Interstitial Granulomatous Dermatitis as a Rare Paraneoplastic Manifestation of Non-small Cell Lung Cancer: A Case Report

Interstitial granulomatous dermatitis (IGD) is an uncommon inflammatory skin disorder, characterised histologically by mid‑dermal non‑caseating granulomas. While often linked to autoimmune diseases and drug reactions, its occurrence as a paraneoplastic phenomenon is rare. We report a case of a 69‑year‑old male with a two‑year history of a progressive erythematous maculopapular rash involving the trunk and extremities, associated with fatigue, weight loss, and myalgia. Initial superficial biopsy suggested granuloma annulare, but a deeper biopsy confirmed IGD, with a preserved Grentz zone and multinucleate giant cells. Laboratory tests revealed elevated ESR, as well as CRP, hypercalcaemia, and IgM paraproteinaemia. Imaging identified a PET‑avid, cavitating left lower lobe lung nodule. CT‑guided biopsy was non‑diagnostic, prompting wedge resection. Histology confirmed non‑small cell lung cancer, squamous subtype. Notably, the rash markedly improved following lung resection. IGD can be a cutaneous marker of occult malignancy. Persistent granulomatous skin eruptions unresponsive to standard therapies should prompt consideration of systemic disease, including neoplasia. This case underscores the value of a multidisciplinary diagnostic approach and highlights the potential for symptom resolution with targeted treatment of the underlying tumour.