Atypical ductal hyperplasia and ductal carcinoma in situ of the breast associated with perineural invasion.

Perineural invasion is a histologic feature usually diagnostic of invasion in malignancies. In the breast, however, it has been associated with benign lesions such as sclerosing adenosis (SA), complex sclerosing lesion/radial scar (CSL/RS), and ductal carcinoma in situ (DCIS). This article describes perineural invasion associated with atypical ductal hyperplasia (ADH), florid hyperplasia without atypia (FH), and DCIS. All cases with a diagnosis of perineural invasion were selected from a series of 10,000 breast consult cases. Invasive mammary carcinomas were excluded. Fourteen cases of perineural invasion were found and associated with the following diagnoses: ADH (5), DCIS (3), FH (5), and ductal adenoma (1). Nine cases developed in CSL/RS, 4 cases in SA, and 1 case in a previous biopsy site of ductal adenoma; lesions were all less than 3 mm. The glands involving nerves showed cytologic and architectural features of the adjacent ADH, DCIS, and FH. Immunostaining for protein gene product (PGP) 9.5 marked nerves, and smooth muscle actin antibody highlighted the myoepithelial cells around glands. Perineural invasion seen in association with DCIS and ADH, in a background of CSL/RS and SA, may pose difficulty in diagnosis, especially in small biopsy specimens. It should be assessed with care to avoid misinterpretation as invasive mammary carcinoma.