What is the recommended treatment for periocular squamous cell carcinoma (SCC) in equines?

Treatment of Periocular Squamous Cell Carcinoma in Equines

Wide surgical excision with 4-6 mm margins is the recommended first-line treatment for periocular squamous cell carcinoma (SCC) in equines, with adjunctive photodynamic therapy significantly reducing recurrence rates compared to other modalities.

Diagnostic Approach

• Biopsy is essential for confirming diagnosis before treatment
• Histopathologic evaluation should assess:
  • Degree of cellular differentiation
  • Depth of invasion
  • Presence of perineural invasion
  • Presence of lymphovascular invasion
  • Margin status

Treatment Options

Surgical Management (Primary Recommendation)

• Wide local excision with 4-6 mm margins is the standard treatment for equine periocular SCC 1
  • Ideally achieve 2 cm margins when anatomically possible 1
  • Depth should extend to mid-subcutaneous adipose tissue
  • Complete histologic margin assessment is critical

Adjunctive Therapies

1. Photodynamic Therapy (PDT)

   • Most effective adjunctive treatment with significantly lower recurrence rates
   • In comparative studies, PDT following surgical excision resulted in no recurrences during 25-50 month follow-up periods, while cryotherapy had 11/14 recurrences (median time: 10 months) 2
   • Can be performed using verteporfin or HPPH (2-[1-hexyloxyethyl]-2-devinylpyropheophorbide-a) 3, 2

2. Other Adjunctive Options (if PDT unavailable):

   • Cryotherapy - higher recurrence rates than PDT 2
   • Mitomycin C (0.4 mg/mL applied intraoperatively) - 90% of treated eyes appeared tumor-free at 11+ months 4
   • Radiofrequency hyperthermia - 80% complete regression, but not recommended for tumors >5cm with deep penetration 5

Treatment Algorithm Based on Tumor Characteristics

Low-Risk Periocular SCC

• First-line: Wide local excision (4-6 mm margins) with complete histologic margin assessment
• Adjunctive: PDT following surgical excision

High-Risk Periocular SCC (large size, poor differentiation, recurrent)

• First-line: Wide local excision with more extensive margins (aim for 2 cm when possible)
• Adjunctive: PDT is strongly recommended to prevent recurrence
• Consider Mohs micrographic surgery or equivalent technique if available for complete margin control 1

Recurrent Periocular SCC

• Re-excision with wider margins
• Mandatory adjunctive PDT
• Consider advanced imaging to rule out perineural spread 6

Post-Treatment Follow-Up

• Regular ophthalmic examinations to monitor for recurrence
• More frequent monitoring during first year (highest risk period for recurrence)
• Long-term follow-up is essential as recurrence can occur years after initial treatment

Important Considerations

• Perineural invasion significantly increases recurrence risk and mortality 6
• Complete surgical excision is critical - incomplete margins dramatically increase recurrence rates
• Functional and cosmetic outcomes must be balanced with oncologic principles
• For tumors with deep invasion or orbital involvement, more radical surgery may be necessary

Pitfalls to Avoid

• Inadequate surgical margins leading to recurrence
• Failure to perform histopathologic margin assessment
• Underestimating the aggressive nature of periocular SCC
• Delaying adjunctive therapy after surgical excision

Surgical excision with adjunctive PDT offers the best outcomes for equine periocular SCC based on current evidence, with significantly lower recurrence rates compared to other treatment modalities.