C21.1 ICD-10-CM Code: Malignant neoplasm of anal canal
C21.1 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)
C21.1
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceMalignant neoplasm of anal canal
Cancer that develops in the anal canal, which is the short passage that connects the rectum to the external opening of the anus.
Buddy Insight
Malignant neoplasm of the anal canal is the most specific and common coding for anal cancer.
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
Official- Malignant neoplasm of anal sphincter
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
Includes
OfficialNo Includes notes are included in this display for C21.1. 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
Code First
OfficialNo Code First sequencing instructions are included in this display for C21.1. Check the code and parent instructions in the Code Book.
Use Additional
OfficialNo Use Additional Code instructions are included in this display for C21.1. Check the code and parent instructions in the Code Book.
Code Also
OfficialNo Code Also instructions are included in this display for C21.1. 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 C21.1 an HCC code?
Yes. C21.1 (Malignant neoplasm of 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: C21.1 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
- C21.1
- Description
- Malignant neoplasm of anal canal
- 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 C21.1 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →
MEAT review for C21.1
For C21.1, 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
C21.1 is the ICD-10-CM diagnosis code for malignant neoplasm of anal canal. Cancer that develops in the anal canal, which is the short passage that connects the rectum to the external opening of the anus. C21.1 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, C21.1 maps to Bladder, Colorectal, and Other Cancers (HCC 22) with a source-labeled community, non-dual, aged reference coefficient of 0.363. 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.
Anal canal cancer is distinct from rectal cancer and often has different etiologies (HPV association is common). For C21.1, 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 C21.1 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Anal canal cancer is distinct from rectal cancer and often has different etiologies (HPV association is common)
- •Ensure documentation specifies anal canal location to distinguish from other anal structures
Clinical Significance
Malignant neoplasm of the anal canal is the most specific and common coding for anal cancer. The anal canal extends from the anorectal junction to the anal verge, and cancers here are predominantly squamous cell carcinomas with strong human papillomavirus association. The standard treatment is concurrent chemoradiation (5-fluorouracil and mitomycin C), with surgery reserved for treatment failure. This distinction from rectal adenocarcinoma is critical for appropriate treatment and risk adjustment.
Documentation Requirements
- ✓Documentation specifying anal canal as the tumor location
- ✓Histological type (squamous cell carcinoma vs adenocarcinoma vs other)
- ✓Human papillomavirus status and p16 immunohistochemistry results
- ✓Stage of disease including tumor size and lymph node status
- ✓Treatment approach (chemoradiation vs surgical)
Commonly Confused Codes
- •C20: Rectum: tumors above the anorectal junction (dentate line) are rectal, not anal; treatment differs fundamentally
- •C21.0: Anus, unspecified: use C21.1 when the canal is specifically documented
- •C21.2: Cloacogenic zone: the transitional zone at the upper anal canal
- •C44.500: Skin of perianal region: cancers of the perianal skin exterior to the anal verge are skin cancers, not anal canal cancers