The ICD-10-CM code for colon cancer is C18.9 (Malignant neoplasm of colon, unspecified), a billable code reported when a malignant neoplasm of the colon is documented without specifying the anatomic site.
Primary codes
Billable
Billable
Extension
—
7th character
—
POA exempt
No
Inclusion terms
- Malignant neoplasm of large intestine NOS
Coding risks
- Assigning when documentation supports a more specific code
Hierarchy
Code comparison
Compare key differences between related codes to select the most specific option.
| Code | Description | Scenario |
|---|---|---|
| C18.9 | Malignant neoplasm of colon, unspecified | Colon cancer documented without specifying the anatomic segment involved. |
| C18.8 | Malignant neoplasm of overlapping sites of colon | Colon cancer spanning two contiguous colonic segments where no single site of origin can be determined. |
| C18.7 | Malignant neoplasm of sigmoid colon | Colon cancer localized to the sigmoid colon, confirmed by pathology or imaging. |
| C18.2 | Malignant neoplasm of ascending colon | Colon cancer with tumor localized specifically to the ascending colon per pathology or imaging. |
| C18.6 | Malignant neoplasm of descending colon | Colon cancer localized to the descending colon, confirmed by imaging or pathology report. |
Select the most specific code supported by the encounter documentation. More specific codes improve reimbursement accuracy and reduce audit risk.
Code ranges
Malignant neoplasm of colon
Category for primary malignant tumors arising in specific anatomic segments of the colon, from cecum through sigmoid, or unspecified site.
Malignant neoplasms of digestive organs
Range covering primary malignant neoplasms of the esophagus, stomach, small and large intestine, rectum, anus, liver, pancreas, and related digestive sites.
Malignant neoplasm of rectosigmoid junction
Category for primary malignant neoplasms located at the anatomic junction between the sigmoid colon and rectum.
Malignant neoplasm of rectum
Category for primary malignant neoplasms arising in the rectum proper, distal to the rectosigmoid junction.
Secondary malignant neoplasm of large intestine and rectum
Category for metastatic tumors deposited in the colon or rectum from a primary malignancy located elsewhere.
Coding risks
Avoid these common coding issues when reporting Colon Cancer.
Impact
Defaulting to C18.9 when a specific segment is supportable may reduce claim specificity and downstream reporting accuracy.
Mitigation
Review pathology and operative notes for anatomic segment language before assigning C18.9 as unspecified.
Related codes
Ancillary and co-coding
Codes commonly reported with the primary diagnosis on the same claim.
Z-code variants
Family history, personal history, or screening encounter codes related to this condition.
Differential codes
Alternative or commonly confused codes to consider when documentation suggests a different condition than the primary diagnosis.
Frequently asked questions
Use C18.9 only when the chart does not specify the anatomic subsite. Documented subsites like sigmoid or ascending require the specific C18 subcategory.
References
Compiled from the following authoritative coding references. Last updated 2026-08-04.