Should a superficially invasive squamous cell carcinoma (SCC) arising in an actinic keratosis be treated with Mohs surgery?

Management of Superficially Invasive Squamous Cell Carcinoma Arising in Actinic Keratosis

Mohs micrographic surgery is strongly recommended for superficially invasive squamous cell carcinoma (SCC) extending to the base of the specimen, even when arising from actinic keratosis, as it provides the highest cure rates and most thorough margin assessment for this high-risk tumor. 1

Risk Assessment and Classification

Superficially invasive SCC extending to the base of the specimen represents a high-risk tumor for several reasons:

• Extension to the specimen base indicates possible deeper invasion
• Incomplete margin assessment with conventional techniques may miss residual tumor
• Actinic keratosis-derived SCCs can be deceptively aggressive 2, 3

Risk factors to consider:

• Extension to specimen base (positive deep margin)
• Inability to determine full depth of invasion
• Potential for clinically invisible tumor extension

Treatment Algorithm

1. First-line treatment: Mohs micrographic surgery

   • Provides complete circumferential peripheral and deep margin assessment (CCPDMA)
   • Allows for highest cure rates while preserving normal tissue 1
   • Particularly valuable when tumor borders are difficult to define clinically 1

2. Alternative if Mohs is unavailable:

   • Excision with intraoperative frozen section assessment that includes complete assessment of all deep and peripheral margins 1
   • Clinical margins of 4-6 mm for standard excision are insufficient when deep margin involvement is present 1

3. Not recommended for this presentation:

   • Curettage and electrodesiccation (inappropriate for tumors extending to specimen base)
   • Standard excision without complete margin assessment
   • Cryotherapy or topical treatments

Evidence Supporting Mohs Surgery

The British Journal of Dermatology guidelines note that "the best cure rates for high-risk SCCs are reported in series treated by Mohs' micrographic surgery" 1. This is particularly relevant when:

• Tumor extends to specimen base (as in this case)
• Margins are difficult to assess clinically
• Complete histological clearance is essential

The NCCN guidelines specifically state that "Mohs surgery is preferred because of its documented efficacy" for high-risk tumors, and emphasize that "intraoperative assessment of all tissue margins is the key to complete tumor removal" 1.

A study of high-risk SCCs treated with Mohs surgery showed only a 1.2% local recurrence rate and 2.3% metastasis rate, demonstrating excellent outcomes even for challenging cases 4.

Important Considerations

• Specimen orientation: When performing any excision, proper orientation with marker sutures is essential to allow accurate identification of involved margins 1

• Histopathological assessment: The pathology report should comment on:

  • Tumor thickness and differentiation
  • Presence of vascular or neural invasion
  • Distance to closest margin 1

• Risk of underdiagnosis: Studies show that 9.8% of transected SCC in situ referred for Mohs surgery harbor invasive components not detected on initial biopsy 5, highlighting the importance of complete margin assessment

• Follow-up surveillance: Patients with one high-risk SCC have a 75% chance of developing another cutaneous SCC and 7.7% risk of developing melanoma 4, necessitating close follow-up

Pitfalls to Avoid

1. Underestimating tumor extent: Conventional "breadloaf" histological techniques examine only a small sample of the specimen and may miss residual tumor at margins 1

2. Inadequate margin assessment: When tumor extends to specimen base, complete margin assessment is essential to ensure complete removal 1

3. Inappropriate treatment selection: Curettage and electrodesiccation should not be used when tumor extends to subcutaneous tissue or specimen base 1

4. Delayed reconstruction: When doubt exists about margin clearance, it is advisable to delay definitive reconstruction until complete tumor removal is confirmed 1

By following these guidelines and selecting Mohs micrographic surgery for superficially invasive SCC extending to the specimen base, you can provide the patient with the highest probability of complete tumor removal while minimizing recurrence risk and preserving normal tissue.