Depth of invasion alone as an indication for postoperative radiotherapy in small oral squamous cell carcinomas: An International Collaborative Study

Background: We aimed to investigate whether depth of invasion (DOI) should be an independent indication for postoperative radiotherapy (PORT) in small oral squamous cell carcinomas (SCC). Methods: Retrospective analysis of DOI (<5, 5 to <10, ≥10 mm) and disease-specific survival (DSS) in a multi-institutional international cohort of 1409 patients with oral SCC ≤4 cm in size treated between 1990-2011. Results: In patients without other adverse factors (nodal metastases; close [<5 mm] or involved margins), there was no association between DOI and DSS, with an excellent prognosis irrespective of depth. In the absence of PORT, the 5-year disease-specific mortality was 10% with DOI ≥10 mm, 8% with DOI 5-10 mm, and 6% with DOI <5 mm (P =.169), yielding an absolute risk difference of only 4%. Conclusion: The deterioration in prognosis with increasing DOI largely reflects an association with other adverse features. In the absence of these, depth alone should not be an indication for PORT outside a clinical trial.