C18.9: Malignant neoplasm of colon, unspecified

C18.9 maps to HCC 22: Bladder, Colorectal, and Other Cancers.

CMS-HCC v28 is fully phased in at 100 percent for payment year 2026.

Two metastatic categories sit above HCC 22 in the v28 cancer hierarchy. HCC 17 covers cancer metastatic to lung, liver, brain and other organs. HCC 18 covers cancer metastatic to bone together with other and unspecified metastatic cancer.

For Medicare Advantage risk adjustment, an active colon malignancy must be documented and assessed at least once each calendar year. The code must appear on a dated encounter note rather than only on a problem list.

MS-DRG 374: Digestive Malignancy with MCC.

MS-DRG 375: Digestive Malignancy with CC.

MS-DRG 376: Digestive Malignancy without CC/MCC.

A medical admission with C18.9 as the principal diagnosis groups to MS-DRG 374, 375 or 376 under the FY2027 MS-DRG v44 grouper.

With C18.9 as the principal diagnosis, an admission with a major bowel procedure such as a colectomy groups instead to the surgical set in MDC 06 under the FY2027 MS-DRG v44 grouper. MS-DRG 329: Major Small and Large Bowel Procedures with MCC. MS-DRG 330: Major Small and Large Bowel Procedures with CC. MS-DRG 331: Major Small and Large Bowel Procedures without CC/MCC.

Section I.C.2.a: when the malignancy is chiefly responsible for the admission or encounter and treatment is directed at the primary site, the primary malignancy is the principal or first-listed diagnosis. The one stated exception is an encounter chiefly for administration of chemotherapy, immunotherapy or external beam radiation therapy. In that case the Z51.- code is first listed and the malignancy is reported as a secondary diagnosis.

Section I.C.2.b covers a primary neoplasm with metastasis where treatment is directed toward the secondary site only. The secondary neoplasm is designated as the principal diagnosis even though the primary malignancy is still present.

Section I.C.2.l: when the encounter is for treatment of the primary malignancy, the primary site is sequenced first, followed by any metastatic sites.

Section I.C.2.m sets three conditions for moving a former colon primary out of C18 and into category Z85. The malignancy has been excised or eradicated from its site. No further treatment is directed to that site. There is no evidence of any existing primary malignancy at that site.

There is no National Coverage Determination that governs diagnosis code selection for colon cancer.

C18.9 supports medical necessity for oncology evaluation and management, chemotherapy administration and surveillance imaging when the malignancy is documented as active.

A screening encounter is reported differently. Z12.11 covers an encounter for screening for malignant neoplasm of colon in a patient with no known malignancy, not an encounter for a diagnosed colon cancer.

These risk adjustment rules apply to Medicare fee for service and to Medicare Advantage plans using CMS-HCC v28 for payment year 2026.

C18.9 states that the colon segment is unknown. When the operative report or the pathology report names a segment, the claim and the record disagree. That mismatch is what a coding audit finds.

Omitting the secondary neoplasm codes in metastatic disease leaves the encounter in HCC 22 when the documented metastases would support HCC 17 or HCC 18 under CMS-HCC v28.

Reporting C18.9 for a resolved colon cancer overstates active disease. Section I.C.2.m directs Z85.038 for a former colon primary with no further treatment and no residual disease.

Assigning C18.7 to a rectosigmoid junction tumor contradicts the Excludes1 note at C18.7. The correct code is C19.

C18.9 is a CC. It appears in Appendix G, diagnoses defined as complications or comorbidities, in the ICD-10 MS-DRG Definitions Manual Version 44.0. As a secondary diagnosis it can move a case to a with-CC tier where the DRG pair offers one.