Melanoma Actually (Part 3)
Melanoma is the smallest, deadliest tumor.
You would think that a tumor invading less than 1 mm is not dangerous, but actually that's not true.
No other tumor is as small and as deadly as a melanoma. While you might notice the spread of the tumor (how big it is on your skin) pathologists are not as interested in the size that you see with your naked eye. What matters most to us dermatopathologists is the depth of the tumor - how deep does the melanoma invade below the skin surface? This is called the Breslow depth.
1 mm is considered a significant depth of invasion. Now that staging parameters have changed, even melanomas that invade just deeper than 0.8 mm are at a worse stage (T1b). We measure invasive melanoma to a tenth of a millimeter, with 0.1 mm invasion into the dermis as the smallest depth. *
While pathologists measure and collect data on all tumors, dermatopathology is the primary field of pathology where we measure tumors with this degree of precision. Despite this precision, there is uncertainty in melanoma.
I think this uncertainty persists because these tumors are so small; it is hard to study them. I don't mean study under the microscope – for over 100 years pathologists have studied and documented the microscopy, histologic description and characterization of melanomas. There is a rich tradition of knowledge that accurately describes the patterns of melanoma. No, I mean it makes it difficult to study these tumors by molecular methods. Most cancers, lymphomas, and sarcomas generate lots of tumor cells; after all, dividing is their main activity. Therefore most tumors tend to be somewhat bulky, and can be measured by pathologists at the bench, grossly, in centimeters. That's harder to do for melanoma, although large and bulky melanomas do occur.
Most melanomas are small. The sample is entirely used up in the process of diagnosis. This means there is little to be sent off for additional, molecular testing. This is rapidly changing, and molecular tests for melanoma are expanding. Still, correlating how the melanoma looks clinically and under the microscope (the phenotype of the melanoma) to the genes expressed (the genotype of the melanoma) is difficult to do with current techniques.
Molecular testing that categorizes melanoma is expanding, and I find this area the most intriguing, because Melanoma is just one address for a whole apartment building full of other residents. There is uveal melanoma (melanoma of the eye) that we already know has it's own unique molecular pattern. Melanoma under the nail has a different appearance and a different molecular pattern from melanomas arising in sun-damaged areas. Rare melanomas can arise internally too - melanoma of the sinuses, melanoma of the anus, vulva and vagina. (I should just state outright that none of these types are sun related. ) Right now we describe these tumors based on their appearance. In the future we can hopefully talk about these tumors from their molecular origins and develop staining markers and therapies based on those molecular markers.
Some clinicians prefer to biopsy moles that are over 4 mm in diameter so that patients are not over-treated. This is because we don't really understand the biological behavior of very small atypical pigmented lesions -- these could be early melanomas or they might not progress further. Although the overall size of any pigmented lesion matters, lots of factors go into a dermatologist's decision to biopsy an atypical looking pigmented (dark) spot. It comes down to 'funny looking' moles. Sometimes it is just a 'gut check' on the part of the patient or doctor - this particular mole is new and/or looks odd.
The bottom line is if you are concerned about a skin condition, you should ask your dermatologist to evaluate it. If they are sufficiently concerned, they can biopsy it and then send it to us, the dermatopathologists.
*Many melanomas are not invasive and instead stay in the epidermis, called in situ melanoma. These do not have the potential to spread (metastasize) but they may locally recur if not totally removed.