C4A.51 ICD-10-CM Code: Merkel cell carcinoma of anal skin
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) / Melanoma and other malignant neoplasms of skin (C43-C44)
C4A.51
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceMerkel cell carcinoma of anal skin
A rare aggressive skin cancer (Merkel cell carcinoma) that develops on the skin around the anus.
Buddy Insight
Merkel cell carcinoma of the anal skin is an exceedingly rare presentation of this aggressive neuroendocrine cutaneous malignancy.
CMS-HCC V28
MappedHCC 21
RAF 0.545
CMS-HCC V24
MappedHCC 12
RAF 0.150
ACA/HHS
00
RAF 0
ESRD/PACE
MappedHCC 12
RAF 0.0
RXHCC
MappedHCC 22
RAF 0.0
Code Trumping
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Code Book Path
Inclusion Terms
Official- Merkel cell carcinoma of anal margin
- Merkel cell carcinoma of perianal skin
Excludes 2
Official- malignant neoplasm of anus NOS (C21.0)
- malignant neoplasm of scrotum (C63.2)
Related Child Codes
Includes
OfficialICD-10-CM does not list Includes notes for C4A.51 in this effective period.
Excludes 1
OfficialICD-10-CM does not list Excludes 1 notes for C4A.51 in this effective period.
Code First
OfficialICD-10-CM does not list Code First sequencing instructions for C4A.51 in this effective period.
Use Additional
OfficialICD-10-CM does not list Use Additional Code instructions for C4A.51 in this effective period.
Code Also
OfficialICD-10-CM does not list Code Also instructions for C4A.51 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 C4A.51 an HCC code?
Yes. C4A.51 maps to Breast, Prostate, Colorectal and Other Cancers and Tumors under the CMS-HCC V28 risk adjustment model (and Breast, Prostate, and Other Cancers and Tumors 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 C4A.51
For C4A.51to 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 C4A.51 during that encounter, not just copy-forwarded from a problem list.
What This Code Means
C4A.51 is the ICD-10-CM diagnosis code for merkel cell carcinoma of anal skin. A rare aggressive skin cancer (Merkel cell carcinoma) that develops on the skin around the anus. C4A.51 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering melanoma and other malignant neoplasms of skin (c43-c44).
Under the CMS-HCC V28 risk adjustment model, C4A.51 maps to Breast, Prostate, Colorectal and Other Cancers and Tumors (HCC 21) with a community, non-dual, aged base RAF weight of 0.545. Under the older CMS-HCC V24 model, C4A.51 maps to Breast, Prostate, and Other Cancers and Tumors (HCC 12) with a community, non-dual, aged base RAF weight of 0.150. 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.
This code is for anal skin involvement; ensure documentation specifies anal skin rather than internal anal canal or rectum. Because C4A.51 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 C4A.51 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code is for anal skin involvement; ensure documentation specifies anal skin rather than internal anal canal or rectum
- •Merkel cell carcinoma in this location may require coordination with colorectal specialists for treatment planning
Clinical Significance
Merkel cell carcinoma of the anal skin is an exceedingly rare presentation of this aggressive neuroendocrine cutaneous malignancy. Perianal Merkel cell carcinoma must be distinguished from more common anal malignancies such as squamous cell carcinoma and adenocarcinoma. Due to the rich lymphatic drainage of the perianal region, early nodal involvement is common and aggressive multimodality treatment is typically required.
Documentation Requirements
- ✓Histopathologic confirmation of Merkel cell carcinoma with neuroendocrine markers
- ✓Documentation that the tumor involves the anal skin (perianal skin) rather than the anal canal mucosa (C21.x)
- ✓Tumor size, depth, and circumferential involvement
- ✓Inguinal and pelvic lymph node status
- ✓Staging per American Joint Committee on Cancer criteria
- ✓Treatment plan (wide local excision vs. abdominoperineal resection, radiation, immunotherapy)
Commonly Confused Codes
- •C21.0: Malignant neoplasm of anus, unspecified; for internal anal canal malignancies, not skin
- •C44.500: Unspecified malignant neoplasm of anal skin; use for non-Merkel cell skin cancers of this area
- •C4A.59: Merkel cell carcinoma of other part of trunk; for trunk sites not specifically the anal skin or breast
- •C21.1: Malignant neoplasm of anal canal; internal canal, not perianal skin
- •C4A.52: Merkel cell carcinoma of breast skin; different trunk subsite