C4A.0
BillableMerkel cell carcinoma of lip
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C4A.0 an HCC code?
Yes. C4A.0 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.0
For C4A.0 to 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.0 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C4A.0 is the ICD-10-CM diagnosis code for merkel cell carcinoma of lip. Merkel cell carcinoma is a rare and aggressive skin cancer that develops on the lip, often appearing as a firm nodule or bump. C4A.0 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.0 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.0 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.
Merkel cell carcinoma codes (C4A) are distinct from other skin cancers; verify the pathology confirms this diagnosis. Because C4A.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 C4A.0 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Merkel cell carcinoma codes (C4A) are distinct from other skin cancers; verify the pathology confirms this diagnosis
- •Document whether the lesion is on the upper or lower lip if specified, though this code applies to either
Clinical Significance
Merkel cell carcinoma of the lip is a rare but highly aggressive neuroendocrine skin cancer with a high rate of local recurrence, regional lymph node metastasis, and distant spread. Merkel cell carcinoma has a worse prognosis than melanoma stage-for-stage, making early accurate identification and coding essential for treatment authorization and risk adjustment.
Documentation Requirements
- ✓Pathology confirming Merkel cell carcinoma (neuroendocrine markers: CK20 positive, TTF-1 negative)
- ✓Specific lip location (upper lip, lower lip, vermilion border, commissure)
- ✓Tumor size and depth of invasion
- ✓Sentinel lymph node biopsy status and results
- ✓Merkel cell polyomavirus status if tested