What Happens When a Mass Remains After Chemotherapy?

You finish chemotherapy, and then you are told a mass is still visible. That is a difficult moment. It sounds like failure. Very often it is not.

Chemotherapy kills tumor cells. But killing tissue is not the same as removing it. The body clears destroyed tumor slowly, and it commonly replaces it with fibrous scar. What the CT scanner sees is density and shape. It does not see whether the cells inside are alive. A mass that is entirely scar can look much like a mass that is entirely cancer.

That is why size alone would misclassify people who have in fact responded completely.

Broadly, four things, sometimes mixed together:

• Fibrosis — scar tissue, no cancer cells present
• Necrosis — dead tumor that has not yet been cleared
• Living cancer — cells that survived treatment
• Teratoma — in germ cell tumors specifically, a benign-looking tissue type that chemotherapy does not eliminate

Which of these is present changes what happens next. So the next step is usually a test designed to tell them apart.

FDG PET uses a radioactive sugar tracer. The tracer builds up in tissue that is metabolically active, meaning tissue that is still using energy. Scar and dead tissue are not active. Living tumor usually is. So a residual mass with no uptake on PET often means a complete response, even though it is still visible on CT.

This works better in some cancers than others. It is well established in lymphoma and in seminoma. It is much less reliable in cancers that are not strongly FDG-avid, and in mucinous or low-grade tumors.

Timing is not a formality. Chemotherapy and radiation cause inflammation, and inflamed tissue takes up the tracer too. Scanning too early produces false positives. The usual guidance is to wait at least three weeks after chemotherapy, preferably six to eight, and roughly eight to twelve weeks after radiation.

Response is assessed with the Deauville five-point scale. It compares uptake in the residual mass against two internal reference points: the mediastinal blood pool and the liver. Scores of 1 or 2 indicate a complete metabolic response. Score 3 is intermediate, and is read alongside everything else. Scores of 4 or 5 suggest residual disease.

Under this system, a lump that persists on CT with a Deauville 1 or 2 is a complete response. The mass is not the finding. The metabolism is.

Here two extra tools come into play.

Tumor markers. AFP, hCG and LDH are measured in blood. Markers that have normalized argue strongly against actively growing marker-producing cancer. Markers that stay high, or that are rising, change the plan substantially.

Surgery. Imaging cannot see inside the mass reliably, so residual masses are often removed and examined. In nonseminoma, a residual retroperitoneal mass larger than about 1 cm with normal markers is commonly resected. When those masses are examined, a large share turn out to be necrosis and fibrosis. Roughly 30 to 40 percent contain mature teratoma. Up to about 20 percent contain living germ cell tumor.

Teratoma is the reason surgery is used rather than watching. It does not respond to chemotherapy. It is marker-negative. And it can grow, or rarely transform, over time. So removing it is treatment, not just diagnosis.

In seminoma the approach differs. Residual masses under 3 cm are usually observed. Larger ones are assessed with PET, where a negative scan supports surveillance rather than surgery.

A residual mass is a question, not an answer. The reasonable next steps are a properly timed PET scan, where that applies to your cancer. Tumor markers, where they are relevant. Repeat imaging over an interval. Or tissue sampling.

Stability over time is itself informative. A mass that stays exactly the same size across several scans behaves differently from one that grows. Has your team recommended watching rather than acting? Ask what they expect to see, and what change would prompt them to do something.

• RadiologyInfo: Positron Emission Tomography (PET)
• Therapeutic Advances in Urology (PMC) — Management of the residual post-chemotherapy retroperitoneal mass in germ cell tumors
• NCI: Tumor Markers Fact Sheet