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Atypical leiomyoma: An unusual variant of cutaneous pilar leiomyoma

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Atypical leiomyoma: An unusual variant of cutaneous pilar leiomyoma
Nocito Mabel Jimena MD, Lustia María Marcela MD, Luna Paula Carolina MD, Cañadas Nadia Guadalupe MD, Castellanos Posse María Laura MD, Marchesi Carolina MD, Carabajal Graciela MD, Mazzini Miguel Angel MD
Dermatology Online Journal 15 (3): 6

Dermatology Department, Hospital Churruca, Buenos Aires, Argentina.


Cutaneous atypical leiomyoma is an unusual benign tumor arising from arrector pili muscle that shares histological features with uterine atypical or symplastic leiomyoma: atypical cellularity with pleomorphic nuclei but minimal or no mitosis. Six other cases have been reported so far and, in spite of its name and of being a smooth muscle proliferation, no recurrences nor metastasis have been reported.


Cutaneous leiomyoma is a rare, benign, smooth muscle tumor. It has been divided into different subtypes according to the skin structures from which it can derive:

1. Cutaneous pilar leiomyoma, arising from arrector pili muscle;
2. Angyoleiomyoma, arising from vessel walls
3. Genital leiomyoma, from smooth muscle of the scrotum, nipples and labia majori [1].

Cutaneous pilar leiomyoma usually presents as multiple tumors and, though sporadic in the vast majority, a familial autosomic dominant pattern of inheritance has been described. A similar pattern of inheritance has been reported in some women with early uterine leiomyomatosis and cutaneous leiomyomas. This condition is known as Reed syndrome or multiple leiomyomatosis [2, 3]. Presentation of the lesion as a solitary tumor is less common.

There have been some previous reports of cutaneous pilar leiomyomas with atypical cellularity, similar to the findings in symplastic uterine leiomyomas. We hereby present the case of a female patient with an atypical leiomyoma.

Clinical case

Figure 1Figure 2
Figure 1. Erythematous papulonodule measuring 1 cm on the right thigh
Figure 2. Fusocellular proliferation arranged in fascicles

68-year-old female patient with a history of hypertension presented with a reddish asymptomatic nodule measuring 1 cm in diameter located in her right thigh of 2 years of evolution (Fig. 1).

Figure 3
Figure 3. Pleomorphic nuclei with no mitosis

Cutaneous biopsy revealed a dermal fusocellular proliferation with pleomorphic and atypical nuclei. There was no evidence of mitosis (Fig. 2 and 3). Stains for actin and desmin were positive whereas S100 and Ki67 were negative (Fig. 4 and 5). The diagnosis of atypical leiomyoma was made. The lesion was surgically removed with free margins, No recurrences were seen in a 6 month follow-up period.

Figure 4Figure 5
Figure 4. Positive desmin stain
Figure 5. Negative Ki67 stain


Cutaneous pilar leiomyomas are benign smooth muscle tumors that appear between the second and third decades of life as multiple, painful red-brown papulonodules. They are located on the face, back and extensor aspect of extremities. A germline gene mutation responsible for the multiple familial form of these tumors has been identified in the gene that encodes a tumor suppressor enzyme, fumarate hydratase; this has been located in chromosome 1q42.3-43 [4]. It must be remembered that this syndrome also includes uterine fibroids and may be associated with renal cell carcinoma [4, 5].

Solitary cutaneous pilar leiomyomas, on the other hand, are usually asymptomatic and larger in size than those encountered in patients with the multiple form.

Leiomyosarcomas account for the malignant variant of the lesion. They can originate from dermis and subcutaneous tissue as solitary tumors located in the extremities. When superficial, metastases after surgical removal are very rare but local recurrences are found in 40 percent. Mohs's micrographic surgery is therefore the treatment of choice [6].

Atypical leiomyoma or symplastic leiomyoma is an unusual variant of cutaneous pilar leiomyoma; only 6 cases are reported in the literature so far. It shares histological features with symplastic leiomyoma of the uterus: atypic cellularity, pleomorphic nuclei, and minimal or no mitosis [7].

This tumor presents as painful or asymptomatic solitary red-brown nodules and the predominant location is on the extremities. It has been more frequently observed in men. The differential diagnosis includes other benign tumors such as schwannomas, fibrohystiocytomas, and neuromas.

Histologically, leiomyoma is a non-encapsulated dermal proliferation composed of myocytes with an abundant eosinophylic cytoplasm without atypical cells or mitoses, whereas leiomyosarcoma is a highly pleomorphic proliferation of atypical myocytes and a significant number of mitoses. A recent review of leiomyosarcomas demonstrated the presence of 9 or more mitoses per 10 high power field in the 84 percent of the tumors [8].

This finding is crucial to distinguish benign leiomyomas from leiomyosarcomas. Atypical cutaneous leiomyoma stands in the middle of the cutaneous leiomyoma spectrum exhibiting atypical myocytes and giant cells but with minimal or absent mitoses.

The tumor stains positive for smooth muscle actin and desmin and negative for S100 and Ki67, which is a histological marker for mitoses. Based on the experience with uterine symplastic leiomyomas (a review of cases with a ten-year follow-up period found no recurrences nor metastasis), surgical excision of the tumor with free margins is the proposed treatment of choice for the cutaneous atypical leiomyoma.

Despite a short follow up period of no more than two years, none of the six cutaneous atypical leiomyomas previously reported recurred or metastasized. Nevertheless, a close follow up of the patients is recommended, especially if two or more mitoses per ten high power fields are present.


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2. Weedon. Tumores del músculo liso, cartílago y hueso en Piel. In Patología. Weedon D, Strutton G, 1era Edic. Marbán, MADRID.2002, 795-6.

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8. Kaddu S, Beham A, Cerroni L et al. Cutaneous leiomyosarcoma. Am J Surg Pathol.1997; 21: 979-87. [PubMed]

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