C4A.21
BillableMerkel cell carcinoma of right ear and external auricular canal
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C4A.21 an HCC code?
Yes. C4A.21 maps to Lymphoma and Other Cancers 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.21
For C4A.21to 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.21 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C4A.21 is the ICD-10-CM diagnosis code for merkel cell carcinoma of right ear and external auricular canal. A rare aggressive skin cancer (Merkel cell carcinoma) that develops on the right ear or in the right ear canal. C4A.21 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.21 maps to Lymphoma and Other Cancers (HCC 21) with a community, non-dual, aged base RAF weight of 0.671. Under the older CMS-HCC V24 model, C4A.21 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 specifies right ear location; ensure documentation clearly indicates right side involvement. Because C4A.21 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.21 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code specifies right ear location; ensure documentation clearly indicates right side involvement
- •This code includes both the external ear surface and the external auditory canal on the right side
Clinical Significance
Merkel cell carcinoma of the right ear and external auricular canal is a rare but highly aggressive neuroendocrine skin cancer. The ear and external auricular canal are sun-exposed areas where Merkel cell carcinoma can develop, and lesions in this location have access to rich lymphatic drainage, increasing metastatic risk. Early detection and treatment are critical as 5-year survival rates drop significantly with advanced staging.
Documentation Requirements
- ✓Histopathologic confirmation of Merkel cell carcinoma with immunohistochemistry (CK20+, TTF-1 negative)
- ✓Laterality: right ear
- ✓Specific location on the ear (pinna, tragus, lobule, external auditory canal) for clinical staging
- ✓Tumor size, depth, and margins
- ✓Regional lymph node involvement (preauricular, postauricular, cervical)
- ✓Merkel cell polyomavirus status if tested
- ✓Treatment plan and response status
Commonly Confused Codes
- •C4A.22 — Merkel cell carcinoma of left ear; laterality is reversed
- •C4A.20 — Merkel cell carcinoma of unspecified ear; use only when laterality is not documented
- •C44.211 — Other malignant neoplasm of right ear skin (basal cell carcinoma, squamous cell carcinoma); not Merkel cell
- •C43.21 — Melanoma of right ear; different tumor type entirely