2027 ICD-10-CM Diagnosis Code C21.0Malignant neoplasm of anus, unspecified

ICD-10-CM Codes›C00–D49›C15-C26›C21

ICD-10-CM C21.0
CMSSource: CMS FY 2027 ICD-10-CM dataset · Effective Oct 1, 2026 – Sep 30, 2027

C21.0 is a billable ICD-10-CM diagnosis code for malignant neoplasm of anus, unspecified. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 374 through 376. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 22 closely related codes. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Gastrointestinal cancers - anus.

For Medicare Advantage risk adjustment, C21.0 maps to CMS-HCC Category 22 (Bladder, Colorectal, and Other Cancers) under the V28 model, adding a risk factor of about 0.363 for a community, non-dual, aged beneficiary in payment year 2027.

Code Identity

ICD-10-CM Code
C21.0
Billable Status
Yes — Valid for Submission
Code Describes
Malignant neoplasm of anus, unspecified
Short Description
Malignant neoplasm of anus, unspecified
Same as the full description in the CMS dataset.
Parent Code
C21
Malignant neoplasm of anus and anal canal

Code Classification

ChapterC00–D49Neoplasms
SectionC15-C26Malignant neoplasms of digestive organs
CategoryC21Malignant neoplasm of anus and anal canal
This CodeC21.0Malignant neoplasm of anus, unspecified

Medicare Risk Adjustment (HCC)Billing

C21.0 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2027.

CMS-HCC V28 Category (Payment Model)
HCC 22— Bladder, Colorectal, and Other Cancers
Payment HCC · PY 2027 one of 126 ICD-10-CM codes in this category
Risk Adjustment Factor (RAF) Weight
+0.363
community, non-dual, aged · ranges 0.314–0.410 across segments
Hierarchy
Superseded by HCC 17, HCC 18, HCC 19, HCC 20, and HCC 21
a more severe related category takes the payment when both are reported
Prior Model (CMS-HCC V24)
HCC 11
V24 retired V28 pays 100% of MA risk scores since PY 2026
Other CMS Models
PACE (CMS-HCC V22): HCC 11 · ESRD (V21): HCC 11 · ESRD (V24): HCC 11
ESRD V21 weights: 0.076 dialysis, 0.284–0.298 functioning graft · ESRD V24 weights: 0.059 dialysis, 0.257–0.362 functioning graft
Part D (RxHCC)
Not mapped
C21.0 does not risk-adjust in the RxHCC prescription drug model

Source: CMS Payment Year 2027 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Adenocarcinoma of anus
  • Anorectal adenocarcinoma
  • Carcinoma of anus
  • Lymphoma of anus
  • Malignant mesenchymal neoplasm of anus
  • Malignant neoplasm of anus
  • Malignant neoplasm of rectosigmoid junction
  • Malignant neoplasm of rectum, rectosigmoid junction and anus
  • Primary adenocarcinoma of anus
  • Primary malignant mesenchymal neoplasm of anus
  • Primary malignant neoplasm of anus
  • Primary sarcoma of perineum
  • Primary squamous cell carcinoma of anus

Instructional NotesGuidance

Instructions from the official ICD-10-CM Tabular List that apply to C21.0: its own notes plus those printed at C21, block C15-C26, and Chapter 2. A note printed at a category, block or chapter applies to every code under it.

Excludes1

Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.

From Block C15-C26 Malignant neoplasms of digestive organs applies to 60 codes
  • Kaposi's sarcoma of gastrointestinal sites (C46.4)

Excludes2

Not included here: the excluded condition is not part of this code, but a patient may have both, so both codes may be reported.

From C21 Malignant neoplasm of anus and anal canal applies to 4 codes
From Block C15-C26 Malignant neoplasms of digestive organs applies to 60 codes
  • gastrointestinal stromal tumors (C49.A-)

Notes

General instructions on how these codes are used.

From Chapter 2 (C00-D49) Neoplasms applies to 1,730 codes
  • Functional activity
  • All neoplasms are classified in this chapter, whether they are functionally active or not. An additional code from Chapter 4 may be used, to identify functional activity associated with any neoplasm.
  • Morphology [Histology]
  • Chapter 2 classifies neoplasms primarily by site (topography), with broad groupings for behavior, malignant, in situ, benign, etc. The Table of Neoplasms should be used to identify the correct topography code. In a few cases, such as for malignant melanoma and certain neuroendocrine tumors, the morphology (histologic type) is included in the category and codes.
  • Primary malignant neoplasms overlapping site boundaries
  • A primary malignant neoplasm that overlaps two or more contiguous (next to each other) sites should be classified to the subcategory/code .8 ('overlapping lesion'), unless the combination is specifically indexed elsewhere. For multiple neoplasms of the same site that are not contiguous, such as tumors in different quadrants of the same breast, codes for each site should be assigned.
  • Malignant neoplasm of ectopic tissue
  • Malignant neoplasms of ectopic tissue are to be coded to the site mentioned, e.g., ectopic pancreatic malignant neoplasms are coded to pancreas, unspecified (C25.9).

