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Occult invasion possible with in situ melanoma

Occult invasion of in situ melanoma is possible, but it is rare, according to study results recently published in JAMA Dermatology.

Researchers used the melanoma database on patients with in situ melanoma at the University of Michigan Comprehensive Cancer Center between Jan. 1, 2012, and July 31, 2014, to identify unequivocal in situ melanoma without associated nevi or regression. There were 33 in situ melanomas in a consecutive cohort plus one nonconsecutive index case identified. A patient who developed a regional lymph node metastasis 7 years after the initial diagnosis in 2007 was used as the index case.

“After cutting deeper into the most representative tissue block, three sequential slides were stained with hematoxylin-eosin (H-E), melanoma antigen (melan-A) and again with H-E,” the researchers wrote. “Melan-A stained slides showing definitive invasion were double-stained with Sry-related HMg-Box gene 10 (SOX1O) to confirm the melanocytic nature of the cells of interest.”

Occult invasion was detected by immunochemistry. Three dermatopathologists independently scored the slides.

Eleven of the 33 consecutive cases had occult invasive melanoma detected. Immunochemistry alone was used in the diagnosis of six of the 11 melanomas, while the other five tumors were diagnosed through melan-A and H-E staining.

A few small melan-A positive cells in the dermis that were not identifiable in the flanking sections were seen in four cases, but it was not sufficient for a diagnosis for invasive melanoma, “and were most consistent on a cytomorphologic basis with occult nevi,” the researchers wrote.

“Occult invasion was detected in 33% of our consecutive in situ melanoma case cohort using immunohistochemistry and deeper sectioning into the tissue block,” the researchers wrote. “We applied unique methods to circumvent potential limitations of prior reports, including the absence of preexisting nevus to minimize false-positive immunostaining and regression with the associated potential for regressed microinvasion.”

“Melanoma in situ may rarely metastasize,” the researchers concluded. “History and physical examination, including regional lymph nodes, education and surveillance recommendations should be based on a very low, but not zero, risk of melanoma metastasis.” – by Bruce Thiel

Disclosure: The researchers report no relevant financial disclosures.