Images in PathologyDedifferentiation in solitary fibrous tumor (SFT) is a rare and relatively recently characterized concept. Dedifferentiation generally indicates the presence of a high-grade neoplasm without evidence of the line of differentiation of the original tumor; it can occur de novo, or be juxtaposed to, or arise as a recurrence of previously well-differentiated tumor. Dedifferentiated SFTs are usually high-grade sarcomas with variable morphologic features and rarely heterologous differentiation, 1-5 with dedifferentiated components typically showing an abrupt transition from areas of classical SFT. SFT 650257I JSXXX10.

Well-differentiated liposarcoma (WDL) can show a morphologic spectrum, including lipoma-like, sclerosing and inflammatory subtypes. It does not metastasize but can dedifferentiate, acquiring metastatic potential. Hibernomas are benign neoplasms that show variable differentiation toward brown fat, and can sometimes occur in the abdomen or retroperitoneum. We illustrate a case of retroperitoneal WDL that showed extensive hibernoma-like morphology, with sheets of multivacuolated adipocytes of varying sizes, with abundant cytoplasm and numerous lipid vacuoles or granular eosinophilic cytoplasm. However, very focally there were fibrous septa containing spindle cells with enlarged, hyperchromatic, mildly pleomorphic nuclei. Further sampling showed areas of typical WDL, with lobules of mature fat intersected by fibrous septa containing atypical, enlarged spindle cells, as well as small foci of dedifferentiation. Immunohistochemistry for CDK4 and p16 showed strong and diffuse nuclear expression in the hibernoma-like areas, and fluorescence in situ hybridization showed MDM2 gene amplification, all in keeping with WDL. We highlight hibernoma-like morphology as part of the histologic spectrum of WDL, and recognition of this variant is important for correct treatment and prognostication.

Soft tissue tumors are diagnostically challenging, and it is recommended that these are reported or reviewed by specialist soft tissue pathologists. We present our experience with second-opinion (consultation) cases in a specialist tertiary sarcoma center. The aim of this study was to determine areas of diagnostic difficulty in soft tissue pathology. We assessed 581 second-opinion cases which were reviewed by two experienced pathologists in a period of one year. There was 62% concordance between the original and the second-opinion diagnosis, with diagnostic discrepancy in 38%. The largest group of soft tissue neoplasms received for second opinion was fibroblastic/myofibroblastic tumors, and most major diagnostic problems were encountered in adipocytic and so-called “fibrohistiocytic” tumors. Major diagnostic errors impacting management were found in 148 cases (25%). Morphologic assessment of tumors, judicious use of molecular techniques, newer immunostains and their interpretation, along with importance of knowledge of rarer entities were found to be most useful in avoiding errors.

Myxoinflammatory fibroblastic sarcoma is a rare malignant soft tissue neoplasm that typically arises on the distal extremities of adults. It usually behaves in a low-grade manner and its characteristic histology is of a lobulated proliferation of moderately atypical spindled to epithelioid cells, vacuolated cells, and enlarged or bizarre cells with prominent nucleoli, dispersed within myxoid stroma containing a mixed inflammatory cell infiltrate. The etiology of myxoinflammatory fibroblastic sarcoma remains unknown with no definite causal factors identified. We describe a case of myxoinflammatory fibroblastic sarcoma arising in the foot of a 77-year-old female, which rapidly recurred locally after initial excision and which arose 10 years after renal transplantation. The neoplasm also showed intermingled areas of hemosiderotic fibrolipomatous tumor. The patient also had multifocal areas of squamous cell carcinoma in situ of the foot and hand, in keeping with the clinical context of immune deficiency. This is the second case of myxoinflammatory fibroblastic sarcoma reported to occur after transplantation, but additionally shows hybrid features of hemosiderotic fibrolipomatous tumor, highlights immunocompromise/immunosuppressive therapy as a possible etiologic factor in their development, and adds to the growing number of myxoinflammatory fibroblastic sarcoma that has demonstrated aggressive behavior.

Superficial CD34-positive fibroblastic tumor (SCPFT) is a recently described entity that, despite significant pleomorphism, carries a good prognosis. We briefly describe this tumor and its principal differential diagnoses. Recognition of SCPFTs, including the clinical context in which they arise, is important to avoid confusion with other pleomorphic soft tissue tumors, particularly neoplasms in the group of pleomorphic sarcomas, which are typically aggressive tumors that could lead to unnecessary overtreatment.