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C18 covers primary malignant neoplasms arising in the colon, including the cecum, appendix, ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, and sigmoid colon. It also includes overlapping or unspecified colonic sites. Rectal and rectosigmoid-junction malignancies are classified separately.
Common Histologies
- Conventional adenocarcinoma, by far the most common histology
- Mucinous adenocarcinoma
- Signet-ring cell carcinoma
- Medullary carcinoma
- Rare squamous, adenosquamous, or undifferentiated carcinomas
- Appendiceal epithelial tumors, including adenocarcinoma and goblet cell adenocarcinoma, when malignant
Malignant well-differentiated neuroendocrine tumors (“malignant carcinoid tumors”) of the colon are excluded from C18 and classified in the C7A.02- family.
Common Symptoms
Presentation varies by location and stage. Findings may include:
- Occult or overt gastrointestinal bleeding (K92.2)
- Iron-deficiency anemia, particularly with right-sided tumors (D50.9)
- Change in bowel habits, constipation, or narrowing of stool caliber (R19.4)
- Abdominal pain (R10.9), bloating (R14.0), or an abdominal mass (R19.00)
- Unintentional weight loss, fatigue, or reduced appetite (R63.4)
- Large-bowel obstruction or, less commonly, perforation (K56.609)
- Appendicitis-like symptoms or incidental discovery for appendiceal tumors (D37.3)
Early disease may be asymptomatic and detected through colorectal cancer screening.
Diagnosis
Evaluation commonly includes colonoscopy with biopsy and histopathologic confirmation. Baseline assessment may include complete blood count, liver tests, carcinoembryonic antigen (CEA), and contrast-enhanced imaging of the chest, abdomen, and pelvis. Complete colonic evaluation is important to identify synchronous lesions when clinically feasible.
Pathology reports generally document tumor type and grade, depth of invasion, lymphovascular and perineural invasion, margins, tumor deposits, lymph-node findings, and relevant molecular results.
Staging
Colon cancer is staged using the TNM system:
- T: depth of invasion through the bowel wall and involvement of adjacent structures
- N: regional lymph-node involvement and tumor deposits
- M: distant metastasis, commonly involving the liver, lungs, peritoneum, or distant lymph nodes
Adequate regional lymph-node evaluation is important after oncologic resection. Appendiceal malignancies may have distinct staging considerations depending on histology and pattern of peritoneal spread.
Molecular Markers
Commonly assessed biomarkers include:
- Mismatch-repair protein expression or microsatellite instability status
- KRAS and NRAS mutations
- BRAF V600E mutation
- HER2 amplification or overexpression in selected advanced tumors
- NTRK fusions and other actionable alterations in selected cases
Mismatch-repair deficiency may indicate Lynch syndrome, inform prognosis, and predict benefit from immune-checkpoint inhibition in advanced disease. Germline evaluation may be appropriate based on tumor findings, age, personal history, and family history.
Common Treatments
Treatment depends on site, histology, stage, molecular profile, resectability, and patient fitness.
- Localized disease: oncologic surgical resection with regional lymphadenectomy
- Selected stage III and high-risk stage II disease: adjuvant fluoropyrimidine-based chemotherapy, often with oxaliplatin
- Metastatic disease: systemic chemotherapy, targeted therapy, and/or immunotherapy according to molecular findings
- Selected limited metastatic disease: surgical resection or local treatment of liver, lung, or peritoneal metastases
- Obstruction or perforation: urgent surgery, diversion, or endoscopic decompression in selected cases
- Appendiceal malignancy: treatment varies substantially by histology and extent; selected peritoneal disease may be managed with cytoreductive surgery and intraperitoneal therapy at specialized centers
Scope Note
C18 identifies the anatomic site of a primary malignant neoplasm of the colon, with child codes distinguishing specific colonic segments, overlapping sites, or an unspecified site. It does not encode histologic subtype, grade, TNM stage, molecular profile, treatment status, recurrence, or metastatic burden. Metastases to the colon from another primary site are not coded as primary C18 malignancies, and distant metastases from a colon primary require appropriate secondary malignant neoplasm codes. Rectal, rectosigmoid-junction, and malignant carcinoid/neuroendocrine tumors are classified in other ICD-10-CM families.