So I had a Stage IA melanoma removed from my left leg. While this is basically one of the “best” invasive cancers (Stage IA means it has invaded skin tissue, albeit not deeply, Stage 0 or melanoma in situ exists on top of the skin but hasn’t invaded the epidermis) in terms of long term survivability rates it still lead me to do a lot of research and ask a lot of questions of my doctors, because hey, it’s cancer–I felt I needed to know.
As I would read about specific cases of melanoma that had spread, I had the same question. “How can they diagnose melanoma when they can’t find a primary on the skin?” And “how do they know someone didn’t have a melanoma on their skin and independently developed say, lung, liver, brain, or lymphoma unrelated to the melanoma?” In my case I have reasonable belief that my melanoma developed ten years prior to its removal, as that is when a mole grew in size, darkened, and then sat like that for ten years. I was one of the lucky ones–my melanoma was apparently not growing, and I’ve heard case stories of people who had a melanoma that sat there for 40+ years without spreading any further. I’ve also read case stories of people who see a weird skin growth and who are in the ground four months later.
The answer as I understand it, is when melanoma metastasizes it first goes to the near lymph nodes, and then on the lymphatic superhighway can spread to far organs. The way they can know that it is a case of metastatic melanoma and not simply independent cancers, is that the melanocytes that normally only exist on the outer layer of the skin and give skin its pigmentation, are actually found in those cancers in the lymph nodes and removed organs. So that would be how, if Carter’s cancer is metastatic melanoma, they’d know that, the tumors they’ve found in his brain or liver would have melanocytes in them. That’s my layman’s understanding based on my understanding of what my personal doctor sand the Internet told me, so I may be getting it wrong.
FWIW I’ve also read that is why there is some hope that melanoma may be the first metastatic cancer that can be truly “cured.” Its behavior in that it spreads “identifiable cells” throughout the body, that are distinct from the cells of surrounding tissue, gives researchers hope that they can engineer drugs that teach the immune system to target these cells specifically and destroy them. With most other cancers the cancerous growths are extremely hard to “mark” as different from healthy tissue.