Plexiform fibrohistiocytic tumor with molecular and cytogenetic analysis

Plexiform fibrohistiocytic tumor: a clinicopathological and immunohistochemical study of 39 tumors, with evidence for a CSF1-producing "null cell" population

Plexiform fibrohistiocytic tumor (PFHT) is a mesenchymal tumor of intermediate malignancy, typically occurring in the superficial soft tissues of young patients and displaying a biphasic pattern, with nodules of histiocytoid cells surrounded by fascicles of myofibroblastic spindled cells. The pathogenesis of PHFT is unknown. We comprehensively studied 39 PFHT, occurring in 25 females (66%) and 13 males (34%), ranging from 2 to 55 years of age (median 21 years). The tumors most often occurred in the upper extremity (n = 16, 41%) and ranged from 0.4 to 6.1 cm in size (median 1.5 cm). One patient with known neurofibromatosis type 1 presented with metachronous tumors of the finger and back. Clinical follow-up (29 patients; range 5-168 months; median 60 months) showed 3 tumors to have recurred locally; none was metastasized. One patient died of an unrelated cause; all others were alive without disease at the time of last follow-up. Immunohistochemistry showed the histiocytoid nodules of all cases to contain CD163/CD11c-positive histiocytes and cells negative for both markers ("null cells"). CSF1 expression was present in "null cells" in 7/10 cases (RNAscope chromogenic in situ hybridization). The Ki-67 labeling index was very low (< 5%); Ki-67-positive cells within histiocytoid nodules appeared to represent "null cells." All tested cases were negative for significant mutations or fusion events (TruSight Mutation Panel, TruSight Fusion Panel, Mayo Clinic Melanoma Targeted Gene Panel). We conclude that PHFT may be even more indolent than has been appreciated, although classification as an "intermediate" tumor is correct. We hypothesize that the CSF1-producing "null cells" of PHFT may represent the neoplastic element, with the bulk of the tumor masses comprising recruited and reactive cell populations.

Malignant Superficial Mesenchymal Tumors in Children

Malignant superficial mesenchymal tumors are a very diverse group of neoplasms with few clinical and radiological discriminatory factors. Hence, some of these cancers are rarely suspected based on clinical and radiological grounds, others may be easily misdiagnosed, and the histological analysis of a biopsy or resection is central in the diagnostic process. In children, the age at presentation is a major element of the differential diagnosis. Some tumors have a very distinct epidemiology, while others may be seen at any age. More recently, the advances in molecular biology have greatly improved the diagnosis of mesenchymal tumors and new entities are still being described. In the present review, we provide an overview of the diversity of malignant superficial mesenchymal tumors in children, including new and/or rare entities. We discuss the important diagnostic features, be they clinical, histological, or molecular. Special attention was given to the genetic features of these tumors, particularly when they were helpful for the diagnosis or treatment.

Molecular Alterations in Pediatric Fibroblastic/Myofibroblastic Tumors: An Appraisal of a Next Generation Sequencing Assay in a Retrospective Single Centre Study

BACKGROUND

Pediatric fibroblastic/myofibroblastic tumors (PFMTs) can be challenging to definitively classify. Large case series or diagnostic updates have not been recently published despite identification of molecular alterations that could improve diagnostic accuracy. Our review of the literature found that over two-thirds of the more than 30 types of PFMTs harbor recurrent molecular alterations. We performed an institutional review of PFMTs to highlight limitations of a predominantly morphological classification, and evaluated the utility of a next-generation sequencing assay to aid diagnosis.

METHODS

PFMTs identified over a period of 12 years were reviewed, categorized per the new WHO classification, and tested using the Oncomine Childhood Cancer Research Assay.

RESULTS

Eighty-seven specimens from 58 patients were reviewed; 50 were chosen for molecular analysis, 16 (32%) lacking definitive classification. We identified alterations, some novel, in 33% of assayed cases. Expected alterations were identified for most known diagnoses and mutations were identified in 6 of 16 tumors (38%) that were initially unclassified.

CONCLUSION

We confirmed a significant subset of PFMTs remain difficult to classify using current criteria, and that a combined DNA/RNA assay can identify alterations in many of these cases, improving diagnostic certainty and suggesting a clinical utility for challenging cases.

Ulcerated congenital plexiform fibrohistiocytic tumor: Case report and literature review

A newborn boy presented with a progressively infiltrating and painful congenital ulcerated plaque on the back of his left foot. A partial excision was performed and histopathologic examination confirmed a diagnosis of a plexiform fibrohistiocytic tumor. This rare tumor usually appears in children and adolescents, with congenital presentations even more uncommon. This case details the exceptional presentation of a congenital ulcerated plexiform fibrohistiocytic tumor with a review of the current literature.

Plexiform fibrohistiocytic tumor of bone

Plexiform fibrohistiocytic tumor is an extremely rare soft tissue tumor with a low malignancy potential. The patient is usually a child or a young adolescent and the tumor is usually localized in the upper extremities. We report on a case of a 21-year old male with a plexiform fibrohistiocytic tumor in the left fibula admitted to our hospital due to a swelling and pain in the left lower extremity. Radiologically a lytic lesion in the distal end of left fibula consistent with a non-aggressive lesion with low biological activity was found. Treated with curettage, the specimen revealed plexiform proliferation of mononuclear histiocyte-like cells, multinucleated osteoclast-like cells, and spindle fibroblast-like cells in variable proportions histopathologically. Immunohistochemical stains were positive for CD68 in scattered fashion in histiocytes and giant cells, and spindle like cells showed positivity for smooth muscle actin. Under electron microscopy, rough endoplasmic reticulum and collagen bundles in the spindle cells suggested fibroblastic differentiation. Also multiple large electron-dense lysosomal granules in histiocytoid cells were found. Multinucleated giant cells exhibited osteoclast-like appearance. All these findings suggested plexiform fibrohistiocytic tumor. Interestingly, the tumor was localized in bone. During the follow up for 27 months after the resection, there was no recurrence or metastasis.

Mitotically active plexiform fibrohistiocytic tumor

Plexiform fibrohistiocytic tumor is an intermediate malignant tumor situated in superficial soft tissues. It affects children and young adults. The tumor is most commonly located on upper extremities, whereas involvement of back region is rare. Mitotic activity is generally low (~3/10 HPF). It is rare, but it can exhibit aggressive behavior, so total excision with clear surgical margins and long-term followup is necessary to detect local recurrence and metastases. We report a child with a solid mass on back region which was found to be a mitotically active plexiform fibrohistiocytic tumor (6/10 HPF) after excision. Plexiform fibrohistiocytic tumor (PFT) is a mesenchymal neoplasm of children, adolescents, and young adults. It is characterized by fibrohistiocytic cytomorphology and multinodular growth pattern. Clinically it is usually a slow-growing mass of upper extremities with frequent local recurrence and rare regional lymphatic and systemic metastasis (Fletcher et al. (2002), Enzinger and Zhang (1988), Remstein et al. (1999)).