lobular Carcinoma In Situ: A Case Report

Background: Lobular carcinoma in situ (LCIS) is a type of lobular neoplasia within the terminal duct lobular unit, involving more than half of acini. It is mostly diagnosed incidentally and although it is not a malignant lesion, it has an association with malignancy in the future. Idiopathic granulomatous mastitis (IGM) is a benign granulomatous disease with few simultaneous reports of malignant lesions in the literature. In this case report, we present a case with LCIS and IGM in a single breast. Case presentation: A ۳۵-year-old female presented to the breast clinic with complaints of pain and a palpable mass in the right breast. The past medical history, habitual history, drug history, and family history of cancer were negative. Physical examinations showed a palpable mass in the right breast without nipple discharge and retraction and lymphadenopathy and the left breast was intact. Breast imaging showed a right UOQ mass. A core needle biopsy was performed showing LCIS and IGM on the right side with a positive cytoplasmic reaction for P۱۲۰ catenin. E-cadherin was negative in LCIS. P۶۳ and CK۵/۶ immunostainings revealed the presence of myoepithelial cells around ductal and glandular structures. The patient developed erythema nodosum at the time of diagnosis. She was put on prednisolone ۵۰ mg per day and NSAID, and after improvement was maintained on ۲۵ mg daily prednisolone for about ۷ months, combined with ۳ months of hydroxychloroquine ۲۰۰ mg twice a day. At the ۱۲-month follow-up, both breasts were completely normal in the physical examination without any mass or skin changes and the right breast mass decreased in size. Conclusion: Radiologic findings of IGM can be suggestive of other possible causes and silent etiologies like the primary phase of breast cancer and LCIS should not be overlooked.