C4A.112 ICD-10-CM Code: Merkel cell carcinoma of right lower eyelid, including canthus
C4A.112 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.112
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceMerkel cell carcinoma of right lower eyelid, including canthus
A rare aggressive skin cancer (Merkel cell carcinoma) that develops on the right lower eyelid or the corner of the right eye.
Buddy Insight
Merkel cell carcinoma of the right lower eyelid, including canthus is a rare, highly aggressive neuroendocrine skin malignancy with a high propensity for local recurrence and distant metastasis.
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.112. Check the code and parent instructions in the Code Book.
Excludes 2
OfficialNo Excludes 2 notes are included in this display for C4A.112. Check the code and parent instructions in the Code Book.
Related Codes
Includes
OfficialNo Includes notes are included in this display for C4A.112. Check the code and parent instructions in the Code Book.
Excludes 1
OfficialNo Excludes 1 notes are included in this display for C4A.112. Check the code and parent instructions in the Code Book.
Code First
OfficialNo Code First sequencing instructions are included in this display for C4A.112. Check the code and parent instructions in the Code Book.
Use Additional
OfficialNo Use Additional Code instructions are included in this display for C4A.112. Check the code and parent instructions in the Code Book.
Code Also
OfficialNo Code Also instructions are included in this display for C4A.112. 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.112 an HCC code?
Yes. C4A.112 (Merkel cell carcinoma of right lower 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.112 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.112
- Description
- Merkel cell carcinoma of right lower 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.112 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →
MEAT review for C4A.112
For C4A.112, 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.112 is the ICD-10-CM diagnosis code for merkel cell carcinoma of right lower eyelid, including canthus. A rare aggressive skin cancer (Merkel cell carcinoma) that develops on the right lower eyelid or the corner of the right eye. C4A.112 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.112 maps to Lymphoma and Other Cancers (HCC 21) with a source-labeled community, non-dual, aged reference coefficient of 0.671. No V24 mapping is shown for C4A.112; use the applicable model and payment year when reviewing the V28 mapping. 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.
This code specifies right side and lower eyelid location; confirm documentation distinguishes between upper and lower eyelid. For C4A.112, 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.112 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code specifies right side and lower eyelid location; confirm documentation distinguishes between upper and lower eyelid
- •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 right lower 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 right side
- ✓Specific eyelid location documented as lower 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.122: Merkel cell carcinoma of left lower eyelid; laterality is reversed: always confirm right vs. left from documentation
- •C4A.111: Merkel cell carcinoma of right upper 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