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Pathologic Staging: Assessing pM
The M category of a TNM stage grouping refers to distant metastatic disease anywhere is the body. The category has been modified to reflect the fact that distant metastasis confined to one organ can be surgically resected for cure in some cases, resulting in long-term survival. The M category for distant metastasis in colorectal cancer staging is subclassified as follows:1
M Category Subclassification
- MX
- Distant metastasis cannot be assessed
- cM0
- No distant metastasis (the category pM0 does not exist)
- M1
- Distant metastasis
- M1a
- Metastasis confined to one organ or site (e.g., liver, lung, ovary, non-regional node).
- M1b
- Metastases in more than one organ/site or the peritoneum.
From the point of view of pathological assessment, this category has two unique caveats. First, the category pM0 does not exist. A designation of pM0 would imply whole-body exclusion of metastasis by pathological examination. This may be done only at autopsy and would be classified using the autopsy-specific nomenclature of the AJCC.1 Second, the designation pMX is discouraged by the CAP and the COC, even though is strictly correct in connoting the unknown pathological status of distance metastasis. Although it applies in most cases of colorectal cancer, the designation pM0 has been deemed confusing and unhelpful, perhaps because its meaning was widely misunderstood.3
For the pathologist, therefore, only the designation pM1 is appropriate when tumor from distant metastatic site is confirmed pathologically. This may occur, for example, in resection specimens of distant metastatic lesions in the liver or lung performed as part of the primary therapy. In such cases, the goal is complete excision, and pathological; examination of the surgical margins is required. Alternatively, pM1 assignment is appropriate when biopsy confirmation of distant lesions observed at operation is performed or peritoneal cytology is found to be positive. Negative biopsies or cytologies to rule out distant metastasis are not assigned pM0 as discussed above.