Mycosis Fungoides — SCE Dermatology MCQ

Correct answer: C — Contact allergic dermatitis (occurs in up to 50% of patients)

The correct answer is C, contact allergic dermatitis (occurs in up to 50% of patients). Topical chlormethine (mechlorethamine) is an alkylating agent used as skin directed therapy for early stage mycosis fungoides, and its major limitation is a delayed type hypersensitivity contact dermatitis at application sites, which can occur in a substantial proportion of patients and often necessitates dose reduction, intermittent dosing, or co-treatment with topical corticosteroid. This reaction is the dominant, dose-limiting toxicity described in the SmPC and clinical literature for both the aqueous and gel formulations, distinguishing chlormethine from systemic chemotherapeutic agents whose main concerns are organ-specific toxicities. Because chlormethine is applied topically with minimal systemic absorption, the clinically relevant adverse effect is local (cutaneous), not systemic. Why the other options are wrong: D. Pulmonary fibrosis: This is a recognised toxicity of systemic alkylating and cytotoxic agents such as bleomycin or busulfan, not of topically applied chlormethine, which has negligible systemic absorption. E. Cardiac arrhythmias: This is not a described effect of chlormethine in any formulation; it is not cardiotoxic when used topically. B. Systemic immunosuppression: Because topical chlormethine is not significantly absorbed systemically, it does not cause clinically meaningful systemic immunosuppression, unlike systemic chemotherapy or extensive PUVA regimens. A. Renal toxicity: Renal impairment is associated with systemic alkylating agents given intravenously, not with a topical formulation that acts locally on skin lesions. Key point: The main practical limitation of topical chlormethine therapy is local contact allergic dermatitis, not systemic organ toxicity, because absorption through skin is minimal.

Reference: electronic Medicines Compendium (emc), Ledaga 160 micrograms/g gel Summary of Product Characteristics, sections 4.4 and 4.8, https://www.medicines.org.uk/emc/product/10384/smpc