Interobserver Variation in Vulvar Squamous Cell cancer Depth of Invasion Using Two Measurement Methods.

Introduction: Depth of invasion (DOI) in vulvar squamous cell carcinoma (vSCC) predicts risk of nodal metastasis, with measurement >1 mm dictating the need for lymph node diagnostic procedures. In 2021, the International Federation of Gynecology and Obstetrics (FIGO) changed its advice on how pathologists measure DOI. Some organizations revised guidelines to this "New" method; others continued to endorse the FIGO 2009 "Old" method. This study compares interobserver variation of vSCC DOI using Old and New FIGO measurement strategies.

Methods: A single representative image was chosen from 50 consecutive vSCC excisions with reported DOI of 0.1-3 mm. Ten pathologists provided 2 electronic measurements for each, using Old and New methods. Statistical evaluation included analyses of variance, Student t -test, and the kappa statistic.

Results: The Old method yielded a larger mean DOI than the New [1.3 vs 0.9 mm; p < .001]. The Old method had a lower proportion of measurement disagreements spanning 1 mm (53%, κ = 0.65% vs 68%, κ = 0.6). Agreement by all pathologists of DOI being either ≤1 mm or >1 mm occurred in 29/50 cases (58%) using the Old and 26 (52%) using the New method. When at least 2 pathologists measured DOI >1 mm, interobserver variation was lower using the Old method in 30 (83%) of 36 cases [mean difference = -0.1 mm, t (280) = -2.78, p = .008].

Conclusions: The FIGO 2021 DOI measurement method has higher interobserver variation than FIGO 2009, with this difference arising from tumors with DOI >1 mm. This finding, combined with inadequate international consensus and scant clinical outcome data, should trigger reconsideration of 2021 FIGO staging guidelines.