BREAST CYTOPATHOLOGY

124 4. Primary Malignant Tumors
Pitfalls and Differential Diagnosis
• Atypical squamous metaplasia. This is an uncommon but
a known cause of “atypical/suspicious” diagnosis on FNA.
It shows moderate cellularity with syncytial sheets of atypical
ductal cells with oval nuclei, coarse chromatin, angulated
macronucleoli, and abundant amphophilic cytoplasm.
There is often anisonucleosis and occasional bizarre cells
with giant nucleoli. Apocrine differentiation is not readily
apparent. Signet-ring forms have also been reported. Histologically,
atypical squamous metaplasia is characterized
by the replacement of ductal epithelium by a single layer
of apocrine cells that display at least a threefold variation
in nuclear size. “Atypical apocrine hyperplasia” is a similar
lesion with hyperplastic features regarded by some as a
form of atypical ductal hyperplasia. Additionally, atypical
apocrine cells have also been reported on FNA in the
setting of sclerosing adenosis (atypical apocrine adenosis).
Although diagnostic confusion with ductal carcinoma NOS
or apocrine carcinoma may arise on FNA, it is helpful to
remember that, unlike carcinoma, in atypical squamous
metaplasia the atypical nuclei are only focally present and
are never a diffuse feature. Also many myoepithelial cells
are present and no mitoses or karyorrhexis is noted. The
true malignant potential of atypical squamous metaplasia
is not known.
• Ductal carcinoma with neuroendocrine differentiation.
• Acinic cell carcinoma.
• Squamous cell carcinoma.
• Granular cell tumor.
• Metastatic tumors (malignant melanoma).
• Apocrine carcinoma arising in axillary apocrine glands
should be distinguished from breast primaries.
Metaplastic Carcinoma
Clinical Features
• Metaplastic carcinomas account for less than 1% of all
invasive breast carcinomas.