Comparison of the conventional and alternative methods of measuring depth of invasion of vulvar squamous cell carcinoma using whole-slide digital pathology images.

The accurate staging of stage 1 vulvar squamous cell carcinoma has been complicated by multiple definitions of how to measure depth of invasion (DOI). These differences create a subset of tumours that by the alternative method would be classified as stage IA but by the conventional method are classified as stage IB. This study evaluates the clinical outcomes of patients who were down-staged from IB to IA using the alternative method as opposed to the conventional method. Another analysis compared DOI measurement using an ocular micrometer on glass slides versus digital ruler with digitised slides. A total of 46 stage I (IA or IB) vulvar squamous cell carcinomas were identified; 11 were classified as stage IB if the conventional method was used but down-staged if the alternative definition was used. Down-staged patients with clinical follow-up data showed no recurrent lymph node involvement, while three of the 22 patients consistently staged as IB showed recurrent lymph node involvement. Digital ruler measurements of DOI were, on average, over 1 ​mm less than the manual ocular micrometer measurements. These data support prior publications that encourage the use of the alternative method of DOI measurement as a more conservative approach to staging vulvar squamous cell carcinoma and avoiding lymph node sampling in a subset of patients who do not show as high a risk of nodal metastasis. Additionally, this study highlights a more conservative DOI measurement when using a digital measuring ruler than while using the manual ocular micrometer.