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Matsumoto and colleagues should be commended for undertaking an important study evaluating the treatment of nail unit melanoma in situ with Mohs micrographic surgery (MMS). Nail unit melanoma is seemingly easy to diagnose for the experienced nail doctor. However, anyone treating these patients is aware of the difficulties involved in obtaining unambiguously clear margins. In principle, it should be evident: The melanoma is pigmented and clearly visible, usually in the setting of longitudinal melanonychia, and in long-standing cases, there may also be a slight nail dystrophy. However, we know from lentigo maligna and particularly from acral lentiginous melanoma that the clinically visible border of the lesion may not represent the histological border. This is the rationale for extirpating these tumors with a border of “normal skin” as a safety margin. However, even this may not be enough. Molecular biologic investigations of in situ melanomas have shown that biochemically altered melanocytes, so-called “field cells,”may extend in the epidermis up to 9 mm beyond the histologically discernable border, although fortunately not extending into the dermis or deeper. These field cells are histologically bland, that is, looking completely normal without atypia or mitoses, are equidistantly distributed, and do not display any immunohistochemical abnormalities, meaning they cannot be identified by light microscopy nor by immunohistochemical methods. Adding controversy, it is not yet proven that these biochemically abnormal field cells are truly melanoma cells, or “premalignant” cells, or whether they simply develop field cells characteristics from local environmental influences. Therefore, the first issue that one must question is whether or not it is possible to get a safer margin withMohs surgery. Clinical follow-up with cure rates beyond 5 and 10 years will help answer this question. A second issue is a technical one. Mohs micrographic surgery examines the entire cut surgical margin in 1 block or, when this is too large or cumbersome, divided into 2 or more blocks. The specimen is laid on a cryostat chuck to allow the specimen to be sectioned with the cryotome en face. This requires sectioning one planar surface of the specimen’s lateral and deep margins. To achieve this, the excision may require a beveled edge. If the specimen margin is not completely even and flat, the cryostat section will exhibit “holes” thatmay be interpreted as a false negative or false positive. Grossing and processing the entire nail apparatus in the Mohs laboratory for frozen section is truly fraught with potential pitfalls. Furthermore, in sites where the tumor extends extremely close to the cut margin (e.g., the apical matrix, a common site for nail melanoma in situ), facing or trimming the block can inadvertently lead to cutting into the tumor itself, creating a true false-positive margin. This is precisely the circumstance discussed by Strickland and colleagues in their discussion. Furthermore, the photomicrographs in the article show residual matrix centrally. Either this represents a false positive (if the nail apparatus was excised in toto) or indicates that the Mohs margins or dissection plane was inadequate. For these reasons, the authors routinely treat nail melanoma in situ with wide local excision (at least 4–6 mm bilaterally and 8–10 mm proximally from the proximal nail fold), carried to periosteum under the matrix and nail bed and the extensor tendon proximally. The specimen must be serially step-sectioned in the longitudinal plane to best assess margins. This technique minimizes development of nail spicules by ensuring complete removal of the lateral matrix horns, which are the commonest sites of recurrence if not completely extirpated. A third issue is how the authors made the diagnosis of melanoma in situ. All were biopsy proven; however, smaller (especially punch) biopsies that are incisional may miss areas of invasive tumor. It is therefore incumbent on the Mohs surgeon to evaluate (by frozen section or permanent section consultation) the central margin to rule out invasive disease, especially in the setting of incisional diagnostic biopsy. If the melanoma is truly in situ, one could excise the deep tumor at bone and evaluate the peripheral margins as described with the spaghetti technique. Still, for these reasons, the authors prefer wide local excision specifically for nail melanoma in situ, with longitudinal processing, and serial step sectioning as a compromise that addresses the anatomic nuances of margin analysis in this region. Fourth, immunohistochemistry is able to demonstrate melanocytes that otherwise remain invisible in normal H&E sections. However, there is still the issue that none of themelanocyte markers is amelanomamarker. Aswe know now, nail melanoma in situ may demonstrate only scattered atypical melanocytes with hyperchromatic nuclei (varying in size and shape) with a range of cellular atypia, without accompanying pagetoid spread and/or increased melanocytic density. This has the potential to further confound the Mohs surgeon, even with exceptionally prepared slides representing the true en face margin. Finally, the authors reported 1 case of recurrence after 130 months “requiring amputation (unknown type).” We know that such recurrences are not uncommon, even after Mohs surgery, and yet reassuringly, the histopathology http://dx.doi.org/10.1097/DSS.0000000000002755