C4A.10 ICD-10-CM Code: Merkel cell carcinoma of unspecified eyelid, including canthus
C4A.10 maps to CMS-HCC V28 21. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · HCC Buddy coding tools
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FY 2026 Apr update / Neoplasms (C00-D49) / Melanoma and other malignant neoplasms of skin (C43-C44)
C4A.10
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceMerkel 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.
Buddy Insight
Merkel cell carcinoma of the eyelid (including canthus) without laterality specification represents a rare aggressive skin cancer in a cosmetically and functionally sensitive area.
CMS-HCC V28
MappedHCC 21
Coefficient HCC 21: 0.671 (Community Non-Dual Aged (CNA))
CMS-HCC V24
HistoricalHistorical
Not used for CY2026 payment
ACA/HHS
MappedHCC 012
Code-level coefficient reference
ESRD/PACE
MappedHCC 12
Code-level coefficient reference
RXHCC
MappedHCC 22
Code-level coefficient reference
Code Book Path
Inclusion Terms
OfficialNo inclusion terms are included in this display for C4A.10. Check the code and parent instructions in the Code Book.
Excludes 2
OfficialNo Excludes 2 notes are included in this display for C4A.10. Check the code and parent instructions in the Code Book.
Related Codes
Includes
OfficialNo Includes notes are included in this display for C4A.10. Check the code and parent instructions in the Code Book.
Excludes 1
OfficialNo Excludes 1 notes are included in this display for C4A.10. Check the code and parent instructions in the Code Book.
Code First
OfficialNo Code First sequencing instructions are included in this display for C4A.10. Check the code and parent instructions in the Code Book.
Use Additional
OfficialNo Use Additional Code instructions are included in this display for C4A.10. Check the code and parent instructions in the Code Book.
Code Also
OfficialNo Code Also instructions are included in this display for C4A.10. Check the code and parent instructions in the Code Book.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →
Is C4A.10 an HCC code?
Yes. C4A.10 (Merkel cell carcinoma of unspecified eyelid, including canthus) maps to HCC 21, Lymphoma and Other Cancers under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.671. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.
Coder answer: C4A.10 is billable and maps to V28 HCC 21, Lymphoma and Other Cancers. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.
- Code
- C4A.10
- Description
- Merkel cell carcinoma of unspecified eyelid, including canthus
- HCC (V28)
- HCC 21 — Lymphoma and Other Cancers
- RAF reference coefficient
- 0.671
- Billable
- Yes
- Payment year
- 2026
HCC Category Mapping
These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. Weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.
Work C4A.10 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →
MEAT review for C4A.10
For C4A.10, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule.
- 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
Coder workflow notes
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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 source-labeled community, non-dual, aged reference coefficient of 0.671. For CY2026 non-PACE Medicare Advantage, CMS uses 100% of the 2024 CMS-HCC model (V28). PACE uses a separate model blend. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.
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. For C4A.10, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule. 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, source-labeled coefficient references, 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
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