Cutaneous Oncology and Tumor Biology
Core concepts
Candidates must know the biology of the tumors Mohs is used for and, equally, the tumors for which it is inappropriate. Basal cell carcinoma subtypes matter: nodular and superficial tumors behave indolently, whereas morpheaform, infiltrative, and basosquamous variants show subclinical extension and higher recurrence after standard excision. Squamous cell carcinoma risk stratifies on differentiation, depth, perineural invasion, and immunosuppression. Melanoma in situ and lentigo maligna are treated with Mohs or staged excision using melanoma antigen immunohistochemistry (MART-1/Melan-A, SOX10).
Field cancerization and margins
Understand that actinically damaged skin harbors genetically altered clones; this drives multifocal recurrence and explains why margin control, not just gross clearance, defines cure. Know the distinction between recurrent and primary tumors: recurrent lesions have distorted anatomy and discontinuous tumor islands, raising the value of Mohs.
Traps
Questions conflate "margin negative on frozen section" with "cured" - residual risk depends on tumor biology. Another trap: assuming all SCCs need Mohs; low-risk SCC on the trunk is often adequately treated by excision. Perineural invasion of large-caliber nerves (named nerves) warrants imaging and multidisciplinary care, not just local excision.
Rule of thumb
For BCC, ask: is it on the face (H-zone), recurrent, large (>2 cm), ill-defined, or an aggressive subtype? Any yes favors Mohs. Example: a 1.5 cm infiltrative BCC on the nasal ala in a 40-year-old - Mohs is preferred because tissue conservation and margin control both matter.