2026 ICD-10-CM Diagnosis Code C18.1Malignant neoplasm of appendix

ICD-10-CM Codes›C00–D49›C15-C26›C18

ICD-10-CM C18.1
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

C18.1 is a billable ICD-10-CM diagnosis code for malignant neoplasm of appendix. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 374 through 376. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Gastrointestinal cancers - colorectal.

Code Identity

ICD-10-CM Code
C18.1
Billable Status
Yes — Valid for Submission
Code Describes
Malignant neoplasm of appendix
Short Description
Malignant neoplasm of appendix
Same as the full description in the CMS dataset.
Parent Code
C18
Malignant neoplasm of colon

Code Classification

ChapterC00–D49Neoplasms
SectionC15-C26Malignant neoplasms of digestive organs
CategoryC18Malignant neoplasm of colon
This CodeC18.1Malignant neoplasm of appendix

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Adenocarcinoma of appendix
  • Carcinoma of appendix
  • Lymphoma of appendix
  • Malignant mesenchymal neoplasm of appendix
  • Malignant neoplasm of appendix
  • Mucinous adenocarcinoma of gastrointestinal tract
  • Primary malignant mesenchymal neoplasm of appendix
  • Primary malignant neoplasm of appendix
  • Primary mucinous adenocarcinoma of appendix
  • Primary mucinous adenocarcinoma of gastrointestinal tract
  • Primary mucinous carcinoma of digestive organ

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Tumor
      • mucocarcinoid
        • unspecified site

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR NEO015
Gastrointestinal cancers - colorectal
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Appendiceal Neoplasms

    tumors or cancer of the appendix.

  • Appendix

    a worm-like blind tube extension from the cecum.

  • Colon

    the segment of large intestine between the cecum and the rectum. it includes the ascending colon; the transverse colon; the descending colon; and the sigmoid colon.

Table of NeoplasmsClinical

Anatomical sites in the Table of Neoplasms that reference this code family.

SiteMalig.
Primary		Malig.
Secondary		Ca
in situ		BenignUncertainUnspec.
appendixC18.1C78.5D01.0D12.1D37.3D49.0
intestine, intestinal › large › appendixC18.1C78.5D01.0D12.1D37.3D49.0

Patient EducationClinical

Colorectal Cancer

Colorectal cancer is cancer that develops in the tissues of the colon or rectum. Your colon and rectum are part of your digestive system:

The full article covers:

  • What is colorectal cancer?
  • What causes colorectal cancer?
  • Who is more likely to develop colorectal cancer?
  • What are the symptoms of colorectal cancer?
  • What is screening for colorectal cancer and who needs it?
  • How is colorectal cancer diagnosed?
  • What are the treatments for colorectal cancer?
  • Can colorectal cancer be prevented?

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert C18.1 to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
153.5 Malignant neo appendix
Exact Match The mapping is direct, with no qualifiers.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About C18.1Overview

Is C18.1 (Malignant neoplasm of colon) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report malignant neoplasm of appendix on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does C18.1 group to?

When malignant neoplasm of appendix is the principal diagnosis on an inpatient stay, it groups to MS-DRG 374, 375, 376, with relative weights from 0.9243 to 2.1387 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of C18.1?

Under the General Equivalence Mappings, malignant neoplasm of appendix converts to ICD-9-CM 153.5 (malignant neo appendix). The mapping is a direct match.

Footnotes

[1] Chronic - a chronic condition code indicates a condition lasting 12 months or longer and its effect on the patient based on one or both of the following criteria:

  • The condition results in the need for ongoing intervention with medical products,treatment, services, and special equipment
  • The condition places limitations on self-care, independent living, and social interactions.