Localized Breast Amyloidosis in a Previously Treated Breast: A Case Report and Literature Review.

Breast amyloidosis is a rare disorder that can mimic malignancy, particularly in patients with a history of breast cancer. We describe a 61-year-old woman with a history of left breast invasive ductal carcinoma who developed an enlarging mass at the mastectomy site eight years post-treatment. Imaging suggested recurrence; however, biopsies revealed Congo red-positive amyloid deposits with admixed necrosis, fibrosis, and inflammation, with no evidence of recurrent carcinoma or lymphoproliferative disease. Mass spectrometry identified only amyloid signature proteins, with no specific amyloid fibril protein or subtype detected, and in the absence of systemic involvement, supporting a diagnosis of localized degenerative amyloidosis possibly related to prior treatment. This not only represents a novel etiopathological category of post-treatment degenerative amyloidosis but also highlights the importance of Congo red staining and mass spectrometry in the evaluation of ambiguous post-treated lesions to prevent misdiagnosis and overtreatment.