Editorial orientation provided by MedAPI — not part of the coding instruction.
C16 covers primary malignant neoplasms of the stomach, with subcodes identifying sites such as the cardia, fundus, body, pyloric antrum, pylorus, and gastric curvatures.
Common Histologies
- Adenocarcinoma is the predominant gastric malignancy, including intestinal, diffuse, signet-ring cell, and mixed patterns.
- Less common epithelial malignancies include adenosquamous and undifferentiated carcinomas.
- Gastrointestinal stromal tumor of the stomach is coded separately to C49.A2.
- Primary gastric lymphoma is classified under the applicable lymphoid neoplasm code; for example, extranodal marginal zone lymphoma of mucosa-associated lymphoid tissue (MALT lymphoma) is coded to C88.4.
- Malignant carcinoid tumor of the stomach is excluded from C16 and coded to C7A.092.
Common Symptoms
Early gastric cancer may be asymptomatic. Presentations can include:
- Persistent epigastric discomfort (R10.13), dyspepsia (K30), or early satiety (R68.81)
- Unintentional weight loss and anorexia (R63.4)
- Nausea (R11.0), vomiting (R11.10), or gastric outlet obstruction (K31.89)
- Iron-deficiency anemia (D50.9), fatigue (E61.1), melena (K92.1), or occult gastrointestinal bleeding (K92.2)
- Dysphagia when the cardia or gastroesophageal junction is involved (R13.19)
- Advanced manifestations such as ascites (R18.8), jaundice (R17), palpable lymph nodes (R59.9), or symptoms of peritoneal disease
Diagnosis
Diagnosis generally requires upper endoscopy with biopsy. Pathology establishes histologic type and grade. Evaluation commonly includes:
- Contrast-enhanced CT of the chest, abdomen, and pelvis
- Endoscopic ultrasound for local depth and regional nodal assessment in selected potentially curable tumors
- Diagnostic laparoscopy with peritoneal cytology for selected locally advanced cancers, because small-volume peritoneal disease may be occult on imaging
- Laboratory assessment for anemia, nutritional status, hepatic function, and treatment fitness
Documentation should clearly identify the primary site as stomach versus esophagus (C15); note that C16.0 (cardia) includes the cardio-esophageal/gastroesophageal junction terms, so junctional tumors are assigned per documented point of origin.
Staging
Gastric cancer is staged using the TNM system:
- T: depth of invasion through the gastric wall and into adjacent structures
- N: extent of regional lymph-node involvement
- M: presence of distant metastatic disease
Common metastatic sites include the liver, peritoneum, distant lymph nodes, lung, bone, and ovaries. Stage strongly influences whether management is endoscopic, surgical, multimodal, or palliative.
Molecular Markers
Biomarker testing is particularly relevant in unresectable, recurrent, or metastatic gastric adenocarcinoma and may include:
- HER2 amplification or overexpression
- Mismatch-repair deficiency or microsatellite instability
- PD-L1 expression, commonly reported as combined positive score
- CLDN18.2 expression
- Rare actionable alterations, such as NTRK fusions, when broad molecular testing is appropriate
Epstein–Barr virus association and other molecular features may provide additional biologic or prognostic context.
Common Treatments
Treatment depends on stage, location, histology, biomarkers, symptoms, and patient fitness.
- Very early, appropriately selected superficial cancers may undergo endoscopic resection.
- Resectable disease is generally treated with subtotal or total gastrectomy and regional lymph-node dissection, often combined with perioperative or adjuvant chemotherapy.
- Chemoradiation may be used in selected postoperative, unresectable, or palliative settings.
- Advanced adenocarcinoma is commonly treated with fluoropyrimidine- and platinum-based systemic therapy.
- Biomarker-directed options may include HER2-targeted therapy, immune-checkpoint inhibition, or CLDN18.2-targeted therapy in eligible patients.
- Palliative interventions may address bleeding, obstruction, pain, malnutrition, or ascites.
Scope Note
C16 identifies a primary malignant neoplasm of the stomach and, through its subcodes, the documented gastric subsite. It does not encode histologic subtype, grade, TNM stage, molecular profile, treatment status, or metastatic sites. C16.8 applies to a single tumor overlapping contiguous gastric subsites when the point of origin cannot be determined; C16.9 indicates an unspecified gastric site. Metastatic involvement of the stomach from another primary is coded as a secondary neoplasm (C78.89), not C16. Gastric gastrointestinal stromal tumors, primary gastric lymphomas, and malignant gastric carcinoid tumors are classified separately under their applicable histology-specific codes.