Unmasking the Supraclavicular Chameleon: A Case Report on the Diagnostic Evolution of a Recurrent Myxoid Mesenchymal Neoplasm

Background: Mesenchymal neoplasms exhibiting myxoid characteristics, including inflammatory myofibroblastic tumours (IMTs) and myxofibrosarcomas, present complex diagnostic and surgical challenges. These difficulties are particularly pronounced when lesions recur within anatomically complex sites, such as the cervicothoracic junction. Case Presentation: This report details the clinical management of an older woman presenting with a painful, recurrent, large left supraclavicular mass, following two previous surgical resections. Contrast-enhanced computed tomography (CECT) demonstrated a large soft-tissue lesion closely abutting the carotid sheath and pectoralis musculature. Although an initial core needle biopsy suggested a solitary fibrous tumour (SFT) based on patchy CD34 reactivity, the lesion’s extensive size and impending epidermal ulceration required wide local excision alongside reconstruction via a pectoralis major myocutaneous (PMMC) flap. Subsequent postoperative complications, specifically a haematoma and partial flap loss, were effectively resolved using negative pressure wound therapy (NPWT). Definitive histopathological and immunohistochemical (IHC) analysis of the resected specimen revealed a spindle cell proliferation set within a myxoid matrix and accompanied by a marked inflammatory infiltrate. Positive immunoreactivity for vimentin and CD68, coupled with CD34 negativity, prompted a revised diagnosis of a myxoid mesenchymal neoplasm, favouring an IMT or myxofibrosarcoma. Conclusion: This case emphasises the diagnostic constraints of core needle biopsies in heterogeneous soft tissue masses and highlights the intricate surgical and reconstructive strategies needed for massive tumours located at the root of the neck.