Similar to MF originating in the head and neck region, the upper limb as a primary tumor site initially was found to be a significant predictor of both OS and DSS on univariate analysis but not on multivariate analysis. By contrast, Su et al18 found survival outcomes were worse for patients diagnosed with MF with the upper limb as the primary tumor site compared with the lower limb on multivariate Cox proportional hazards analysis but not on pairwise propensity score matching. The difference in our results compared with Su et al18 may be because the National Cancer Database only reports OS, while DSS may be more useful in determining prognostic factors associated with poorer survival, especially in an older patient population with greater comorbidities. Furthermore, the nonsignificance of the upper limb as a primary tumor site on further multivariate analysis may be due to similar reasonings as for the head and neck, including more advanced T staging and an anatomic location close to lymph nodes.
Study Limitations—The SEER database is a national registry, which lends itself to potential data heterogeneity in recording and miscoding. Additionally, there may be higher rates of unconfirmed or missing information given the retrospective nature of the SEER database; the database also does not delineate facility type, insurance status, or Charlson/Deyo comorbidity index as demographic factors, which could influence the multivariable analysis. Finally, the SEER database does not further demarcate subtypes of MF, such as the aggressive folliculotropic variant commonly seen in head and neck MF lesions, which precludes independent analysis of disease course by subtype.
Conclusion
Our study evaluated primary disease site as a prognostic factor for OS and DSS in patients with MF. Although head and neck and upper limb as primary disease sites were found to be significant on univariate analysis, they were found to be an insignificant prognostic variable for OS or DSS in our multivariable analysis, potentially due to the aggressive nature of advanced-stage MF and localization close to lymph nodes. Further research including a deeper dive into MF of all stages and subtypes is needed to fully investigate primary disease site as a prognostic indicator. Older age, larger tumor size, a higher T stage, lymph node involvement, and presence of metastasis were associated with worse DSS, and patients with these attributes should be counseled regarding expected disease course and prognosis.