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C4A.121 ICD-10-CM Code: Merkel cell carcinoma of left upper eyelid, including canthus

C4A.121 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

ICD-10-CM Code View

HCC Buddy Code Card

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Code lookupC4A.121

FY 2026 Apr update / Neoplasms (C00-D49) / Melanoma and other malignant neoplasms of skin (C43-C44)

C4A.121

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

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.

Buddy the Bee presenting code insight

Buddy Insight

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.

OverviewRisk ModelsCode BookDocumentationAuditNotesEdition comparison

CMS-HCC V28

HCC 21

Coefficient HCC 21: 0.671 (Community Non-Dual Aged (CNA))

CMS-HCC V24

Historical

Historical

Not used for CY2026 payment

ACA/HHS

HCC 012

Code-level coefficient reference

ESRD/PACE

HCC 12

Code-level coefficient reference

RXHCC

HCC 22

Code-level coefficient reference

Code Book Path

Official
C4A.1Merkel cell carcinoma of eyelid, including canthusC4A.12Merkel cell carcinoma of left eyelid, including canthusC4A.121Merkel cell carcinoma of left upper eyelid, including canthus

Inclusion Terms

Official

No inclusion terms are included in this display for C4A.121. Check the code and parent instructions in the Code Book.

Excludes 2

Official

No Excludes 2 notes are included in this display for C4A.121. Check the code and parent instructions in the Code Book.

Related Codes

Official
C4A.122Merkel cell carcinoma of left lower eyelid, including canthus

Includes

Official

No Includes notes are included in this display for C4A.121. Check the code and parent instructions in the Code Book.

Excludes 1

Official

No Excludes 1 notes are included in this display for C4A.121. Check the code and parent instructions in the Code Book.

Code First

Official

No Code First sequencing instructions are included in this display for C4A.121. Check the code and parent instructions in the Code Book.

Use Additional

Official

No Use Additional Code instructions are included in this display for C4A.121. Check the code and parent instructions in the Code Book.

Code Also

Official

No Code Also instructions are included in this display for C4A.121. Check the code and parent instructions in the Code Book.

Buddy Documentation Tip

HCC Buddy guidance
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

MEAT Support

HCC Buddy guidance
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

Audit Caution

HCC Buddy guidance
Assigning a C44 (other skin cancer) code instead of C4A for Merkel cell carcinoma — C4A has its own dedicated subcategory
Failing to distinguish upper vs. lower eyelid — the 5th character differentiates these locations
Confusing right and left laterality — verify from operative/pathology reports, not just progress notes
Using unspecified eyelid code C4A.10 when laterality and position are documented in the record

Common Mistakes

HCC Buddy guidance
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

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.121 an HCC code?

Yes. C4A.121 (Merkel cell carcinoma of left upper 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.121 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.121
Description
Merkel cell carcinoma of left upper eyelid, including canthus
HCC (V28)
HCC 21 — Lymphoma and Other Cancers
RAF reference coefficient
0.671
Billable
Yes
Payment year
2026

HCC Category Mapping

V28HCC 21, Lymphoma and Other Cancers
0.671
ESRDHCC 12, Breast/Prostate/and Other Cancers and Tumors
Not separately weighted
RxHCCHCC 22, Prostate, Breast, Bladder, and Other Cancers and Tumors
Not separately weighted

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.121 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT review for C4A.121

For C4A.121, 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.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 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.121; 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 left side and upper eyelid location; ensure documentation clearly indicates left upper eyelid involvement. For C4A.121, 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.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

Child Codes

C4A.121Merkel cell carcinoma of left upper eyelid, including canthus
HCC 21
C4A.122Merkel cell carcinoma of left lower eyelid, including canthus
HCC 21

Code Hierarchy

└C4AMerkel cell carcinoma└C4A.1Merkel cell carcinoma of eyelid, including canthus└C4A.12Merkel cell carcinoma of left eyelid, including canthus└C4A.121Merkel cell carcinoma of left upper eyelid, including canthus
└C4A.121Merkel cell carcinoma of left upper eyelid, including canthus

C4A.121 code history

Code setChange
FY2019 (effective Oct 1, 2018)Added to the code set

Source: official CMS ICD-10-CM order and addenda files, FY2016 through FY2027.

Also searched as

  • C4A 121
  • C4A121

For C4A.121, 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.

C4A.121 maps to CMS-HCC V28 category 21, Lymphoma and Other Cancers. See the ICD-10 to HCC mapping hub for how the V28 crosswalk works. The mapping identifies a payment HCC category for C4A.121. Review its source-labeled HCC coefficient above, check the RAF Calculator with complete member context for CMS-HCC V28 PY2026, and confirm the documentation the chart needs before the code is submitted. HCC Buddy shows no RAF score unless every required source and calculation check passes.

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