C21.0 ICD-10-CM Code: Malignant neoplasm of anus, unspecified
HCC Buddy Code Card
Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.
FY 2026 Apr update / Neoplasms (C00-D49) / Malignant neoplasms of digestive organs (C15-C26)
C21.0
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceMalignant neoplasm of anus, unspecified
Cancer of the anus where the specific type or exact location within the anal region is not specified.
Buddy Insight
Unspecified anal cancer captures malignancy in the anal region without documentation of the specific subsite.
CMS-HCC V28
MappedHCC 22
RAF 0.0
CMS-HCC V24
MappedHCC 11
RAF 0.306
ACA/HHS
00
RAF 0
ESRD/PACE
MappedHCC 11
RAF 0.0
RXHCC
00
RAF 0
Code Trumping
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Code Book Path
Inclusion Terms
OfficialICD-10-CM does not list inclusion terms for C21.0 in this effective period.
Excludes 2
Official- malignant carcinoid tumors of the colon (C7A.02-)
- malignant melanoma of anal margin (C43.51)
- malignant melanoma of anal skin (C43.51)
- malignant melanoma of perianal skin (C43.51)
- other and unspecified malignant neoplasm of anal margin (C44.500, C44.510, C44.520, C44.590)
Related Child Codes
Includes
OfficialICD-10-CM does not list Includes notes for C21.0 in this effective period.
Excludes 1
OfficialICD-10-CM does not list Excludes 1 notes for C21.0 in this effective period.
Code First
OfficialICD-10-CM does not list Code First sequencing instructions for C21.0 in this effective period.
Use Additional
OfficialICD-10-CM does not list Use Additional Code instructions for C21.0 in this effective period.
Code Also
OfficialICD-10-CM does not list Code Also instructions for C21.0 in this effective period.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
Last updated: FY2026 ICD-10-CM Apr update, Apr 1, 2026 through Sep 30, 2026. CMS-HCC V28 is 100% phased in for payment year 2026.
Is C21.0 an HCC code?
Yes. C21.0 maps to Colorectal, Bladder, and Other Cancers under the CMS-HCC V28 risk adjustment model (and Colorectal, Bladder, and Other Cancers under V24).
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. Actual per-patient RAF contribution depends on member segment, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains in use during the transition and for historical data.
MEAT Criteria for C21.0
For C21.0to count as a valid HCC diagnosis in a given encounter, the provider's documentation must show MEAT: Monitor, Evaluate, Assess, or Treat. A diagnosis from a prior year does not carry forward automatically, it has to be re-documented and supported each calendar year.
- 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
Only one of M/E/A/T is required to support the code, but the documentation must be specific enough to show that the provider actually addressed C21.0 during that encounter, not just copy-forwarded from a problem list.
What This Code Means
C21.0 is the ICD-10-CM diagnosis code for malignant neoplasm of anus, unspecified. Cancer of the anus where the specific type or exact location within the anal region is not specified. C21.0 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, C21.0 maps to Colorectal, Bladder, and Other Cancers (HCC 22) with a community, non-dual, aged base RAF weight of 0.000. Under the older V24 model, C21.0 mapped to the same category but with a base RAF weight of 0.306, V28 recalibrated weights across the entire model. V28 is the CMS-HCC risk adjustment model that reached 100% phase-in for payment year 2026, replacing V24 which was used during the PY2024–PY2025 transition.
Query the provider to determine if the cancer is in the anal canal (C21.1) or other anal structures. Because C21.0 maps to a payment HCC, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's Medicare Advantage risk adjustment score. 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, RAF weights, and MEAT documentation criteria for C21.0 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Query the provider to determine if the cancer is in the anal canal (C21.1) or other anal structures
- •Anal cancer is relatively rare and may have different treatment approaches than colorectal cancer
Clinical Significance
Unspecified anal cancer captures malignancy in the anal region without documentation of the specific subsite. Anal cancer is relatively uncommon and has a strong association with human papillomavirus infection. The primary treatment for anal cancer is typically chemoradiation (Nigro protocol) rather than surgery, which is fundamentally different from rectal cancer management. Accurate distinction between anal and rectal cancer is critical because misclassification can lead to inappropriate treatment planning.
Documentation Requirements
- ✓Confirmed malignancy in the anal region
- ✓Attempt to determine if the cancer is in the anal canal (C21.1) versus perianal skin or other anal structures
- ✓Histological type (squamous cell carcinoma is most common for anal cancer)
- ✓Human papillomavirus status if tested
- ✓Stage of disease
Commonly Confused Codes
- •C21.1 — Anal canal: the anal canal is the most common specific site; use when documentation specifies canal involvement
- •C20 — Rectum: critical distinction — rectal cancer is above the anorectal junction; treatment differs significantly
- •C21.2 — Cloacogenic zone: specific transitional zone at the anorectal junction
- •C44.500 — Unspecified malignant neoplasm of skin of anal region: perianal skin cancer has different codes from anal canal cancer