Article Review: Reliability of Mohs Slides Diagnosis

Each month I post a short summary of an article from the literature that has caught my eye. Every week I scan all journals relevant to skin cancer, and look out for articles that are likely to be of interest to general practitioners who see patients with skin cancer.


This month’s article is from the British Journal of Dermatology and is by van Lee J et al. It is titled “Reliability of Mohs slides diagnosis: interpersonal and intrapersonal agreement on BCC presence and histological subtype.

The study is from The Netherlands and the authors looked at how closely pathologists and surgeons agreed when looking at slides taken from Mohs surgery patients with BCC. The study looked at 50 different slides, twice over a 2-month period, using 6 raters, and 2 panel experts. In all, 800 slide assessments were done.

This type of study always draws my eye, because it reminds me how dermato-pathology is not a precise and exact science. The answer is rarely “yes or no”. This truth is often hard for us, as general practitioners, to realise and remember. In our courses we teach that pathology is NOT the gold standard.

This study reminds us of that.

Key findings – at the interpersonal level (between experts) there was disagreement on whether BCC was present or not in 23/100 cases.

At the intrapersonal level (that is, the same person viewing the same slide, 2 months later) there was disagreement in 9/100 cases.

At the group consensus meeting of experts there was disagreement in 5/50 cases.


I share this paper to remind us all that pathology is not an exact or precise science. Experts can disagree with each other on an apparently simple issue such as “is BCC present or not”. Also (especially in tricky situations like Mohs surgery slides) the same expert may provide a different result on the same specimen when viewed at a different time.

None of this is a criticism of pathologists or experts. It simply serves to remind us of the uncertainty that is inherent in our work, and that pathology is not the gold standard.

So, what is the gold standard? Well, we are! The GP is the gold standard: only the GP has all the information – history, clinical examination, dermoscopy, and pathology report. Only the GP – who did the procedure, knows margins were used.

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2 comments on “Article Review: Reliability of Mohs Slides Diagnosis

  1. Without looking at slides myself I often wonder about the subgroup that BCCs are classified in by the histopathologist. How arbitrary are these decisions? How may BCC have a mixed morphology?
    Would it be more realist to classify them as aggressive and non aggressive?
    The management decision between topical, cryotherapy or excisional surgery often rests with the histological classification. These histological opinions are probably not all perfectly reliable.

  2. David, You are spot on here. The levels of agreement on sub type of BCC are even lower. As such, mixed sub type is common, and studies do show that there is quite poor concordance between biopsy BCC results and results on the same lesion when fully excised. So, as you imply, my practice is to group BCC as non aggressive or aggressive, and treat accordingly. All aggressive BCC should be excised.