C4A.121
BillableMerkel cell carcinoma of left upper eyelid, including canthus
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C4A.121 an HCC code?
Yes. C4A.121 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.121
For C4A.121to 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.121 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C4A.121 is the ICD-10-CM diagnosis code for merkel cell carcinoma of left upper eyelid, including canthus. A rare aggressive skin cancer (Merkel cell carcinoma) that develops on the left upper eyelid or the corner of the left eye. C4A.121 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.121 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.121 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 left side and upper eyelid location; ensure documentation clearly indicates left upper eyelid involvement. Because C4A.121 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.121 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code specifies left side and upper eyelid location; ensure documentation clearly indicates left upper eyelid involvement
- •The 'including canthus' notation means this code covers the inner corner of the eye as well as the eyelid itself
Clinical Significance
Merkel cell carcinoma of the left upper eyelid, including canthus is a rare, highly aggressive neuroendocrine skin malignancy with a high propensity for local recurrence and distant metastasis. Periocular Merkel cell carcinoma requires urgent multidisciplinary management due to the risk of orbital invasion and lymph node spread, and carries a worse prognosis than many other periocular malignancies.
Documentation Requirements
- ✓Histopathologic confirmation of Merkel cell carcinoma (neuroendocrine differentiation on immunohistochemistry — CK20 positive, TTF-1 negative)
- ✓Laterality documented as left side
- ✓Specific eyelid location documented as upper eyelid (not just 'eyelid')
- ✓Tumor size and depth of invasion
- ✓Involvement or sparing of the canthus (medial or lateral corner of the eye)
- ✓Sentinel lymph node biopsy status and results
- ✓Current treatment status (active treatment, surveillance, or recurrence)
Commonly Confused Codes
- •C4A.111 — Merkel cell carcinoma of right upper eyelid; laterality is reversed — always confirm right vs. left from documentation
- •C4A.122 — Merkel cell carcinoma of left lower eyelid; verify whether tumor is on upper or lower eyelid
- •C44.111-C44.122 — Other malignant neoplasms of eyelid skin (basal cell carcinoma, squamous cell carcinoma); Merkel cell carcinoma requires C4A codes, not C44
- •C43.111-C43.122 — Melanoma of eyelid; entirely different histology — melanocytic vs. neuroendocrine origin
- •C69.0x — Malignant neoplasm of conjunctiva; periocular Merkel cell carcinoma involves the skin, not conjunctival mucosa