C18.1 ICD-10-CM Code: Malignant neoplasm of appendix
C18.1 maps to CMS-HCC V28 22. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · HCC Buddy coding tools
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FY 2026 Apr update / Neoplasms (C00-D49) / Malignant neoplasms of digestive organs (C15-C26)
C18.1
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceMalignant 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.
Buddy Insight
Malignant neoplasm of the appendix is a relatively uncommon cancer that may be discovered incidentally during appendectomy for presumed appendicitis.
CMS-HCC V28
MappedHCC 22
Coefficient HCC 22: 0.363 (Community Non-Dual Aged (CNA))
CMS-HCC V24
HistoricalHistorical
Not used for CY2026 payment
ACA/HHS
MappedHCC 011
Code-level coefficient reference
ESRD/PACE
MappedHCC 11
Code-level coefficient reference
RXHCC
N/A—
Not mapped
Code Book Path
Inclusion Terms
OfficialNo inclusion terms are included in this display for C18.1. Check the code and parent instructions in the Code Book.
Excludes 2
Official- gastrointestinal stromal tumors (C49.A-)Inherited from C15-C26
Related Codes
Includes
OfficialNo Includes notes are included in this display for C18.1. Check the code and parent instructions in the Code Book.
Excludes 1
Official- Kaposi's sarcoma of gastrointestinal sites (C46.4)Inherited from C15-C26, C18
- malignant carcinoid tumors of the colon (C7A.02-)Inherited from C15-C26, C18
Code First
OfficialNo Code First sequencing instructions are included in this display for C18.1. Check the code and parent instructions in the Code Book.
Use Additional
OfficialNo Use Additional Code instructions are included in this display for C18.1. Check the code and parent instructions in the Code Book.
Code Also
OfficialNo Code Also instructions are included in this display for C18.1. Check the code and parent instructions in the Code Book.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
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 C18.1 an HCC code?
Yes. C18.1 (Malignant neoplasm of appendix) maps to HCC 22, Bladder, Colorectal, and Other Cancers under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.363. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.
Coder answer: C18.1 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
- C18.1
- Description
- Malignant neoplasm of appendix
- HCC (V28)
- HCC 22 — Bladder, Colorectal, and Other Cancers
- RAF reference coefficient
- 0.363
- Billable
- Yes
- Payment year
- 2026
HCC Category Mapping
These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. 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 context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.
Work C18.1 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →
MEAT review for C18.1
For C18.1, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule.
- 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
Coder workflow notes
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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 Bladder, Colorectal, and Other Cancers (HCC 22) with a source-labeled community, non-dual, aged reference coefficient of 0.363. No V24 mapping is shown for C18.1; use the applicable model and payment year when reviewing the V28 mapping. For CY2026 non-PACE Medicare Advantage, CMS uses 100% of the 2024 CMS-HCC model (V28). PACE uses a separate model blend. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.
Verify the histological type (adenocarcinoma, neuroendocrine, mucinous, etc.) as this may require additional coding with laterality or behavior codes. For C18.1, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule. 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, source-labeled coefficient references, 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
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 malignancy
- •C18.0: Cecum: the appendix arises from the cecum; ensure primary site is appendix, not cecum
- •C7A.020: Malignant carcinoid tumor of appendix: carcinoid/neuroendocrine tumors of the appendix have their own specific code
- •D3A.020: Benign carcinoid tumor of appendix: benign carcinoid is different from malignant