<p><b>PURPOSE: </b>The seventh edition of tumor staging by the American Joint Committee on Cancer (AJCC) includes extrinsic muscle involvement to define stage T4a tongue carcinomas. The anatomic location of extrinsic muscles predisposes them to early involvement even in superficial tumors. The purpose of this study was to expose a fallacy in this staging system for extrinsic muscle involvement.</p><p><b>MATERIALS AND METHODS: </b>This was a prospective cohort study of 87 patients with oral tongue squamous cell carcinoma. Magnetic resonance imaging (MRI) parameters were 1) the distance of the extrinsic muscles from the surface measured on the normal side in millimeters (range, mean, and standard deviation); 2) maximum transverse, craniocaudal, and anteroposterior tumor dimensions (range, mean, and standard deviation); and 3) tumor involvement of the muscles recorded on the involved side for the number and percentage of each muscle involved. Histopathologic depth of invasion also was recorded.</p><p><b>RESULTS: </b>Sixty-seven patients were men and 20 were women (age range, 18 to 74 yr; mean age, 51 yr). The mean distances of the most superficial part of the muscle to the normal surface at MRI for the genioglossus (anteroventral), hyoglossus, and styloglossus were 3.98, 2.13 and 0.66 mm, respectively. The patterns of extrinsic muscle involvement showed hyoglossus, styloglossus, and genioglossus involvement in 79 (90.8%), 58 (66.76%), and 31 (35.6%), respectively. In patients with a pathologic depth of invasion shallower than 10 mm, involvement of the hyoglossus, styloglossus, and genioglossus was seen in 80, 35, and 15%, respectively.</p><p><b>CONCLUSION: </b>The extrinsic muscles of the tongue are not deep. Even superficial thin tumors can involve these muscles. The eighth edition of tumor staging by the AJCC, which includes tumor thickness in the staging system, is in the process of being implemented. The present study justifies the removal of extrinsic muscle involvement in defining stage T4 of the oral cavity.</p>
S. P. Murthy, Thankappan, K., Jayasankaran, S. Chirukanda, Milind, K., Prasad, C., Balasubramanian, D., and Iyer, S., “"Deep Extrinsic Muscle Involvement" Is a Fallacy in the American Joint Committee on Cancer's Seventh Edition of Tumor Staging of Oral Cavity Cancers.”, J Oral Maxillofac Surg, vol. 76, no. 1, pp. 206-212, 2018.