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C18.6 ICD-10-CM Code: Malignant neoplasm of descending colon

C18.6 maps to CMS-HCC V28 22 (RAF 0.363). Documentation must support MEAT. MEAT criteria · RAF calculator · free HCC coding tools

ICD-10-CM Code View

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FY 2026 Apr update / Neoplasms (C00-D49) / Malignant neoplasms of digestive organs (C15-C26)

C18.6

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Malignant neoplasm of descending colon

Cancer that occurs in the descending colon, which is the portion of the large intestine that runs downward on the left side of the abdomen.

Buddy the Bee presenting code insight

Buddy Insight

Malignant neoplasm of the descending colon occurs in the left-sided portion of the large intestine running vertically from the splenic flexure to the sigmoid colon.

CMS-HCC V28

HCC 22

RAF 0.363

CMS-HCC V24

HCC 11

RAF 0.307

ACA/HHS

HCC 11

Varies by metal level

ESRD/PACE

HCC 11

RAF 0.059

RXHCC

N/A

—

Not mapped

Code Book Path

Official
C18Malignant neoplasm of colon
C18.6Malignant neoplasm of descending colon

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for C18.6 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for C18.6 in this effective period.

Related Child Codes

Official
C18.0Malignant neoplasm of cecum
C18.1Malignant neoplasm of appendix
C18.2Malignant neoplasm of ascending colon
C18.3Malignant neoplasm of hepatic flexure
C18.4Malignant neoplasm of transverse colon

Includes

Official

ICD-10-CM does not list Includes notes for C18.6 in this effective period.

Excludes 1

Official
  • malignant carcinoid tumors of the colon (C7A.02-)

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for C18.6 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for C18.6 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for C18.6 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance
Documentation confirming descending colon as the tumor location
Histological type and grade
Stage of disease with lymph node status
KRAS/NRAS/BRAF mutation status if tested (affects targeted therapy eligibility)

MEAT Support

HCC Buddy guidance
Documentation confirming descending colon as the tumor location
Histological type and grade
Stage of disease with lymph node status
KRAS/NRAS/BRAF mutation status if tested (affects targeted therapy eligibility)

Audit Caution

HCC Buddy guidance
Confusing descending colon with sigmoid colon — the sigmoid begins at the pelvic brim where the colon curves
Using C18.9 when left hemicolectomy pathology specifies descending colon involvement
Coding a tumor at the junction of descending and sigmoid as descending colon when documentation suggests the sigmoid (or overlapping)

Common Mistakes

HCC Buddy guidance
C18.5 — Splenic flexure: the splenic flexure is above the descending colon; confirm documentation specifies the straight descending segment
C18.7 — Sigmoid colon: the sigmoid begins where the descending colon curves into the S-shape; verify anatomical boundary
C18.9 — Colon, unspecified: use C18.6 when descending colon is documented

Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →

Is C18.6 an HCC code?

Yes. C18.6 (Malignant neoplasm of descending colon) maps to Bladder, Colorectal, and Other Cancers under the CMS-HCC V28 risk adjustment model (and Colorectal, Bladder, and Other Cancers under V24), with a community non-dual aged RAF of 0.363. It is billable for payment year 2026.

Coder answer: C18.6 is billable and maps to V28 HCC 22, Bladder, Colorectal, and Other Cancers. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code
C18.6
Description
Malignant neoplasm of descending colon
HCC (V28)
HCC 22 — Bladder, Colorectal, and Other Cancers
RAF
0.363
Billable
Yes
Payment year
2026

HCC Category Mapping

V28HCC 22, Bladder, Colorectal, and Other Cancers
0.363
V24HCC 11, Colorectal, Bladder, and Other Cancers
0.307
ESRDHCC 11, Colorectal, Bladder, and Other Cancers
0.059

Each model's RAF is its CMS base weight for that model's standard population, so weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. 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.

Work C18.6 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for C18.6

For C18.6 to 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 C18.6 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

C18.6 is the ICD-10-CM diagnosis code for malignant neoplasm of descending colon. Cancer that occurs in the descending colon, which is the portion of the large intestine that runs downward on the left side of the abdomen. C18.6 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, C18.6 maps to Bladder, Colorectal, and Other Cancers (HCC 22) with a community, non-dual, aged base RAF weight of 0.363. Under the older CMS-HCC V24 model, C18.6 maps to Colorectal, Bladder, and Other Cancers (HCC 11) with a community, non-dual, aged base RAF weight of 0.307. 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.

Distinguish between descending colon (C18.6) and sigmoid colon (C18.7) based on anatomical documentation. Because C18.6 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 C18.6 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

  • •Distinguish between descending colon (C18.6) and sigmoid colon (C18.7) based on anatomical documentation
  • •Confirm the tumor does not extend into the sigmoid colon, which would require a different code

Clinical Significance

Malignant neoplasm of the descending colon occurs in the left-sided portion of the large intestine running vertically from the splenic flexure to the sigmoid colon. Left-sided colon cancers have distinct molecular profiles compared to right-sided cancers, with different responses to targeted therapies. These tumors are more likely to present with obstructive symptoms due to the narrower lumen of the descending colon compared to the right colon.

Documentation Requirements

  • ✓Documentation confirming descending colon as the tumor location
  • ✓Histological type and grade
  • ✓Stage of disease with lymph node status
  • ✓KRAS/NRAS/BRAF mutation status if tested (affects targeted therapy eligibility)
  • ✓Colonoscopy or surgical pathology confirmation

Commonly Confused Codes

  • •C18.5: Splenic flexure: the splenic flexure is above the descending colon; confirm documentation specifies the straight descending segment
  • •C18.7: Sigmoid colon: the sigmoid begins where the descending colon curves into the S-shape; verify anatomical boundary
  • •C18.9: Colon, unspecified: use C18.6 when descending colon is documented

Child Codes

C18.0Malignant neoplasm of cecum
HCC 22
C18.1Malignant neoplasm of appendix
HCC 22
C18.2Malignant neoplasm of ascending colon
HCC 22
C18.3Malignant neoplasm of hepatic flexure
HCC 22
C18.4Malignant neoplasm of transverse colon
HCC 22
C18.5Malignant neoplasm of splenic flexure
HCC 22
C18.6Malignant neoplasm of descending colon
HCC 22
C18.7Malignant neoplasm of sigmoid colon
HCC 22
C18.8Malignant neoplasm of overlapping sites of colon
HCC 22
C18.9Malignant neoplasm of colon, unspecified
HCC 22

Code Hierarchy

└C18Malignant neoplasm of colon└C18.6Malignant neoplasm of descending colon
└C18.6Malignant neoplasm of descending colon

Because C18.6 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score.

C18.6 maps to CMS-HCC V28 category 22, Bladder, Colorectal, and Other Cancers. See the ICD-10 to HCC mapping hub for how the V28 crosswalk works. Because C18.6 carries a payment HCC, you can see what it adds to a RAF score and check the documentation the chart needs before it is submitted.

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