C4A.10
BillableMerkel cell carcinoma of unspecified eyelid, including canthus
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C4A.10 an HCC code?
Yes. C4A.10 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.10
For C4A.10to 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.10 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C4A.10 is the ICD-10-CM diagnosis code for merkel cell carcinoma of unspecified eyelid, including canthus. A rare aggressive skin cancer (Merkel cell carcinoma) that develops on or around the eyelid or corner of the eye, but the specific eyelid is not specified. C4A.10 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.10 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.10 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.
Use this code only when the specific eyelid location (upper/lower/right/left) is not documented; if location is specified, use more specific codes C4A.111, C4A.112, C4A.121, or C4A.122. Because C4A.10 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.10 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Use this code only when the specific eyelid location (upper/lower/right/left) is not documented; if location is specified, use more specific codes C4A.111, C4A.112, C4A.121, or C4A.122
- •Merkel cell carcinoma codes require documentation of the exact anatomical site for proper specificity
Clinical Significance
Merkel cell carcinoma of the eyelid (including canthus) without laterality specification represents a rare aggressive skin cancer in a cosmetically and functionally sensitive area. Eyelid location presents unique challenges for surgical excision with adequate margins while preserving ocular function, often requiring Mohs surgery or oculoplastic reconstruction.
Documentation Requirements
- ✓Pathology confirming Merkel cell carcinoma with immunohistochemistry (CK20+, TTF-1-)
- ✓Laterality (right or left eye) — query if not documented
- ✓Specific eyelid location (upper vs. lower eyelid, medial vs. lateral canthus)
- ✓Tumor size, depth, and margin status
- ✓Sentinel lymph node biopsy of parotid/periauricular nodes (draining basin for eyelid)
Commonly Confused Codes
- •C4A.111/C4A.112 (Right upper/lower eyelid) and C4A.121/C4A.122 (Left upper/lower eyelid) — use when laterality and upper/lower are documented
- •C44.10_ (Other malignant neoplasm of skin of eyelid) — basal cell or squamous cell carcinoma of eyelid
- •C43.1_ (Melanoma of eyelid) — melanoma, not Merkel cell carcinoma
- •C69.0_ (Malignant neoplasm of conjunctiva) — conjunctival malignancy, not eyelid skin