Case 1 History
The patient is a 54-year-old female with a painful, circumscribed, skin-colored papule on the chest.
Microscopic Findings
Sections show a dermal proliferation of spindle cells with a fascicular arrangement ( Fig. 15.1 ). These spindled cells contain blunt-ended nuclei and eosinophilic cytoplasm. Mitotic figures are not identified.
Diagnosis
Pilar Leiomyoma
Clinical Presentation
Leiomyomas are indolent, frequently painful, firm, red to skin-colored, circumscribed papules. Cutaneous leiomyomas emulate either follicular smooth muscle (pilar leiomyoma or piloleiomyoma) or vascular smooth muscle (angioleiomyoma). The presentation of pilar leiomyoma favors the trunk and upper extremities, whereas angioleiomyoma commonly presents on the leg. Multiple cutaneous leiomyomas may indicate hereditary leiomyomatosis and renal cell cancer, which results from mutations in fumarate hydratase ( FH ). Affected patients develop cutaneous and uterine leiomyomas as well as renal cell carcinoma.
Histopathology
Pilar leiomyomas are composed of fascicles of spindled cells with blunt-ended nuclei and pale eosinophilic cytoplasm that are similar to perifollicular smooth muscle. Pilar leiomyomas are nonencapsulated and incompletely circumscribed; by contrast, angioleiomyomas show sharp circumscription. Scattered cytomegaly may be seen in long-standing tumors (representing so-called symplastic, degenerative, or ancient change), but mitotic figures are uncommon. Leiomyomas invariably express smooth muscle determinants, including desmin, caldesmon, calponin, and smooth muscle actin.
Differential Diagnosis
The differential diagnosis includes other spindle cell tumors with myoid or myofibrocytic differentiation, including angioleiomyoma, dermatomyofibroma, and dermatofibroma ( Table 15.1 ).
| Pilar Leiomyoma | Angioleiomyoma | Dermatofibroma | Dermatomyofibroma | |
|---|---|---|---|---|
| Composition | Bland spindle cells with blunt-ended nuclei and eosinophilic cytoplasm | Bland spindle cells with blunt-ended nuclei and eosinophilic cytoplasm | Spindled and stellate cells | Bland spindled cells (myofibroblasts and fibroblasts) |
| Dermal configuration | Incompletely circumscribed collection of fascicles | Well-circumscribed nodule | Nodule with peripheral keloidal collagen | Horizontally arrayed collection of fascicles |
| Epidermal changes | None | None | Acanthosis with hyperpigmentation | None |
| Immunohistochemical studies | Desmin, calponin, and SMA all positive | Desmin, calponin, and SMA all positive | Nonspecific; factor XIIIa is expressed by the epithelioid subtype | Nonspecific |
Angioleiomyoma
Clinical Presentation
Angioleiomyomas emulate the smooth muscle differentiation of subcutaneous vessels. These are benign neoplasms that can present widely but commonly involve the lower extremity. The clinical morphology is not distinctive but typically is nodular. Pain is a common symptom.
Histopathology
Angioleiomyomas are composed of smooth muscle cells ( Fig. 15.2 ). Angioleiomyomas are circumscribed smooth muscle proliferations with fascicular internal architecture. Embedded and integrated within the proliferation are numerous small vascular spaces. A fibrous pseudocapsule is typically present, and stromal fat may be present.
Dermatofibroma
Clinical Presentation
Dermatofibroma presents as a small, brown to skin-colored, firm papule or nodule. Associated dimpling of the cutaneous surface can often be identified by the clinician.
Histopathology
Classically, dermatofibroma shows a proliferation of spindled and stellate cells arranged interstitially or as short fascicles in the mid to upper dermis ( Fig. 15.3 ). There are associated altered collagen, including peripheral keloidal collagen in some instances. Associated surface acanthosis with hyperpigmentation and flattening of rete ridges is characteristic. Dermal cytologic features are highly varied. The fibroma cells may be stellate, fusiform, histiocytoid, foamy, or epithelioid. Associated hemosiderosis is not uncommon.
Dermatomyofibroma
Clinical Presentation
Dermatomyofibromas typically present on the upper trunk of young adults as red-brown plaques. ,
Histopathology
These myofibroblastic proliferations represent ill-defined nonencapsulated dermal and superficial subcutaneous plaques composed of fascicles of spindle cells oriented in a horizontal configuration ( Fig. 15.4 ). The fascicles maintain ahorizontal orientation. By contrast to dermatofibroma, there is no associated acanthosis. The myofibrocytic cells within fascicles may variably or weakly express actin. Curiously, associated coarse elastic fibrils can be identified in association with dermatomyofibroma using elastic tissue staining.
Case 2 History
The patient is a 68-year-old male with an enlarging nodule on the lower leg.
Microscopic Findings
Sections show a poorly circumscribed, infiltrative proliferation of spindle cells arranged as fascicles ( Fig. 15.5 ). The spindle cells show enlarged, elongated nuclei with pleomorphism and mitotic figures.
Diagnosis
Leiomyosarcoma
Clinical Presentation
Leiomyosarcomas typically present in later adulthood. An associated preexistent piloleiomyoma can be found in some instances. Several series have noted a distinction between leiomyosarcomas restricted to the dermis versus those involving the subcutis. When subcutaneous involvement has occurred, the prognosis decreases, and metastatic risk increases. Some authors use the term atypical smooth muscle tumor with respect to wholly intradermal leiomyosarcoma because its metastatic potential is low.
Histopathology
Sections show an infiltrative, poorly circumscribed proliferation of atypical spindle cells arranged in fascicles. Blunt-ended nuclei, as is typical of smooth muscle, can be found. Nuclei are enlarged and hyperchromatic. Nuclear pleomorphism and multinucleation can be observed in some tumors. The mitotic index is elevated. Predictably, leiomyosarcomas express desmin, smooth muscle actin, and caldesmon.
Differential Diagnosis
The differential diagnosis includes other spindle cell tumors, including the entities listed in Case 1 above.
