ATYPICAL LEIOMYOMA (PILOLEIOMYOMA) WITH LENTIGINOUS PIGMENTATION AND EPIDERMAL HYPERPLASIA
Piloleiomyomas  are characterized by the presence of fascicles of smooth muscle  within the dermis. Cytologic atypia is present in a small percentage of these tumors, and cytologic atypia of the degree illustrated in this case is of no consequence in the absence of significant mitotic activity. False mitoses, that may represent pyknotic nuclei, can lead to a false diagnosis of leiomyosarcoma. It should be noted that most of the cases that have been thought to be leiomyosarcomas that have been restricted to the dermis have not metastasized. Leiomyosarcomas of the subcutis and deep tissue have metastasized.

Sometimes the atypical nuclei have the smudged appearance that is seen in other benign but atypical tumors such as pleomorphic lipomas and pleomorphic fibromas.

Benign lentiginous hyperpigmentation and epidermal acanthosis can be seen over a variety of dermal tumors,  the most common being dermatofibromas. The presence of lentiginous hyperpigmentation over a smooth muscle tumor or smooth muscle hamartoma raises the possibility of Becker's nevus (Becker's melanosis). It might be best to restrict that diagnosis to the classical hypertrichotic, hyperpigmented lesion.

Scan view of part of this lesion.
Low power of upper left side of above showing epidermal hyperplasia and lentiginous hyperpigmentation..
Low power of dermal fascicles.
High power of above. Note the clear zones around the nuclei of the cells that are cut in cross-section. Smooth muscle has this appearance, but this is not found exclusively in smooth muscle tumors.
High power of another area. Large, irregulary shaped nuclei that are sometimes hyperchromatic account for the atypia. Note the smudged appearance of some of the hyperchromatic nuclei.
High power of another area. It should be noted that no true mitoses were found, and the lesion was confined to the dermis. Note the the smudged, hyperchromatic nuclei.

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