What is ICD-10 code C21.8?

C21.8 is the ICD-10-CM diagnosis code for Malignant neoplasm of overlapping sites of rectum, anus and anal canal. It belongs to the ICD-10-CM chapter for neoplasms (c00-d49). C21.8 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. C21.8 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. C21.8 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. C21.8 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

Which HCC does C21.8 map to under V28?

C21.8 maps to CMS-HCC V28 HCC 22 (Bladder, Colorectal, and Other Cancers). The category numbers, labels, and conditions come from the CMS-HCC V28 model file for payment year 2026.

What documentation supports C21.8?

Use this code only when the malignant tumor clearly involves two or more of these three sites (rectum, anus, anal canal); if the cancer is limited to a single site, use the more specific site-specific code instead Verify documentation specifies the overlapping nature of the neoplasm and confirm the tumor is not better described by codes C20 (rectal cancer) or C21.0-C21.2 (anus/anal canal specific codes) before assigning this overlapping sites code For C21.8, 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. See the MEAT criteria checklist at /meat-criteria for what Monitor, Evaluate, Assess, or Treat looks like in a note. One mnemonic element does not establish that a diagnosis is reportable.

2026 ICD-10-CM C21.8: Malignant neoplasm

C21.8 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 coding software

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Code lookupC21.8

Effective periodFY2026 Apr Update (Apr 1-Sep 30)

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

C21.8

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

Malignant neoplasm of overlapping sites of rectum, anus and anal canal

This code describes cancer that involves multiple overlapping areas of the lower digestive tract, including the rectum (the last part of the large intestine), the anus (the opening at the end of the digestive tract), and the anal canal (the short passage leading to the anus). When a tumor spans across these adjacent areas rather than being confined to just one location, this code is used.

Buddy Insight

Overlapping sites of rectum, anus, and anal canal indicates a tumor spanning multiple structures in the lowest portion of the gastrointestinal tract.

Overview Risk Models Code Book Documentation Audit Notes Edition comparison

CMS-HCC V28

Mapped

HCC 22

Bladder, Colorectal, and Othe...

Code-level coefficient reference

CMS-HCC V24

Historical

Not used for standard CY2026 Medicare Advantage payment. Select a historical period to review V24.

Not used for CY2026 payment

ACA/HHS

Mapped

HCC 011

HHS-HCC 011 adult, RAF varies...

Code-level coefficient reference

ESRD/PACE

Mapped

HCC 11

Colorectal/Bladder/and Other ...

Code-level coefficient reference

RXHCC

N/A

—

Not risk-adjusted for this model/period.

Not mapped

Code Book Path

Official

C21Malignant neoplasm of anus and anal canal C21.8Malignant neoplasm of overlapping sites of rectum, anus and anal canal

Inclusion Terms

Official

Malignant neoplasm of anorectal junction
Malignant neoplasm of anorectum
Primary malignant neoplasm of two or more contiguous sites of rectum, anus and anal canal

Excludes 2

Official

gastrointestinal stromal tumors (C49.A-)Inherited from C15-C26, C21
malignant carcinoid tumors of the colon (C7A.02-)Inherited from C15-C26, C21
malignant melanoma of anal margin (C43.51)Inherited from C15-C26, C21
malignant melanoma of anal skin (C43.51)Inherited from C15-C26, C21
malignant melanoma of perianal skin (C43.51)Inherited from C15-C26, C21
other and unspecified malignant neoplasm of anal margin (C44.500, C44.510, C44.520, C44.590)Inherited from C15-C26, C21
other and unspecified malignant neoplasm of anal skin (C44.500, C44.510, C44.520, C44.590)Inherited from C15-C26, C21
other and unspecified malignant neoplasm of perianal skin (C44.500, C44.510, C44.520, C44.590)Inherited from C15-C26, C21

Related Codes

Official

C21.0Malignant neoplasm of anus, unspecified C21.1Malignant neoplasm of anal canal C21.2Malignant neoplasm of cloacogenic zone

Includes

Official

No Includes notes are included in this display for C21.8. 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

Code First

Official

No Code First sequencing instructions are included in this display for C21.8. Check the code and parent instructions in the Code Book.

