TNM Classification: Are Cancers With Extrinsic Tongue Muscle Involvement Systematically T4a?

In the Union for International Cancer Control classification, extrinsic muscle involvement in oropharyngeal cancers is systematically defined as a T4a tumor, although the term extrinsic may incorrectly imply that these muscles are outside the tongue. Our aim was to describe the topography of extrinsic tongue muscles and show that their involvement in oropharyngeal cancers does not always correspond to T4a staging.

This cadaveric study was based on dissections of the tongues of ten healthy subjects. Dissections were carried out using sections, and careful macroscopic examination with a 3.5-diopter magnifying glass allowed the identification of muscles from their origin to their termination. Imagery and histology were excluded to stay as close as possible to the clinical evaluation.

The sample comprised seven men and three women, with a mean age 82 years. In all the ten cases, the extrinsic muscles were located at the periphery of the tongue. The genioglossus was the only deep extrinsic muscle, extending to the periphery and under the mucous membrane of the tongue. As a result, a T1 tumor can invade the mucous membrane and affect the extrinsic muscles without transforming into a T4a tumor.

As extrinsic tongue muscles are peripheral and submucosal, the use of the term "deep" is erroneous, as is the systematic usage of extrinsic muscles to define T4a oropharyngeal cancers. This study attempts to explain this error, which is recurrent in the different editions of the Union for International Cancer Control and American Joint Committee on Cancer.