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A large protuberant abdominal mass was discovered in an 83-year-old man who was examined by the Emergency Department for transient ischemic attack. Subsequent exploratory laparotomy revealed a solid mass that replaced the omentum. Received was a 27.0 x 23.0 x 13.0 cm solid, rubbery, tan and white mass with irregular borders that infiltrated omental adipose tissue. The tumor had the following immuno-histochemical profile: S100(-), CD117(-), CD34(+), CK(-), SMA(-), Factor VIII (-), Calretinin(-), BCL-2(+), EMA(-), WT-1(-), CD31(-).
Archive Case and Diagnosis:
This case first appeared as Performance Improvement Program in Surgical Pathology (PIP) 2006, Case 4 and is a Malignant solitary fibrous tumor
Criteria for Diagnosis and Comments:
The tumor is a malignant solitary fibrous tumor (MSFT). Like virtually all solitary fibrous tumors (SFT) it expresses CD34. It also has a hemangiopericytoma-like pattern, which is a common feature in SFTs. Some slides show areas of hyalinization, which is also a frequent characteristic of SFTs, although less so in MSFTs. Classification as a malignant, rather than benign SFT, is based on its relatively high degree of cellularity, mitotic rate, and nuclear atypia. Vallat-Decouvelaere, et al, reported that, similar to intrathoracic SFTs, approximately 10% of extrathoracic SFTs demonstrate atypical histologic features and appear to follow an aggressive clinical course. Their pathologic criteria for extrathoracic SFT associated with aggressive behavior were: nuclear atypia, hypercellularity, >4 mitosis/10HPF, and necrosis. These criteria are similar to those used in predicting malignancy for intrathoracic SFTs. The authors stress, however, that "rarely SFTs without atypical histologic features have recurred or metastasized so that it is probably unwise to regard any [SFT] as definitely benign."
The most difficult diagnosis in the differential diagnosis is that of a hemangiopericytoma. Both hemangiopericytoma and solitary fibrous tumors can occur in body cavities, can have staghorn vascular pattern, myxoid areas and can be CD34 positive. Similar to solitary fibrous tumors, hemangiopericytomas can also be associated with hypoglycemia due to the production of insulin-like growth factors. Incidence of and criteria for malignancy is also similar for both tumors. In hemangiopericytomas, however, the cells demonstrate a more striking pericytic pattern with fewer areas of hyalinization than are seen in SFTs. In contrast to the diffuse and strong CD34 positivity present in SFTs, hemangiopericytomas may show only weak and patchy staining. Enzinger and Weiss give the following table as a summary to distinguish between hemangiopericytoma and solitary fibrous tumor.
|Parameter||Hemangiopericytoma||Solitary Fibrous Tumor|
|Location||Usually extremity||Usually body cavity, particularly pleura|
|Association with hypoglycemia||Yes||One-fourth|
|Pericytic vascular pattern||+++ (Definitional)||Focal|
|Spindling||Not typical ||Yes|
|Broad zone of hyalinization||Variable to focal ||Typical|
|Histologic malignant forms||Small number||Small number|
|CD34||Most positive||Virtually all positive|
|Cytogenetic abnormality||Abnormalities of 12q||Trisomy 21|
|Comparative genomic hybridization||No gains/losses||Frequent gains/losses|
Angiosarcomas can occur in the abdominal cavity although they are typically a tumor of skin and/or superficial soft tissues. The vascular pattern seen in angiosarcomas is typically complex and anastamosing in comparison with the large staghorn type vessels seen in solitary fibrous tumors. In contrast to SFTs, the neoplastic cells in angiosarcoma are usually either slender and spindle shaped in low-grade tumors or more pleomorphic and atypical in high-grade tumors. Large areas of hyalinization, such as is frequently seen in SFTs, are also unusual in angiosarcomas. Both tumors are CD34 positive, but angiosarcomas will also be positive for other vascular endothelial markers such as CD31 and Factor VIII.
An intra-abdominal fibrous (sarcomatoid) diffuse malignant mesothelioma is a consideration, although these predominantly arise from the peritoneal surface. Mesotheliomas are also usually positive for cytokeratin, EMA, WT-1 and calretinin. They are usually negative for CD34.
Anatomic location would also be important in the differential diagnosis with gastrointestinal stromal tumor, since these tumors arise from the bowel wall. Hemangiopericytoma-like patterns have been described in GISTs and two-thirds are CD34 positive, but unlike SFTs they are uniformly CD-117 positive.
When dealing with malignant spindle cell tumors, sarcomatoid carcinomas are always in the differential diagnosis. In this particular case, a sarcomatoid renal cell carcinoma would be one of the more likely carcinomas to consider. The vascular staghorn pattern and the relatively uniform neoplastic cells would be unusual for a sarcomatoid renal cell carcinoma, but, more importantly, the immunophenotype of negative EMA, negative cytokeratin and positive CD34 is opposite of what would be expected in a renal cell carcinoma. Finally, malignant peripheral nerve sheath tumor (MPNST) may have a somewhat similar histologic pattern. These are tumors of the deep soft tissues of the extremities or trunk. Approximately 25-50% of malignant peripheral nerve sheath tumors occur in association with neurofibromatosis 1. The cells of MPNST typically are elongated with pale cytoplasm and slender wavy nuclei. In contrast to the staghorn-like vascular pattern of an SFT, MPNSTs frequently demonstrate a whorling pattern around small or thick walled blood vessels. MPNSTs usually show S100 protein, Leu-7 and myelin basic protein immunoreactivity. Although CD34 positivity has been reported in MPNSTs, it is rare and not as diffuse and strong as in a SFT.