SS may have a prominent vascular pattern very similar to that seen in hemangiopericytoma (HPC); however, differently from the latter, SS is negative for CD34 and is positive for keratins and EMA. Additionally, SS has a consistent t(X; 18) (p11.2; q11.2) translocation involving the SYT gene on chromosome 18 and one of the SSX genes on chromosome X. This translocation is never found in HPC.
HPC is a mesenchymal neoplasm of fibroblastic and not hemangiopericytic origin that shows a characteristic vascular pattern characterized by dilated to compressed vessels with thin walls and irregular “staghorn” profiles. In the 2002 WHO classification of soft tissue tumors [Guillou L, Fletcher JA, Fletcher CDM, Mandahl N: Extrapleural solitary fibrous tumour and haemangiopericytoma. In: Fletcher CDM, Unni KK, Mettens F (eds):
Pathology and Genetics of Tumours of Soft Tissue and Bone.
Lyon: IARC Press 2002, 86-93], HPC is classified as a neoplasm in the spectrum of solitary fibrous tumor (SFT) on the basis of histological similarities, identical immunophenotype and similar biological potential. Despite these similarities, extra-pleural SFT and classical HPC have clear differences. SFT is characterized by alternating hypocellular and hypercellular regions containing thick collagenous bands, hemangiopericytic vasculature, and bland spindle cells arranged in a so-called “patternless pattern.” On the other hand, classical HPC is a cellular neoplasm with scant matrix, composed of uniform spindle cells, and showing the characteristic vascular pattern. When these strict criteria are applied, HPC is a characteristic mesenchymal neoplasm that affects adults with a peak incidence in the fourth and fifth decade of life and female predominance. HPC is most commonly seen in the deep soft tissue, mainly in retroperitoneum and proximal extremities, with a subset affecting the meninges. The majority of the cases in the peripheral soft tissues are slow-growing and biologically benign with low recurrence rate. Approximately 10% to 15% of the cases behaves aggressively and metastasizes. Histological features that may be associated with malignant behavior are large size (more than 8 cm), more than 4 mitoses per 10 HPF, and increased cellularity with fascicular architecture, cytologic atypia, and tumor necrosis. Many soft tissue tumors, benign and malignant, have a hemangiopericytic vascular pattern. These include myofibroma and myopericytoma in the benign lesions, and SS and mesenchymal chondrosarcoma in the malignant. HPC is not associated with recurrent cytogenetic abnormalities, but some cases have shown t(12; 19) (q13; q13) and t(13; 22) (q22; q11) translocations.
Cellular schwannoma, like HPC has a peak incidence in the fourth and fifth decade of life and affects many of the anatomical sites of the latter, such as retroperitoneum and extremities. Pathologically it also has some superficial resemblance with HPC due to his cellularity and circumscription; however, cellular schwannoma is fascicular in architecture and lacks the characteristic vascular pattern of HPC. Additionally, cellular schwannoma is consistently S100 protein positive and only focally positive for CD34, mainly at the periphery of the lesion.
LGFMS may affect the same anatomical sites and grossly appear as circumscribed as HPC: however, differently form HPC, LGFMS affects younger patients, usually between the second and fourth decade of life, is multinodular and infiltrative, and has a myxoid to fibrous matrix containing spindle cells arranged in a fascicular architecture. Additionally, the characteristic vascular pattern of HPC is not present, replaced by prominent curvilinear vessels with perivascular crowding of tumor cells. LGFMS is usually positive for vimentin only, with occasional weak staining for smooth muscle actin (SMA) and CD34. LGFMS has a characteristic t(7; 16) (q34; p11) translocation that is not present in HPC.
Leiomyosarcoma of deep soft tissue affects adult patients with a predilection for older individuals. The most common sites of involvement are the lower extremity and the trunk wall. Grossly, they appear as circumscribed masses and lower grade tumors may be firm with a whorled cut surface and the higher grade tumors fleshy and necrotic. In many cases an origin from a vein may be demonstrated. They do not show the characteristic vascular pattern of HPC, but are characterized by discrete criss-crossing fascicles composed of spindle cells with fibrillary and eosinophilic cytoplasm with oval nuclei with blunt ends and often contain a vacuole that may indent one of the nuclear poles. The majority of leiomyosarcomas are positive for SMA, HHF35, desmin, and heavy-caldesmon with a small fraction also expressing CD34.
