The microscopic measurement of tumor thickness or depth of invasion (DOI) has long been considered a valuable parameter
for predicting regional nodal involvement and survival in oral cavity squamous cell carcinoma.
Thickness is usually measured from the mucosal surface of the tumor to the deepest point of tissue invasion in a perpendicular fashion with an optical micrometer or transparent ruler overlaid on the slide,
while DOI is measured from the basement membrane of adjacent normal to the deepest point of invasion of the tumor.