Why Imaging and Pathology May Describe Cancer Differently

Scans and pathology reports often describe what sounds like the same tumor in incompatible terms. The scan says 3.2 centimeters; the pathology report says 2.4. The scan mentions one area; the pathology finds two. The scan calls lymph nodes enlarged; the pathology calls them negative. This is expected, and it is usually not an error by either party.

Imaging measures how tissue looks from the outside: density, shape, edges, and how it takes up contrast. Pathology measures what the cells actually are, under a microscope, in a specimen that has been cut, processed and stained. The two are answering different questions with different tools, so their numbers rarely match exactly.

Several mundane factors move the measurement.

A scan measures the whole disturbed area. Around many tumors sits inflammation, swelling, compressed normal tissue and the body's reaction to the tumor's presence. On imaging these blend into the mass; under the microscope they are clearly not cancer, so the pathologist excludes them and reports a smaller number.

Tissue changes once removed. Specimens shrink when they lose their blood supply, and again during fixation in formalin. A tumor measured inside a living body and the same tumor measured on a cutting board are not measured under the same conditions.

The planes are different. A radiologist measures the longest axis visible on a particular slice. A pathologist measures the longest axis on whatever plane the specimen was cut. Neither is wrong; they are simply not the same line through the same object.

Contrast timing and image resolution matter too. On MRI in particular, the enhancing area can overstate or understate the true extent of disease, which is well described rather than surprising.

Staging reflects this hierarchy directly. Clinical stage, written with a lowercase c as in cT2 cN1, is the estimate assembled before treatment from physical examination, imaging and any early biopsy. Pathologic stage, written with a lowercase p as in pT2 pN0, comes from tissue actually examined after surgery. Pathologic stage is generally the more precise of the two, because it rests on direct examination of cells rather than on interpretation of shadows.

This is why a treatment plan can shift after surgery even though the cancer itself has not changed. What changed is the quality of the information.

Imaging judges lymph nodes largely by size and shape, since a scan cannot see inside one. Nodes enlarge for many reasons unrelated to cancer, including recent infection, inflammation, and the immune response to a nearby biopsy. Meanwhile a normal-sized node can contain a small deposit of cancer that no scan could detect.

So imaging can call nodes suspicious that pathology finds clean, and imaging can call nodes normal that pathology finds involved. Both directions happen, and both are limits of resolution rather than mistakes.

When treatment is given before surgery, the two tests diverge further, and the divergence is informative. Treated tumors leave behind fibrous scar tissue, dead cells and inflammation, all of which can still appear as a mass on imaging even when little or no living cancer remains. The reverse also happens: scattered surviving cells can persist in an area that imaging now calls resolved. Staging after such treatment carries a y prefix, as in ypT1 ypN0, precisely because everyone understands it to be a different measurement.

Rather than trying to reconcile the numbers yourself, ask which report your treatment plan is based on and whether the difference changes anything. In most situations the answer is that the pathology governs and the discrepancy is routine. If a difference is large enough to matter, that is a specific conversation worth having, and it is also the situation in which a second review of the slides or the images is sometimes arranged, which is a normal part of oncology practice rather than a sign that someone made an error.

• NCI - Pathology Reports Fact Sheet
• NCI Dictionary - Clinical Staging
• NCI Dictionary - Pathologic Staging