Melanoma Actually (Part 1)
Updated: Sep 20, 2018
***This represents my experience diagnosing melanoma, and does not indicate an opinion of any specific tumor.***
I’m a dermatopathologist, a pathologist who studies diseases of the skin, which means I can diagnose up to several melanomas in one day. And even with this experience there is a lot about melanoma I just don’t know because there are many unknowns about the meaning of our findings.
I know what a melanoma looks like and what is not a melanoma. But there is a lot of uncertainty as to the biological nature of melanoma itself.
You would think that dermatopathologists could easily tell you what a melanoma cell looks like, but actually, we're not always sure.
To know if something is melanoma we look at it in context - what melanocytes are around it, where is it in the dermis (up high or down low). Sometimes large and atypical (we would say 'ugly') melanocytes are entirely benign (they could be Spitzoid melanocytes, or part of a inverted pattern). Sometimes small melanocytes are melanoma cells.
This is best demonstrated when a single cell metastasizes to a sentinel lymph node.
In a lymph node we must consider the cell out of context - we can't rely on the architecture of the skin, the melanocyte's neighbors or the melanoma growth pattern. We rely on the cell size (which here is large relative to the background small blue lymphocyte nuclei), and the staining pattern. This staining pattern is cellular and appears specific to this cell.
The stakes of this decision are high - one cell in a sentinel lymph node makes it positive. But does one cell indicate a worse prognosis? We don’t really yet know*. A positive node is considered positive no matter how big or how small the metastasis is. This is a major issue for melanoma patients and oncologists. The problem for pathologists looking at these single cells is deciding if it is a real melanoma cell. Is it a nodal nevus (approximately 25% of lymph nodes have nodal nevi, benign collections of melanocytes in the capsule)? Is it a basophil or a histiocyte (cells that might pick up staining pigment)? Once we have decided that the staining is real, and that it is not a nodal nevus, we must call these positive. For all tumors, pathologists scrutinize every slide and every positive cell.
In general, this category of information is called the disease prognosis. How will your diagnosis behave - aggressively or indolently? How we answer this question is based upon data and research from a large number of cases in aggregate, and also upon the genetic findings from specific tumors.
Part 2 coming soon - more about Melanoma Unknowns.
*Studies are coming, perhaps in 2022, to clarify the meaning of metastases to nodes. When there is a metastasis, we measure the largest deposit and describe the extent of the tumor. Hopefully this will have more meaning with studies like the melanoma lymph node study.