Newly Diagnosed With Stomach Cancer: First Steps

This page is for people just diagnosed with stomach cancer and those helping them. It explains the main tumor types, the biomarker tests to confirm, treatment by stage, and when to get help now.

Stomach cancer is not one disease. The National Cancer Institute splits adenocarcinoma into two main patterns, and they behave differently.

Intestinal type. Well differentiated, forming tubular and glandular structures that still look organized under the microscope.

Diffuse type. Undifferentiated or poorly differentiated, with no gland formation. NCI notes this type can spread through the stomach wall and produce linitis plastica, a stiff, thickened stomach that no longer expands.

Some tumors show mixed features. Ask which pattern your report describes, because the diffuse type changes both surgical planning and expectations.

These are worth confirming early.

HER2 status has to be known. So does CLDN18.2, a protein called claudin 18.2 that sits on some stomach cancer cells.

The reason is a specific FDA approval. Zolbetuximab-clzb is approved for first-line treatment of adults with locally advanced unresectable or metastatic gastric or gastroesophageal junction adenocarcinoma that is HER2-negative and CLDN18.2 positive. Positivity is determined by a companion test, the VENTANA CLDN18 (43-14A) RxDx Assay.

Note how the two markers interact. A HER2-positive tumor does not qualify for that drug. A tumor never tested for CLDN18.2 cannot qualify either.

Ask directly: has my tumor been tested for HER2 and for CLDN18.2, and when do the results come back?

NCI lists Helicobacter pylori infection of the stomach first among risk factors. Others include older age, male sex, and a diet low in fruits and vegetables and high in salted or smoked foods. The list also includes chronic atrophic gastritis (long-term thinning of the stomach lining), intestinal metaplasia (stomach lining changing to look like intestine), pernicious anemia, adenomatous polyps, family history, cigarette smoking, Ménétrier disease, Epstein-Barr virus infection, and inherited syndromes.

H. pylori matters because it can be tested for and treated. Ask whether you have been tested. Ask whether close relatives should be.

NCI's SEER program gives 5-year relative survival for stomach cancer, using cases from 2016 to 2022:

• Localized — 78.1%.
• Regional — 39.0%.
• Distant — 8.1%.

The stage distribution explains the overall figure of 39.8%. Only 32% are found localized, while 35% are already distant. For 2026 the American Cancer Society projects 31,510 new cases and 10,740 deaths, the figures SEER carries. The most common age group at diagnosis is 65 to 74, at about 30% of new cases.

These are group averages across many years and older treatments. They are not a forecast for you.

For stage 0 disease, NCI reports over 90% 5-year survival with surgery. It also describes endoscopic mucosal resection as an option for selected patients with good-risk features.

Those features are specific: Tis or T1a depth (limited to the stomach's inner lining), a diameter of 2 cm or less, and a predominantly differentiated type. If a doctor offers an endoscopic approach, ask whether your tumor meets all three.

This is a real change from how stomach cancer used to be treated, and it is worth understanding.

FLOT. Docetaxel, oxaliplatin, and fluorouracil with leucovorin, given before and after surgery. NCI reports the AIO-FLOT4 trial: median overall survival was 50 months with FLOT against 35 months with ECF or ECX, with a hazard ratio of 0.77.

CLASSIC. Capecitabine with oxaliplatin after surgery raised 3-year disease-free survival to 74% from 59%. These figures come from the National Cancer Institute’s “Gastric Cancer Treatment.”

SWOG-9008. Adjuvant chemoradiation gave a median survival of 35 months against 27 months.

Surgery is a gastrectomy, meaning removal of part or all of the stomach, together with removal of regional lymph nodes.

If someone proposes going straight to surgery for stage II or III disease, ask why chemotherapy is not being given first.

Fluorouracil and capecitabine are both fluoropyrimidines. Some people cannot break them down normally.

NCI states that an estimated 1% to 2% of the population carries germline pathogenic variants in DPYD, the gene for the enzyme that clears these drugs. Testing costs less than $200. NCI also notes it may delay therapy by about 2 weeks.

That trade-off is worth discussing out loud, because nearly every regimen above contains a fluoropyrimidine.

Two trials support zolbetuximab, and the numbers are modest but real.

SPOTLIGHT enrolled 565 patients, comparing zolbetuximab plus mFOLFOX6 against placebo plus mFOLFOX6. Median progression-free survival was 10.6 months against 8.7. Median overall survival was 18.2 months against 15.5.

GLOW enrolled 507 patients, using CAPOX as the chemotherapy backbone. Median progression-free survival was 8.2 months against 6.8. Median overall survival was 14.4 months against 12.2.

It starts with a larger loading infusion into a vein, then continues either every 3 weeks or every 2 weeks. The amounts are worked out from your body size by the team giving it.

The FDA lists these among the serious adverse reactions occurring in 2% or more: vomiting, nausea, neutropenia, diarrhea, decreased appetite, pneumonia, and pulmonary embolism. Nausea and vomiting are common enough with this drug to be planned for in advance, not managed after the fact.

Call 911 or go to an emergency department for:

• Vomiting blood, or material that looks like coffee grounds.
• Black, tarry stools.
• Feeling lightheaded or dizzy with any bleeding.
• Severe abdominal pain that comes on suddenly.
• Chest pain or sudden shortness of breath.

Call the oncology team right away, at any hour, for:

• A fever of 100.4°F (38°C) or higher during chemotherapy. CDC says fever may be the only sign of an infection, and an infection during chemotherapy can be life-threatening. If you cannot reach the team, go to an emergency department and say you are on chemotherapy.
• Being unable to keep any fluids down for 12 hours or more.

Call the oncology team the same day for:

• Vomiting that stops you taking your oral medicines.
• New swelling or pain in one leg.

• Is my tumor intestinal type or diffuse type?
• Have HER2 and CLDN18.2 been tested, and when will results arrive?
• Have I been tested for H. pylori?
• Is chemotherapy planned before surgery, and which regimen?
• Will I be tested for DPYD variants before fluorouracil or capecitabine?
• How much of my stomach would surgery remove, and when do I meet a dietitian?

Stomach Cancer is the full overview. Stomach Cancer Treatment goes deeper on the options above. Stomach Cancer Symptoms covers what led here.

• NCI PDQ — Gastric Cancer Treatment (Health Professional Version)
• NCI SEER — Cancer Stat Facts: Stomach Cancer
• U.S. Food and Drug Administration — FDA approves zolbetuximab-clzb with chemotherapy for gastric or gastroesophageal junction adenocarcinoma
• National Cancer Institute — Infection and Neutropenia during Cancer Treatment
• CDC — Watch out for fever during cancer treatment
• CDC — Cancer and blood clots
• MedlinePlus — Gastrointestinal bleeding
• MedlinePlus — Nausea and vomiting in adults