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C18.1

Billable

Malignant neoplasm of appendix

Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)

Is C18.1 an HCC code?

Yes. C18.1 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

V28HCC 22 — Colorectal, Bladder, and Other Cancers
0.000
V24HCC 11 — Colorectal, Bladder, and Other Cancers
0.306
ESRDHCC 11 — Colorectal, Bladder, and Other Cancers
0.000

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 C18.1

For C18.1 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 C18.1 during that encounter — not just copy-forwarded from a problem list.

What This Code Means

C18.1 is the ICD-10-CM diagnosis code for malignant neoplasm of appendix. Cancer that starts in the appendix, a small tube-shaped organ connected to the large intestine. This is a type of malignant tumor that can spread to other parts of the body if not treated. C18.1 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, C18.1 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, C18.1 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.

Verify the histological type (adenocarcinoma, neuroendocrine, mucinous, etc.) as this may require additional coding with laterality or behavior codes. Because C18.1 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 C18.1 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

  • •Verify the histological type (adenocarcinoma, neuroendocrine, mucinous, etc.) as this may require additional coding with laterality or behavior codes
  • •Ensure you have documented confirmation of malignancy and that this is not a benign neoplasm (C80 series) or carcinoma in situ (D12.3)

Clinical Significance

Malignant neoplasm of the appendix is a relatively uncommon cancer that may be discovered incidentally during appendectomy for presumed appendicitis. Appendiceal cancers include several histological types with very different prognoses, from low-grade mucinous neoplasms to aggressive adenocarcinomas and neuroendocrine tumors. Accurate coding is essential because treatment ranges from appendectomy alone for early-stage disease to cytoreductive surgery with hyperthermic intraperitoneal chemotherapy for mucinous tumors with peritoneal spread.

Documentation Requirements

  • ✓Confirmation of malignancy (not carcinoma in situ or benign neoplasm)
  • ✓Histological type: mucinous adenocarcinoma, goblet cell carcinoid, neuroendocrine, or non-mucinous adenocarcinoma
  • ✓Stage of disease including peritoneal involvement
  • ✓Presence or absence of pseudomyxoma peritonei
  • ✓Whether diagnosis was incidental at appendectomy or clinically suspected

Commonly Confused Codes

D12.1 — Benign neoplasm of appendix: benign tumors do not map to HCCs; confirm malignancyC18.0 — Cecum: the appendix arises from the cecum; ensure primary site is appendix, not cecumC7A.020 — Malignant carcinoid tumor of appendix: carcinoid/neuroendocrine tumors of the appendix have their own specific codeD3A.020 — Benign carcinoid tumor of appendix: benign carcinoid is different from malignant

Code Hierarchy

└C18Malignant neoplasm of colon└C18.1Malignant neoplasm of appendix
└C18.1Malignant neoplasm of appendix

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