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

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.