Discordance of pathological thin melanoma thickness and T stage in SEER registry: impacts on clinical management and research directions

Background Ultrathin melanoma was previously demonstrated to have higher risk for melanoma-specific mortality using SEER database. However, these guideline-changing conclusions has been recently challenged by miscoding of thickness. This present study was performed to assess the prognosis of thin and ultrathin melanoma using only surgically-treated, pathologically confirmed and after removal of discordant cases. Methods Melanoma patients from SEER database who were initially diagnosed with histologically confirmed and surgically treated melanoma from 1998 to 2012 were included. Subjects with discordance between T stage and tumor thickness were excluded. Kaplan-Meier curves, log-rank test and multivariate Cox proportional hazards regression models were used. Results 55,754 patients met the strict inclusion criteria, but 16 (0.02%) and 803 (1.4%) patients were removed due to T0 stage and discordance between T stage and thickness, respectively. Therefore, 54,935 patients entered the analyses, among which 52,751 were LN negative and 2,184 were LN positive. In either overall or LN-negative patients, a straightforward dose-effect relationship of larger thickness with increasing mortality was observed. In contrast, in LN positive patients, the T1 subgroup demonstrated a similar survival with tumors in T2 mm subgroup. Multivariable analysis revealed same pattern, and significant interaction between T stage and LN involvement was found. Further categorizing T1 melanoma into 10 equal 0.10 mm increments demonstrated an unexpected "N"-shaped pattern of mortality in overall and LN negative ultrathin melanoma but not in LN positive melanoma. Conclusions No difference in mortality was observed in T1-3 tumors with LN involvement. External and independent validation studies are warranted.