A 68-year-old man with a history of chronic sun exposure presents with a 2 cm indurated nodule on the dorsum of his hand with central ulceration and raised, rolled edges. Biopsy confirms well-differentiated squamous cell carcinoma (SCC) of the skin. The lesion is localized without evidence of regional lymph node involvement or distant metastasis. What is the drug of choice for systemic treatment if surgery is contraindicated or refused?