A 72-year-old woman with a history of hypertension and well-controlled diabetes presents for Mohs micrographic surgery to treat a recurrent infiltrative basal cell carcinoma (BCC) on the medial canthus. On examination, the lesion is 1.1 cm in diameter, with ill-defined borders.

During the second Mohs stage, histology reveals tumor extension along a small nerve, approximately 0.12 mm in diameter, approaching the junction with the infraorbital nerve. The patient remains asymptomatic.

Which of the following is the most appropriate next step in management?

A. Complete Mohs surgery with clear margins and schedule regular follow-up

B. Refer to radiation oncology for adjuvant therapy after Mohs clearance

C. Convert to wide local excision with frozen sections

D. Halt Mohs surgery and obtain a high-resolution MRI prior to further excision

E. Initiate systemic hedgehog pathway inhibitor therapy

Rationale:

This question addresses perineural invasion (PNI), an important risk factor for recurrence and aggressive behavior in cutaneous malignancies, especially in high-risk locations like the medial canthus. PNI involving a nerve >0.1 mm in diameter is considered clinically significant, even in asymptomatic patients.

Correct answer: B. Refer to radiation oncology for adjuvant therapy after Mohs clearance

For clinically significant PNI (>0.1 mm) in high-risk sites, such as the face, especially the medial canthus, adjuvant radiation therapy is recommended after Mohs clearance. Radiation treatment improves local control and reduces recurrence rates.

Incorrect answer choices:

A. Complete Mohs surgery with clear margins and schedule regular follow-up.

While appropriate for superficial or incidental PNI involving nerves <0.1 mm, this patient has clinically significant PNI (>0.1 mm). Observation alone is not sufficient in this high-risk anatomical location.

C. Convert to wide local excision with frozen sections

Mohs surgery is superior to standard excision in terms of margin control and preservation of function and cosmesis, especially in complex facial areas. Converting to wide local excision would not be helpful in this situation. 

D. Halt Mohs surgery and obtain a high-resolution MRI prior to further excision

MRI may be considered in symptomatic patients or when large-caliber nerves (e.g., facial or trigeminal) are involved, or if clinical concern exists for deeper spread. In this asymptomatic patient, continuing with Mohs micrographic surgery until clearance then considering adjuvant therapy is the most appropriate choice. 

E. Initiate systemic hedgehog pathway inhibitor therapy

Hedgehog inhibitors (e.g., vismodegib) are reserved for locally advanced or metastatic BCCs that are not amenable to surgery or radiation. This would not be a first line treatment in a surgically resectable tumor with localized PNI.

Additional reading at Fitzpatrick's Dermatology Chapter 200: Radiotherapy