Source: CMS ICD-10-CM Tabular List. How to read instructional notes.

Referenced in Other NotesGuidance

Instructional notes printed at other codes that name C21.0, its category, or a range that includes it.

Applicable To 2

These notes mention C21.0.

  • Z80.0 Family history of malignant neoplasm of digestive organs
    Conditions classifiable to C15-C26 names C15-C26, which includes this code
  • Z85.048 Personal history of other malignant neoplasm of rectum, rectosigmoid junction, and anus
    Conditions classifiable to C19-C21 names C19-C21, which includes this code

Excludes1 1

These codes carry an Excludes1 note naming C21.0: they are not reported together with it, unless the two conditions are unrelated.

  • C44.5 Other and unspecified malignant neoplasm of skin of trunk
    anus NOS (C21.0)

Excludes2 2

These codes carry an Excludes2 note naming C21.0: its condition is not part of those codes, and both may be reported when the patient has both.

  • C43.5 Malignant melanoma of trunk
    malignant neoplasm of anus NOS (C21.0)
  • C4A.5 Merkel cell carcinoma of trunk
    malignant neoplasm of anus NOS (C21.0)

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

Clinical ClassificationClinical

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

CCSR NEO016
Gastrointestinal cancers - anus
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Malignant Anal Neoplasm

    a primary or metastatic malignant neoplasm that affects the anal canal or perianal skin.

Table of NeoplasmsClinical

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

SiteMalig.
Primary		Malig.
Secondary		Ca
in situ		BenignUncertainUnspec.
anus, analC21.0C78.5D01.3D12.9D37.8D49.0

Patient EducationClinical

Anal Cancer

Your anus is the opening at the end of your large intestine. It is where stool (poop) leaves your body. The anus is formed partly from your outer layers of skin and partly from your intestine. Anal cancer is a type of cancer that forms in the tissues of your anus.

The full article covers:

  • What is anal cancer?
  • Who is more likely to develop anal cancer?
  • What are the symptoms of anal cancer?
  • How is anal cancer diagnosed?
  • What are the treatments for anal cancer?
  • Can anal cancer be prevented?

Read the full article at MedlinePlus

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

Convert C21.0 to ICD-9-CMHistory

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

ICD-9-CM
154.3 Malignant neo anus NOS
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 2017–2026No changes
FY 2027CurrentCurrent code set, no changesEffective October 1, 2026 through September 30, 2027.

Questions About C21.0Overview

What is the ICD-10 code for malignant neoplasm of anus, unspecified?

The ICD-10-CM code for malignant neoplasm of anus, unspecified is C21.0 (sometimes written as C210). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

Is C21.0 (Malignant neoplasm of anus and anal canal) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report malignant neoplasm of anus, unspecified on HIPAA-covered claims from October 1, 2026 through September 30, 2027.

What MS-DRG does C21.0 group to?

When malignant neoplasm of anus, unspecified is the principal diagnosis on an inpatient stay, it groups to MS-DRG 375, 374, 376, with relative weights from 0.8925 to 2.1101 depending on complications. Higher weights mean higher Medicare reimbursement.

Is C21.0 a CC or MCC?

CMS lists C21.0 as a CC (complication or comorbidity) for FY 2027. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 22 closely related codes in its exclusion list.

Can C21.0 be reported with C46.4?

Not as a rule. The Excludes1 note at block C15-C26, which applies to C21.0, lists "Kaposi's sarcoma of gastrointestinal sites (C46.4)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).

What HCC is C21.0?

C21.0 (malignant neoplasm of anus, unspecified) maps to CMS-HCC Category 22 (Bladder, Colorectal, and Other Cancers), commonly written as HCC 22, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2027. It mapped to HCC 11 under the retired V24 model. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. It does not map to any RxHCC in the Part D prescription drug model.

Does C21.0 risk-adjust for Medicare Advantage payment?

Yes. When documented and reported on a Medicare Advantage encounter, C21.0 adds a risk adjustment factor of about 0.363 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 0.314 to 0.410 depending on the payment segment). A more severe related category (HCC 17, HCC 18, HCC 19, HCC 20, and HCC 21) supersedes it when both are reported. See the full factor table on the HCC 22 category page.