Histopathologic Versus Clinical Margins in Melanoma

To the Editor: We thank Drs Ian Katz and Katy Bell for expressing their interest1 in our study “Differences between recommended surgical margins and measured histological margins in malignant melanoma in situ and malignant melanomas.”2 In their letter, they highlighted the importance of measurement uncertainty in histopathologic margin measurement and called into question estimates of tissue shrinkage. Although we agree that there is uncertainty in evaluating histopathologic margin measurements, this uncertainty does not imply that clinical margins could or should be more adequate than represented in the article. Instead, it highlights that frequently malignant melanocytic proliferations appear to extend beyond that which is appreciated clinically. This is best exemplified by the fact that we found 8.5% of melanoma in situ and 5.9% of malignant melanoma cases with positive margins. The literature also appears to be highly variable in terms of actual formalin shrinkage in cutaneous specimens ranging from the 37.5%, as mentioned by Dr Katz and Bell, all the way to 9.5%.3 For the purposes of this study, a specimen correction factor of 15% was used to mirror the proposed clinically useful fixation and shrinkage estimate proposed in 2018 by Friedman et al4 on evaluation of 252 specimens. However, we acknowledge that it would be next to impossible to consider exact shrinkage per every specimen in a retrospective study. Although the corrected margins to account for shrinkage may be slightly farther away depending on what percentage is used, it is clear that we are not achieving 5 mm or 10 mm margins consistently. As indicated by the study, further prospective studies (possibly with gross measurements taken of the impending specimen while still on the patient) are needed. The purpose of our article was to note the difference between the clinical/surgical margin and the histologic margin and possibly to induce future prospective studies. This was not an article on interobserver variability between dermatopathologists and how best to make precise histologic measurements, although such a article could be very useful but likely would need to involve multiple academic institutions to carry any significant weight.