Classification of aerodigestive tract invasion from thyroid cancer

Widely invasive extrathyroidal thyroid cancer invading the aerodigestive tract (ADT) including larynx, trachea, hypopharynx, and/or esophagus occurs in 1-8 % of patients with thyroid cancer and is classified as T4a (current UICC/AJCC system). The T4a stage is associated with impaired tumor-free survival and increased disease-specific mortality. Concerning prognosis and outcome, further subdivisions of the T4a stage, however, have not been made so far. This study is based on a systematic review of the relevant literature in the PubMed database. Retrospective studies suggest a better outcome in patients with invasion of the trachea or the esophagus when compared to laryngeal invasion. Regarding surgical strategies, ADT invasion can be classified based on a three-dimensional assessment determining surgical resection options. Regardless of the invaded structure, tumor infiltration of the ADT can be subdivided into superficial, deep extraluminal, and intraluminal invasion. In contrast to superficial ADT invasion, allowing tangential incomplete wall resection (shaving/extramucosal esophagus resection), deeper wall and intraluminal invasions require complete wall resection (either window or sleeve). Based on the Dralle classification (types 1-6), particularly airway invasion, can be further classified according to the vertical and horizontal extents of tumor invasion. The Dralle classification can be considered as a reliable subdivision system evaluated regarding surgical options as well as oncological outcome. However, further studies determining the prognostic impact of this technically oriented classification system are required.