Rare Variants of Dermatofibrosarcoma Protuberans: Clinical, Histologic, and Molecular Features and Diagnostic Pitfalls

Fibrosarcomatous Dermatofibrosarcoma Protuberans of the Head

This article highlights the case of a 37-year-old male who presented with a recurrent, exponentially enlarging head mass, emphasizing the diagnostic and therapeutic challenges associated with a very rare type of tumor, fibrosarcomatous dermatofibrosarcoma protuberans (DFSP) of the head. Our patient presented with a rapidly growing head mass, initially diagnosed as a spindle cell tumor, and was managed with surgical excision and skin flap grafting. Follow-up revealed relapse and interval development of hemiparesis and hemisensory loss. MRI revealed tumor recurrence, with compression of the right parietal lobe and superior sagittal sinus. Histopathology revealed stroma with fascicles of spindle cells homogenous to fibrillar cytoplasm, with oval vesicular nuclei. Immunohistochemical staining showed positivity for CD34 and SMA. Oral chemotherapy with imatinib 800 mg/day was started. Follow-up imaging showed a marked reduction in the size of the tumor and resolution of the compression of the underlying brain parenchyma with cystic degeneration and decreased contrast enhancement. Future plans include possible surgical tumor debulking and/or radiation therapy. Although extremely rare, awareness of this tumor, with a multi-disciplinary approach to the management of the case, is vital to maximize treatment outcomes.

Spindle cell neoplasms with novel LTK fusion - Expanding the spectrum of kinase fusion-positive soft tissue tumors

AIMS

Kinase fusion-positive soft tissue tumors represent an emerging, molecularly defined group of mesenchymal tumors with a wide morphologic spectrum and diverse activating kinases. Here, we present two cases of soft tissue tumors with novel LTK fusions.

METHODS AND RESULTS

Both cases presented as acral skin nodules (big toe and middle finger) in pediatric patients (17-year-old girl and 2-year-old boy). The tumors measured 2 and 3 cm in greatest dimension. Histologically, both cases exhibited bland-looking spindle cells infiltrating adipose tissue and accompanied by collagenous stroma. One case additionally displayed perivascular hyalinization and band-like stromal collagen. Both cases exhibited focal S100 staining, and one case had patchy coexpression of CD34. Targeted RNA-seq revealed the presence of novel in-frame MYH9::LTK and MYH10::LTK fusions, resulting in upregulation of LTK expression. Of interest, DNA methylation-based unsupervised clustering analysis in one case showed that the tumor clustered with dermatofibrosarcoma protuberans (DFSP). One tumor was excised with amputation with no local recurrence or distant metastasis at 18-month follow-up. The other case was initially marginally excised with local recurrence after one year, followed by wide local excision, with no evidence of disease at 10 years of follow-up.

CONCLUSIONS

This is the first reported case series of soft tissue tumors harboring LTK fusion, expanding the molecular landscape of soft tissue tumors driven by activating kinase fusions. Furthermore, studies involving a larger number of cases and integrated genomic analyses will be warranted to fully elucidate the pathogenesis and classification of these tumors.

Superficial CD34-Positive Fibroblastic Tumor: A Newly Recognized Entity With Overlapping Features of PRDM10-Rearranged Soft Tissue Tumors

Superficial CD34-positive fibroblastic tumor (SCPFT) is a recently identified, infrequent, low-grade mesenchymal neoplasm, first identified in 2014. Although it is relatively new to the field, SCPFT has been gaining prominence in surgical pathology practice because of its distinctive features. As of now, there are limited reported cases of SCPFT, with fewer than 100 instances documented in scientific literature. This distinctive blend of rarity and intriguing variability in presentation emphasizes the significance of identifying and understanding this uncommon entity, facilitating precise diagnosis and optimal management. In this article, we aimed to present a notable case of SCPFT in a male in his 20s who presented with a distinct subcutaneous mass measuring 2.4 × 1.8 cm at the medial aspect of the knee joint. The patient reported no significant medical history or trauma to the affected area. MRI of the knee showed a well-defined 2.4 × 1.8 cm subcutaneous mass with no definite communication with the underlying ligament or meniscus. The histopathological examination revealed spindle cell neoplasm arranged in intersecting fascicles, accompanied by arborizing blood vessels. Neoplastic spindle cells exhibited marked nuclear pleomorphism, and abundant and eosinophilic cytoplasm, with focal areas of granular, glassy, and lipidized cytoplasm. Nuclear pseudo inclusions and a few mitotic figures (1-2 per high-power field) were noted. Inflammatory infiltrates were identified within the neoplasm, comprising eosinophils and lymphocytes, highlighting an immune response within the tumor microenvironment. The surgical margin exhibited involvement with the tumor infiltrates, with the neoplastic cells extending into the adjacent fat tissue. This finding indicates local tumor spread and potential challenges in achieving complete resection. Immunohistochemical staining showed positive staining for CD34, corroborating the diagnosis of a CD34-positive fibroblastic tumor. Focal positive staining for pan-CK was noted. Staining for CD31, smooth muscle actin (SMA), desmin, S100, and anaplastic lymphoma kinase (ALK) was negative, supporting the diagnosis. The Ki67 proliferation index was low.