Regarding the acoustic coupling medium for the estimation of the depth of invasion in tongue squamous cell carcinoma on intraoral sonography with special reference to the interpretation of normal mucosal structure: A literature review

Objective This study reviewed the literature addressing the use of a coupling medium to evaluate the extent of oral tongue squamous cell carcinoma (OTSCC) in intraoral sonography to assess the potentiality of this technique for the precise estimation of the depth of invasion (DOI). Methods Studies for this paper were identified by a search of PubMed and manual searching of references from articles, using the search terms “lingual cancer” and “intraoral sonography” and their synonyms in the title and fields. We excluded studies with no description of the coupling medium. Results Fourteen studies were included in this study. Sterile or non‐sterile gel was used in 11 studies; however, no description about the surrounding normal mucosa was provided in these studies. In two studies, a dedicated acoustic coupling polymer gel was used, and the normal mucosal layer was demonstrated as a hypoechoic line underlying the mucosal surface in one study. Although the first report of the DOI measurement was performed with degassed water, no clear interpretation of the basement level of the surrounding mucosa was presented. Conclusions Insufficient studies were available in the appropriate use of acoustic coupling medium from the viewpoint of the interpretation of the detailed normal anatomical structure of the tongue mucosa. Further studies are required to establish a standardized technique for the estimation of DOI in OTSCC.