Outcomes on Excision and Clinical Follow-up of Isolated Atypical Apocrine Adenosis Diagnosed on Core Needle Biopsy: A Multi-institutional Study.

Context.—: Atypical apocrine adenosis (AAA) is a rare breast lesion defined as presence of cytologic atypia in apocrine adenosis. World Health Organization Classification of Tumours, 5th edition, defines cytologic atypia as at least 3-fold variation in nuclear size and prominent nucleoli. Currently, owing to the rarity of the lesion, the clinical behavior of AAA is not well understood.

Objective.—: To further investigate the risk of upgrade to malignant histology on follow-up excision after a diagnosis of isolated atypical apocrine adenosis (iAAA) on core needle biopsy (CNB).

Design.—: We identified 22 female patients with diagnosis of iAAA on CNBs across 3 institutions between 2000 and 2024, with an average age of 58 years. The most common indication for CNB was presence of a mass. We reviewed pathology reports and available histology slides of CNBs and subsequent surgical excisions.

Results.—: Of 22 patients, 17 underwent surgical excision and 5 were followed up with mammogram for an average of 38 months. The diagnosis for 2 of 17 patients (12%) who underwent excision was upgraded to malignancy (ductal carcinoma in situ), 3 had atypical ductal hyperplasia (1 with AAA), and 2 additional cases had residual iAAA. iAAA was the target lesion in 7 of 21 cases (33%); among these 7 cases, 5 presented with a mass: 2 of 5 (40%) were upgraded to DCIS and 1 of 5 had residual iAAA. The 5 patients who were followed up did not have any adverse outcomes.

Conclusions.—: We recommend surgical excision of iAAA when found on CNB, especially when the target lesion is a mass. However, larger studies are necessary to further understand this entity.