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TO THE EDITOR: As a family physician turned dermatologist, I always read with interest the articles in American Family Physician that deal with the diagnosis and management of skin diseases. I enjoyed reading "Punch Biopsy of the Skin,"1 for its discussion of punch biopsy indication and techniques. I found the article to be informative and well presented, with the exception of the described use of punch biopsy in the diagnosis of melanoma. The pitfalls related to the interpretation of a punch biopsy, with the limited length of skin available to examine, have been previously discussed in American Family Physician.2 Recent data show that, of the three basic techniques of suspected melanoma biopsy--excision, scoop shave, and punch-punch biopsy is the most likely to yield an uncertain result.3 This applies to determining the depth of melanoma, on which prognosis and definitive therapy is based, and correctly diagnosing the lesion. The only exception is the punch biopsy of a lesion that fits entirely within the punch.
"Doing the math" explains the difficulty. A suspicious lesion 1 cm in diameter has an area of 0.79 cm^sup 2^. A "representative" 4-mm punch will sample 0.13 cm^sup 2^, which is only 16 percent of the surface of the lesion. Three 2-mm punches, sometimes advocated for large facial lesions, sample less than 0.1 cm^sup 2^ combined. Furthermore, the advice that one can sample the most suspicious part of the lesion is flawed because the most papular component of a melanoma may indeed represent the thickest part of the cancer or may, instead, be the benign melanocytic lesion out of which the melanoma arose. Since melanoma is diagnosed more by histologic appearance of what the melanocytes are doing than by what any individual melanocyte looks like, the most diagnostic portion of a lesion...