Pseudomyogenic hemangioendothelioma: a distinctive, often multicentric tumor with indolent behavior.

A 1992 report described 5 keratin-positive spindle cell neoplasms with multifocal presentation in a single limb, which were proposed at that time to be a variant of epithelioid sarcoma. This tumor type is not widely recognized and is incompletely characterized. We examined 50 cases of this distinctive tumor to evaluate histologic, immunophenotypic, and clinical features. There was a 4.6:1 male predominance (mean age, 31 y; 82% ≤40 y). Half of the patients presented with painful nodules and the other half with painless nodules. Mean tumor size was 1.9 cm (range, 0.3 to 5.5 cm). Tumors arose in the lower limb (54%), the upper limb (24%), trunk (18%), or head and neck (4%). Thirty-three (66%) were multifocal lesions (ranging from 2 to 15 lesions), including 32 cases with involvement of multiple tissue planes. Of 205 total lesions, 64 (31%) involved the dermis, 42 (20%) involved the subcutis, 70 (34%) lesions involved muscle, and 29 (14%) lesions involved bone; all the lesions had infiltrative margins. The tumors were composed of loose fascicles and sheets of plump spindle cells with vesicular nuclei, variably prominent nucleoli, and abundant brightly eosinophilic cytoplasm, some with a strikingly rhabdomyoblast-like appearance. In all cases, a minority of cells were epithelioid. Twenty-seven tumors contained a prominent neutrophilic inflammatory infiltrate. Most tumors showed only mild nuclear atypia; 6 tumors contained foci of notably pleomorphic cells. The median mitotic rate was 1 per 10 HPF (range, 1 to 10). Seven tumors showed vascular invasion; 7 tumors had areas of necrosis. By immunohistochemistry, all tumors were diffusely positive for AE1/AE3 and FLI1; 22 of 47 tumors were variably positive for CD31. Focal positivity was seen for CAM5.2 (21 of 35), smooth muscle actin (14 of 42), epithelial membrane antigen (7 of 49 weak), and PAN-K (MNF116) (1 of 47). All were negative for CD34, desmin, and S100 protein and showed intact INI1 expression. Follow-up was available for 31 patients and ranged from 9 months to 17 years (mean, 4 y). Most lesions were treated by local excision. Eighteen (58%) patients had local recurrence or developed additional nodules in the same region, all but one, within 1 year of first presentation. Eight patients had postoperative radiation therapy and 6 patients had chemotherapy. Four patients had amputations for multifocal disease. One patient had a regional lymph node metastasis, and, thus far, only 1 patient has developed distant metastases (disseminated), 16 years after primary tumor excision. At the time of the last follow-up, 27 patients were alive with no evidence of the disease, 1 patient was alive with unknown disease status, 2 patients were alive with recurrent disease, and 1 patient died of the disease. In summary, we describe a distinctive type of rarely metastasizing ("intermediate") tumor affecting mainly young men and usually characterized by multifocality in different tissue planes of a limb. Although sharing some features with epithelioid sarcoma (skin/soft tissue of distal extremities, young adults, keratin positive), it differs by having predominantly myoid-appearing spindle cell morphology, expression of FLI1, common reactivity for CD31, lack of epithelial membrane antigen, CD34, and PAN-K expression, and intact INI1. The overall immunophenotypic findings favor endothelial differentiation. Despite the ominous presentation, follow-up thus far suggests an indolent clinical course with a small risk of distant metastasis. Although the precise nosologic status of this tumor type is uncertain, we propose the interim designation "pseudomyogenic hemangioendothelioma."