When You're Told 'There's No Change in Size' — Why Chemo Response in Peritoneal Stomach Cancer Isn't Measured by Size Alone, and Understanding Conversion Surgery and Intraperitoneal Chemo

When stomach (gastric) cancer cells break through the stomach wall and scatter like seeds across the peritoneum — the thin membrane lining the abdominal cavity — this is called peritoneal metastasis. Sometimes the abdomen is opened for surgery only to reveal these small deposits; because removing just the visible tumor would leave too much behind, chemotherapy is often continued instead of surgery. Two common questions follow: how many cycles until the cancer shrinks, and whether the drug should be changed when a CT scan shows 'no change in size.'

It helps to know that peritoneal metastasis behaves differently from other spread. Metastases in the liver or lungs usually appear as round masses whose diameter can be measured and compared over time. Peritoneal deposits, however, are often scattered as tiny grains or spread thinly along the membrane, so they frequently do not show up as a clear 'so many centimeters' on CT. Medicine calls this 'non-measurable' disease. For that reason, 'no change in size' is not the same as 'the drug isn't working.'

Doctors do not look at size alone. They also watch whether fluid buildup in the abdomen (ascites) has decreased, whether symptoms such as pain, appetite, and weight have improved, and the trend of tumor markers (such as CEA and CA19-9). When several signals stay stable, this may be read as disease control — meaning the illness is being held in check rather than worsening. The first response check often comes around cycle 2 or 3, but a single scan rarely decides everything.

A decision to switch drugs comes not from 'the size didn't shrink once' but from mounting evidence of clear progression. The first combination used is called first-line therapy and the next is second-line; treatment moves forward when the disease progresses or side effects become too hard to bear. In stomach cancer, platinum-based agents (such as oxaliplatin) and 5-FU–type drugs form the backbone, while targeted or immune therapies may be chosen based on tests such as HER2 and PD-L1.

For peritoneal metastasis, approaches such as intraperitoneal chemotherapy (delivering drugs directly into the abdomen) and conversion surgery (reassessing whether an operation becomes possible after chemo shrinks the disease enough) are being studied and tried. Eligibility, however, depends on the extent of disease, overall condition, and whether a hospital runs a relevant clinical trial — so if you are interested, it is best to ask your treating physician specifically whether your case qualifies.

This article is for general information only and does not replace individual diagnosis or treatment. Chemotherapy schedules, drug changes, and clinical trial participation differ from person to person, so please decide in consultation with your medical team.