I work in a hospital and went to ask one of our pathologists about this. Don't know if it's what you're looking for.
She said to go to the CAP (College of American Pathologists) website (www.CAP.org) and look under cancer protocols. Select the organ site of disease and look for the staging. It's possible that "m" stands for multifocal (were there 2 separate cancers?).
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