Use Additional

Official

No Use Additional Code instructions are included in this display for C21.8. Check the code and parent instructions in the Code Book.

Code Also

Official

No Code Also instructions are included in this display for C21.8. Check the code and parent instructions in the Code Book.

Buddy Documentation Tip

HCC Buddy guidance

Documentation confirming tumor involvement of two or more structures (rectum, anus, anal canal)

Specific structures involved and extent of disease

Histological type (squamous, adenocarcinoma, or mixed)

Sphincter involvement assessment

MEAT Support

HCC Buddy guidance

Documentation confirming tumor involvement of two or more structures (rectum, anus, anal canal)

Specific structures involved and extent of disease

Histological type (squamous, adenocarcinoma, or mixed)

Sphincter involvement assessment

Audit Caution

HCC Buddy guidance

Using C21.8 when the tumor is confined to a single site — only use for true multi-structure involvement

Confusing overlapping (one tumor spanning multiple structures) with synchronous tumors at different sites

Not distinguishing between anorectal and rectosigmoid (C19) overlap — these are different codes for different anatomical boundaries

Common Mistakes

HCC Buddy guidance

C20 — Rectum: use when tumor is confined to the rectum

C21.0 — Anus, unspecified: use when tumor is confined to the anal region without overlapping

C21.1 — Anal canal: use when tumor is confined to the canal

C18.8 — Overlapping sites of colon: different anatomical region; this code is for the distal gastrointestinal tract

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 C21.8 an HCC code?

Yes. C21.8 (Malignant neoplasm of overlapping sites of rectum, anus and anal canal) 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: C21.8 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: C21.8
Description: Malignant neoplasm of overlapping sites of rectum, anus and anal canal
HCC (V28): HCC 22 — Bladder, Colorectal, and Other Cancers
RAF reference coefficient: 0.363
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 22, Bladder, Colorectal, and Other Cancers

0.363

ESRDHCC 11, Colorectal/Bladder/and Other Cancers

Not separately weighted

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 C21.8 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT review for C21.8

For C21.8, 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

C21.8 is the ICD-10-CM diagnosis code for malignant neoplasm of overlapping sites of rectum, anus and anal canal. This code describes cancer that involves multiple overlapping areas of the lower digestive tract, including the rectum (the last part of the large intestine), the anus (the opening at the end of the digestive tract), and the anal canal (the short passage leading to the anus). When a tumor spans across these adjacent areas rather than being confined to just one location, this code is used. C21.8 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, C21.8 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 C21.8; 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.

Use this code only when the malignant tumor clearly involves two or more of these three sites (rectum, anus, anal canal); if the cancer is limited to a single site, use the more specific site-specific code instead. For C21.8, 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 C21.8 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Use this code only when the malignant tumor clearly involves two or more of these three sites (rectum, anus, anal canal); if the cancer is limited to a single site, use the more specific site-specific code instead
•Verify documentation specifies the overlapping nature of the neoplasm and confirm the tumor is not better described by codes C20 (rectal cancer) or C21.0-C21.2 (anus/anal canal specific codes) before assigning this overlapping sites code

Clinical Significance

Overlapping sites of rectum, anus, and anal canal indicates a tumor spanning multiple structures in the lowest portion of the gastrointestinal tract. These advanced tumors present significant treatment challenges because they may involve the anal sphincter complex, requiring careful assessment of sphincter preservation versus abdominoperineal resection. The overlapping nature typically indicates larger or more advanced disease with complex treatment considerations.

Documentation Requirements

✓Documentation confirming tumor involvement of two or more structures (rectum, anus, anal canal)
✓Specific structures involved and extent of disease
✓Histological type (squamous, adenocarcinoma, or mixed)
✓Sphincter involvement assessment
✓Stage of disease and treatment plan

Commonly Confused Codes

•C20: Rectum: use when tumor is confined to the rectum
•C21.0: Anus, unspecified: use when tumor is confined to the anal region without overlapping
•C21.1: Anal canal: use when tumor is confined to the canal
•C18.8: Overlapping sites of colon: different anatomical region; this code is for the distal gastrointestinal tract