Practical Challenges in Measurement of Depth of Invasion in Oral Squamous Cell Carcinoma: Pictographical Documentation to Improve Consistency of Reporting per the AJCC 8th Edition Recommendations

Abstract

Depth of invasion (DOI) and tumour thickness (TT) are known prognostic indicators in oral squamous cell carcinoma (OSCC), but varying definitions have been used by pathologists for reporting. The American Joint Committee on Cancer (AJCC) has proposed adoption of a uniform definition of DOI and incorporated this measurement in the revised TNM staging (8th edition); however, unambiguous DOI determination can be a challenge in clinical practice. We reviewed archived slides of 95 cases of T1/T2N0 OSCC and listed the challenges in accurate DOI measurement with pictographical documentation. The impacts of DOI and TT on disease-free survival (DFS) were also assessed. The mean DOI and TT was 5.89 mm and 7.32 mm respectively. Challenge in horizon estimation for DOI measurement was experienced in 75/95 cases (78.9%). The most common challenges were lack of adjacent uninvolved mucosa in sections or presence only on one side, rounded/convoluted nature of the tumour surface for tongue and polypoidal tumours, and angulation of adjacent mucosa for alveolar or lip tumours. In cases with very thin epithelium, DOI was equal to TT. In spite of the challenges, Kaplan-Meier analysis showed DOI > 5 mm significantly predicted poorer DFS while TT did not. We recommend various guidelines to help improve consistency in measuring DOI and recording of TT in ambiguous cases for accurate staging of OSCC.

Keywords: Depth of invasion; Oral squamous cell carcinoma; Tumour thickness.

Fig. 1

Difficulty in horizon estimation due to convolutions of adjacent mucosa which is not in a straight plane. The different lines show the various possibilities for drawing the horizon

Fig. 2

a Difficulty in accurate horizon estimation due to adjacent uninvolved mucosa present on one side only. The arcuate yellow line represents the most likely natural contour and the most appropriate horizon. b Angulated adjacent mucosa limiting horizon estimation. c Arcuate line of horizon replicates the natural rounded contour of the lateral border of tongue rather than drawing a straight line from the edge of the adjacent uninvolved mucosa. d In a polypoidal non-verrucous/non-papillary tumour, the mucosa is heaped up by the tumour and thickness (calculated from yellow line) is a better measure of tumour bulk than depth of invasion

Fig. 3

a Difficulty in horizon estimation due to extended zone of dysplasia adjacent to the tumour. The estimation of cut-off point of basement membrane of adjacent uninvolved mucosa is impaired. b Horizon estimation is not possible in small tumours post-biopsy which appear purely submucosal even after study of multiple deeper sections. c In cases with irregular hyperplasia of adjacent epithelium, the current AJCC guidelines do not specify whether measurement should be from the level of the tip of the submucosal papilla (yellow line) or the deepest point of the rete peg. This difference can sometimes vary by 1 mm or more. d When adjacent mucosa is very thin, depth of invasion measurements equal the tumour thickness

Fig. 4

Kaplan–Meier curve showing disease-free survival stratified by depth of invasion