ALK-rearranged CD34 positive superficial spindle cell tumour

Anaplastic lymphoma kinase (ALK) rearranged spindle cell mesenchymal tumours are an emerging category of soft tissue neoplasm with variable clinical and pathological features.1–3 We describe a case of a 6-year-old female who presented with a pigmented lesion on her abdomen. Biopsy of the lesion showed a low-grade dermally based spindle cell lesion with prominent storiform architecture, some entrapped collagen and focal extension into the subcutis. There was no mitosis or anaplasia. On immunohistochemistry, tumour cells were CD34+ and ALK+ and showed no staining for S100, smooth muscle actin (SMA) or epithelial membrane antigen (EMA). RNA sequencing performed on the biopsy detected a 5’ to 3’ in-frame fusion of LRRFIP1 exon 17 to ALK exon 20. The findings of a CD34+/ALK+ bland cutaneous spindle cell lesion with a confirmed LRRFIP1-ALK fusion was consistent with an ALK-rearranged superficial spindle cell mesenchymal tumour (aka superficial ALK-rearranged myxoid spindle cell neoplasm). There was no recurrence following complete excision of the lesion. The case emphasises the importance of awareness of this evolving entity with its unique morphologic, immunophenotypic and molecular cytogenetic features and potential for targeted therapy. References 1. Dermawan JK, Azzato EM, Goldblum JR, Rubin BP, Billings SD, Ko JS. Superficial ALK-rearranged myxoid spindle cell neoplasm: a cutaneous soft tissue tumor with distinctive morphology and immunophenotypic profile. Mod Pathol 2021; 34: 1710–1718. 2. Dermawan JK, DiNapoli SE, Mullaney KA, et al. ALK-rearranged mesenchymal neoplasms: a report of 9 cases further expanding the clinicopathologic spectrum of emerging kinase fusion positive group of tumors. Genes Chromosomes Cancer 2023; 62: 75–84. 3. Tan SY, Al-Ibraheemi A, Ahrens WA, et al. ALK rearrangements in infantile fibrosarcoma-like spindle cell tumours of soft tissue and kidney. Histopathology 2022; 80: 698–707.