Tumor thickness versus depth of invasion – Analysis of the 8th edition American Joint Committee on Cancer Staging for oral cancer
Introduction
One of the most important changes in the recently proposed 8th edition of the American Joint Committee on Cancer (AJCC) staging system for oral cavity squamous cell carcinoma (OSCC) is the addition of depth of invasion (DOI) as a modifier for the T category [1]. The new staging manual emphasizes that it is important to distinguish between tumor thickness and DOI and stipulates the use of the latter for staging purposes (Table 1).
There have been numerous studies examining the impact of both DOI and tumor thickness in OSCC [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17]. This has led to a general consensus that tumors with a larger DOI or thickness are associated with an increased risk of nodal metastasis and worse survival outcomes [5], [11], [18].
Many institutions have measured tumor thickness instead of DOI historically and recorded these in large prospective research databases. It is unknown whether on a population basis it is reasonable to substitute tumor thickness for DOI when retrospectively analyzing the effect of T category or AJCC stage, either as the variable of interest or when adjusting for its effect as a confounder (covariate) in multivariable analyses. The primary aim of this study is to compare the effect of using tumor thickness versus DOI to determine the 8th edition AJCC T-category on survival in a large cohort of OSCC.
Section snippets
Study population
Clinicopathological data were extracted from the Sydney Head and Neck Cancer Institute (SHNCI) database for all patients with OSCC treated with curative intent between 1987 and 2016 with surgery ± adjuvant therapy. Collection of data on thickness started in 1987 and collection of depth of invasion in 1995. Exclusion criteria were absence of follow-up data, perioperative mortality or incomplete data on primary tumor features or nodal status to permit staging.
Depth of invasion and tumor thickness
Depth of invasion and tumor thickness
Patient demographics
Our unit database search identified 927 patients with oral SCC. There were 663 patients with data available on thickness. Of these, 207 were missing data on depth and were therefore excluded from the main analysis. The final study population includes 456 patients with complete information on DOI, tumor thickness, T and N staging and follow-up.
There were 260 men and 196 women, with a median age of 63.9 years (range, 18.2–97.6 years) and median follow-up of 2.1 years. All patients underwent surgery
Discussion
The newly proposed 8th edition of the AJCC TNM staging manual has incorporated depth of invasion to improve the prognostic accuracy of T staging in oral cancer [1]. This is based on numerous studies that have shown depth to be an important predictor of prognosis in oral SCC [11], [16]. The staging manual highlights the distinction between thickness and depth, stipulating that the latter must be used for staging purposes. However, there are also many institutions that have demonstrated the
Conclusion
The T category and TNM stage prognostic performance of 8th edition AJCC staging of oral cancer is similar regardless of whether DOI or thickness is used as the T-category modifier.
In centers without complete DOI data it is reasonable to impute thickness for retrospective survival analyses using the 8th edition of the AJCC staging system.
Conflict of interest
None declared.
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