3665 - Impact of Depth of Invasion on Locoregional Failure in RTOG 0920-Eligible Early-Stage Oral Tongue Squamous Cell Carcinoma and Implications for Adjuvant Therapy

S. Iganej¹, J. Abrahams², M. Modzelewski², A. Beighley³, J. Chen¹, A. Larson⁴, A. Lin⁴, P. D. G. Kim⁵, M. Zhi¹, and O. Bhattasali¹; ¹Southern California Permanente Medical Group, Los Angeles, CA, ²Kaiser Permanente School of Medicine, Pasadena, CA, ³Kaiser Permanente Los Angeles Medical Center, Los Angeles, CA, ⁴Southern California Permanente Group, Los Angeles, CA, ⁵Southern California Permanente Medical Group, Fontana, CA

Purpose/Objective(s): RTOG 0920, which compared postoperative radiotherapy (RT) with or without Cetuximab in patients with intermediate-risk disease, included patients with AJCC 7^th edition cT2N0/pT2N0-X oral tongue squamous cell carcinoma (OTSCC) with at least one of the following adverse pathologic features: depth of invasion (DOI) >5mm, margins within 5mm, lymphovascular invasion (LVI) or perineural invasion (PNI). Some of these patients would now be categorized as having pT3 disease in the AJCC 8^th edition if the DOI exceeded 10mm. We sought to assess the outcomes of all patients with AJCC 7^th edition cT2N0/pT2N0-X disease with or without adjuvant radiotherapy regardless of DOI, so long as they would have qualified for enrollment on RTOG 0920. Materials/

Methods: A retrospective search of patients with OTSCC diagnosed from January 2015 through December 2021 rendered 52 patients with cT2N0/pT2N0-X AJCC 7^th edition disease who were RTOG 0920 eligible. Median age was 68 (range: 31-90). All patients underwent surgery with final negative margins, and 43 (83%) also had elective neck dissections with negative lymph nodes. Forty-nine (94%) patients had DOI >5mm, 22 (42%) of whom also had DOI>10mm. Twenty-seven (52%) patients had PNI, 7 (13%) had LVI, and 6 (12%) had margins within 5mm; 29 (56%) patients had multiple adverse pathologic features. Twenty-one (40%) patients underwent adjuvant RT, two of whom also received concurrent systemic therapy, one getting weekly cisplatin and the other Cetuximab. The remaining 32 (60%) did not have any form of adjuvant therapy
Results: For the entire cohort, with a median follow up 37 months (range: 16-76) for the surviving patients, 3-year locoregional control (LRC) and overall survival rates were 79% and 84%, respectively. Of all the adverse pathologic factors, only DOI>10mm was predictive of inferior LRC (3-year: 70% vs. 93%, p=0.037). Furthermore, adjuvant RT improved LRC only in those with DOI>10mm (3-year: 91% vs. 47%, p=0.035). Amongst the 30 patients with DOI <10mm, 10 underwent adjuvant RT with 3-year LRC of 90% with and 95% without adjuvant RT (P=0.63); within this subgroup, 12 patients had multiple adverse pathologic factors, none of whom had a locoregional recurrence (LRR), with (N=6) or without (N=6) adjuvant RT.
Conclusion: This study suggests that the rate of LRR is relatively low in RTOG 0920-eligible patients with early stage OTSCC measuring >2cm but <4cm with DOI <10mm (AJCC 8^th edition stage pT2N0 disease) and postoperative RT may not be warranted. However, DOI >10mm (AJCC 8^th edition pT3 disease) in these patients predicts for a high rate of LRR which is mitigated by adjuvant radiotherapy. We do, nonetheless, realize the limitations of a small retrospective study and believe additional research is necessary to confirm our findings, particularly in those with multiple pathologic risk factors.