In melanoma staging, M1a refers to metastasis to surrounding soft tissue, M1b to the lung, M1c to any internal organ, and M1d to the central nervous system.

“m1as are Cancers that have metastasized to um surrounding soft tissue or soft tissue anywhere in the body so anywhere else on the skin and you might think well that's kind of crazy like how does that happen and it's really bizarre you can have a patient who had a melanoma that showed up in one part of their body and then they have metastases on other parts of their skin m1b B is uh and I always get B and C confused I think B is the lung so m1b is to the lung m1c is to any internal organ so liver Etc and m1d is to the CNS”