C18.9 ICD-10-CM Code: Malignant neoplasm of colon, unspecified
HCC Buddy Code Card
Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.
FY 2026 Apr update / Neoplasms (C00-D49) / Malignant neoplasms of digestive organs (C15-C26)
C18.9
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceMalignant neoplasm of colon, unspecified
Cancer of the colon where the specific location within the colon is not specified or documented.
Buddy Insight
Unspecified colon cancer is used when documentation confirms malignancy in the colon but does not identify the specific segment.
CMS-HCC V28
MappedHCC 22
RAF 0.0
CMS-HCC V24
MappedHCC 11
RAF 0.306
ACA/HHS
00
RAF 0
ESRD/PACE
MappedHCC 11
RAF 0.0
RXHCC
00
RAF 0
Code Trumping
Basket needed
Code Book Path
Inclusion Terms
Official- Malignant neoplasm of large intestine NOS
Excludes 2
OfficialICD-10-CM does not list Excludes 2 notes for C18.9 in this effective period.
Related Child Codes
Includes
OfficialICD-10-CM does not list Includes notes for C18.9 in this effective period.
Excludes 1
Official- malignant carcinoid tumors of the colon (C7A.02-)
Code First
OfficialICD-10-CM does not list Code First sequencing instructions for C18.9 in this effective period.
Use Additional
OfficialICD-10-CM does not list Use Additional Code instructions for C18.9 in this effective period.
Code Also
OfficialICD-10-CM does not list Code Also instructions for C18.9 in this effective period.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.
Is C18.9 an HCC code?
Yes. C18.9 maps to Colorectal, Bladder, and Other Cancers under the CMS-HCC V28 risk adjustment model (and Colorectal, Bladder, and Other Cancers under V24).
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.
MEAT Criteria for C18.9
For C18.9to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.
- MMonitor: signs, symptoms, disease progression, or lab trending documented in the note
- EEvaluate: test results, medication response, or physical findings reviewed by the provider
- AAssess: explicit mention in the assessment or plan with acknowledgment of status
- TTreat: medication, referral, procedure, therapy, or counseling tied to the diagnosis
Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed C18.9 during that encounter, not just copy-forwarded from a problem list.
What This Code Means
C18.9 is the ICD-10-CM diagnosis code for malignant neoplasm of colon, unspecified. Cancer of the colon where the specific location within the colon is not specified or documented. C18.9 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering malignant neoplasms of digestive organs (c15-c26).
Under the CMS-HCC V28 risk adjustment model, C18.9 maps to Colorectal, Bladder, and Other Cancers (HCC 22) with a community, non-dual, aged base RAF weight of 0.000. Under the older V24 model, C18.9 mapped to the same category but with a base RAF weight of 0.306, V28 recalibrated weights across the entire model. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.
This is a less specific code; query the provider if the exact colon location can be determined from pathology or imaging reports. Because C18.9 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. When documentation is ambiguous, coders should issue a provider query rather than assume the highest-specificity variant.
HCC Buddy maintains structured V28 and V24 mapping, RAF weights, and MEAT documentation criteria for C18.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is a less specific code; query the provider if the exact colon location can be determined from pathology or imaging reports
- •Use only when the anatomical site cannot be determined from available documentation
Clinical Significance
Unspecified colon cancer is used when documentation confirms malignancy in the colon but does not identify the specific segment. This code represents a documentation improvement opportunity, as the anatomical location of colon cancer affects both treatment planning and prognostic assessment. Despite being unspecified, this diagnosis still maps to the same HCC as specified colon cancers, but lacks the audit defensibility of more specific codes.
Documentation Requirements
- ✓Confirmed malignancy of the colon
- ✓Review of colonoscopy, pathology, surgical, and imaging reports for segment identification before defaulting to unspecified
- ✓Histological type if known
- ✓Stage of disease
- ✓Treatment status
Commonly Confused Codes
- •C18.0-C18.7 — Specific colon segments: always assign the most specific code available from documentation
- •C18.8 — Overlapping sites: use when multi-segment involvement is confirmed, not when location is unknown
- •C19 — Rectosigmoid junction: if the cancer involves the junction, use this more specific code
- •C26.0 — Intestinal tract, part unspecified: even less specific; use C18.9 when colon is confirmed