Is it just a scar? A case report of a diagnostic and management challenge of desmoplastic melanoma

A 73-year-old female presented with a progressively enlarging pink nodule on the left anterior upper arm resembling a keloidal scar with accelerated growth and pain in recent months. Clinically, the lesion resembled a keloid scar. Ultrasound imaging revealed a well-demarcated, hypoechoic lesion located within the dermis and extending into the subcutaneous tissue. An excision biopsy confirmed the diagnosis of mixed desmoplastic melanoma, characterised by a spindle cell morphology and significant stromal desmoplasia, with a Breslow thickness of 8.1 mm. Notably, perineural invasion was identified, a recognised adverse prognostic feature associated with this subtype. Immunohistochemical analysis showed strong positivity for SOX-10 (SRY-box transcription factor 10) and minimal staining for HMB-45 (Human Melanoma Black), thereby supporting the diagnosis. Molecular genetic analysis did not reveal pathogenic variants in BRAF, NRAS, or KIT, thereby limiting options for targeted therapy. Staging imaging studies, including positron emission tomography–computed tomography (PET-CT) and Magnetic resonance imaging (MRI) scan of the brain, did not detect any metastatic disease. The wide local excision and sentinel lymph node biopsy were negative for residual disease, local spread, and regional nodal involvement. 
This case exemplifies the diagnostic challenges posed by desmoplastic melanoma, an uncommon and often amelanotic subtype that can clinically mimic benign scar-like lesions. Such similarities may contribute to delayed diagnosis and result in an advanced Breslow thickness upon presentation. Histopathological evaluation remains paramount in establishing an accurate diagnosis, particularly given the overlap in clinical features with benign conditions. The absence of actionable molecular mutations underscores the importance of surgical intervention and the necessity of a multidisciplinary clinical approach.