Basaloid (matrical) cells transition to ghost cells, which are pink cell remnants that retain their outlines.
Clusters of multinucleated giant cells surround the hypocellular pink material.
The tumor shows a random admixture of pink material (ghost cells) and cellular areas.
A Diff-Quik stain of an FNA demonstrates a cellular blue cell neoplasm. There is a suggestion of a multinucleated giant cell on the bottom right image.
Oval hyperchromatic blue cells with relatively scant cytoplasm may be alarming, however, corresponding histology shows a benign pilomatricoma.
Pilomatricoma (a.k.a calcifying epithelioma of Malherbe) is a benign tumor arising in the lower dermis or subcutis. These lesions arise in the dermis and are composed of basaloid (matrical) cells and distinct "ghost" cells. Often the lesion elicits a foreign body giant cell reaction. Calcifications are also common.
They appear as a tan or purplish nodule on the face and upper arms although other sites may be affected as well. The nodules can be hard and bony due to calcification. Sometimes there is cutaneous erosion. Most are seen in young children but they may also affect adults. Multiple tumors may arise in the setting of myotonic dystrophy.
Benign lesions. Exceptionally, there may be malignant transformation to pilcomatrix carcinoma, which is infiltrative rather than circumscribed, with cytologic atypia and necrosis.
Basam KJ. Dermatopathology: Foundations in Diagnostic Pathology 1st Ed. Philadelphia, PA: Elsevier; 2010: 387-9.