Micrometastases vs. Macrometastases Explained

If your report uses the words micrometastasis or macrometastasis, it is describing the size of a cancer deposit found inside a lymph node. Nothing more than that. The pathologist measured the largest cluster of cancer cells in the node and placed it into a category defined by millimetres.

The word metastasis inside these terms is what causes alarm, because in ordinary use it means cancer that has spread to distant organs. Here it is being used in its technical sense — cancer cells that have travelled from where they started — and the distance travelled is to a node beside the tumour.

Staging conventions draw two lines.

Isolated tumour cells — clusters no larger than 0.2 mm, or fewer than 200 cells. Recorded as pN0(i+). Read that notation carefully: it begins with pN0, meaning that for staging purposes the nodes are still counted as negative. The (i+) records that something was seen.

Micrometastases — deposits larger than 0.2 mm but not larger than 2 mm. Recorded as pN1mi. These count as node involvement, but are recorded separately from larger deposits.

Macrometastases — deposits larger than 2 mm. These drive the main pN categories. In breast cancer, for example, pN1 means one to three nodes with deposits larger than 2 mm, pN2 means four to nine, and pN3 means ten or more.

The thresholds themselves are agreed conventions rather than biological cliffs. They exist so that a report written in one hospital means the same thing as a report written in another.

They are a consequence of better looking, not of worse disease.

When surgeons removed whole groups of lymph nodes, a pathologist facing twenty or thirty nodes could examine only a slice or two of each. Sentinel node biopsy changed that. With one or two nodes to assess, the pathologist can cut many thin levels through each one and add immunohistochemical stains that make individual cancer cells visible.

The result is that deposits which would simply never have been found a generation ago are now routinely reported. People being diagnosed today are not being found with more disease than people diagnosed decades ago; they are being examined more closely.

Isolated tumour cells have limited effect on outcome and, in most staging systems, do not change the stage group. Their presence is recorded rather than acted on.

Micrometastases occupy a genuine middle ground. They represent real node involvement and are recorded as such, but their effect on outcome is considerably smaller than that of larger deposits.

Macrometastases are the category known to influence outlook and to change the N stage, and they carry the most weight in treatment decisions.

Perhaps less than the terminology suggests. Practice has moved away from automatically removing the remaining lymph nodes when only isolated tumour cells or micrometastases are found in a sentinel node. Trials found that in selected patients, further surgery did not improve outcomes while it did add the risk of lasting swelling and stiffness.

Whether that applies to you depends on your cancer type, tumour size, grade, biomarker results and what other treatment is planned. The finding is one input among several, not a switch.

Outside pathology reports, micrometastasis is sometimes used more loosely to mean microscopic cancer that has spread somewhere in the body but is too small for any test to find. That concept is the reasoning behind giving chemotherapy after apparently complete surgery — treating disease that may be present but cannot be seen.

The two uses are related but not the same. On your report, the word has the precise, measured meaning above.

Ask what size the deposits were and exactly how they are classified in your report. Ask whether the finding changed your stage. Ask whether it changed the treatment recommendation, and whether any further node surgery is being considered. If the notation on your report is pN0(i+), it is worth asking your team to say out loud what that pN0 means.

• NCI: Sentinel Lymph Node Biopsy Fact Sheet
• NCI: Cancer Staging
• NCI: Pathology Reports Fact Sheet