What is ICD-10 code C4A.8?

C4A.8 is the ICD-10-CM diagnosis code for Merkel cell carcinoma of overlapping sites. Merkel cell carcinoma is a rare and aggressive skin cancer that affects multiple overlapping areas of the body. This code is used when the cancer involves two or more adjacent or nearby skin regions. C4A.8 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. C4A.8 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. C4A.8 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. C4A.8 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

Which HCC does C4A.8 map to under V28?

C4A.8 maps to CMS-HCC V28 21 (Lymphoma and Other Cancers). The category number and label come from the CMS-HCC V28 model file for payment year 2026.

What does C4A.8 pay in RAF?

C4A.8 carries a community, non-dual, aged (CNA) RAF weight of 0.671 under CMS-HCC V28 for payment year 2026. The weight comes from the CMS V28 relative-factor file, not a hand-typed table.

What is the V28 vs V24 difference for C4A.8?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. For C4A.8, V28 maps to Lymphoma and Other Cancers at RAF 0.671 while V24 mapped to Breast, Prostate, and Other Cancers and Tumors at RAF 0.150. Both models can still appear in legacy data and in payer-specific lookback periods.

What documentation supports C4A.8?

When Merkel cell carcinoma involves multiple body sites, use C4A.8 only if the sites are overlapping or contiguous; if they are separate/non-contiguous sites, code each site separately Always verify the pathology report confirms Merkel cell carcinoma diagnosis and document which specific overlapping anatomical sites are involved for complete medical record documentation Because C4A.8 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score. See the MEAT criteria checklist at /meat-criteria for what Monitor, Evaluate, Assess, or Treat looks like in a note. MEAT is generic documentation evidence, not a payer-specific rule.

2026 ICD-10-CM C4A.8: Merkel cell carcinoma

C4A.8 maps to CMS-HCC V28 21 (RAF 0.671). Documentation must support MEAT. MEAT criteria · RAF calculator · HCC coding software

ICD-10-CM Code View

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Code lookupC4A.8Effective periodFY2026 Apr Update (Apr 1-Sep 30)

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

C4A.8

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

Merkel cell carcinoma of overlapping sites

Merkel cell carcinoma is a rare and aggressive skin cancer that affects multiple overlapping areas of the body. This code is used when the cancer involves two or more adjacent or nearby skin regions.

Buddy Insight

Merkel cell carcinoma of overlapping sites indicates a tumor that spans two or more contiguous anatomical regions that cannot be classified to a single C4A subsite.

CMS-HCC V28

Mapped

HCC 21

Lymphoma and Other Cancers

RAF 0.671

CMS-HCC V24

Mapped

HCC 12

Breast, Prostate, and Other C...

RAF 0.150

ACA/HHS

Mapped

HCC 12

HHS-HCC 12

Varies by metal level

ESRD/PACE

Mapped

HCC 12

Breast, Prostate, and Other C...

RAF 0.045

RXHCC

Mapped

HCC 22

Cancer, Other Specified Sites

RAF 0.124

Code Book Path

Official

C4AMerkel cell carcinoma

C4A.8Merkel cell carcinoma of overlapping sites

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for C4A.8 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for C4A.8 in this effective period.

Related Child Codes

Official

C4A.0Merkel cell carcinoma of lip

C4A.1Merkel cell carcinoma of eyelid, including canthus

C4A.2Merkel cell carcinoma of ear and external auricular canal

C4A.3Merkel cell carcinoma of other and unspecified parts of face

C4A.4Merkel cell carcinoma of scalp and neck

Includes

Official

ICD-10-CM does not list Includes notes for C4A.8 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for C4A.8 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for C4A.8 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for C4A.8 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for C4A.8 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Histopathologic confirmation of Merkel cell carcinoma

Documentation of the specific overlapping anatomical sites involved (e.g., face extending to scalp, trunk crossing to limb)

Confirmation that the sites are contiguous/overlapping, not separate discrete lesions

Tumor dimensions across the overlapping regions

MEAT Support

HCC Buddy guidance

Histopathologic confirmation of Merkel cell carcinoma

Documentation of the specific overlapping anatomical sites involved (e.g., face extending to scalp, trunk crossing to limb)

Confirmation that the sites are contiguous/overlapping, not separate discrete lesions

Tumor dimensions across the overlapping regions

Audit Caution

HCC Buddy guidance

Using C4A.8 when the tumor can be classified to a single specific site — overlapping codes are for truly borderline anatomical locations

Confusing overlapping sites (single contiguous tumor) with multifocal disease (separate discrete tumors at different sites)

Using C4A.9 (unspecified) instead of C4A.8 when overlapping sites are actually documented

Not documenting which specific sites overlap, making future coding and staging difficult

Common Mistakes

HCC Buddy guidance

C4A.0-C4A.7 site-specific codes — if the tumor can be classified to a single site, use that specific code instead

C4A.9 — Merkel cell carcinoma, unspecified; use C4A.8 when sites ARE known but overlap, not when site is unknown

C80.0 — Disseminated malignant neoplasm, unspecified; for widely metastatic disease, not contiguous skin sites

Multiple separate C4A codes — if lesions are non-contiguous (separate tumors), code each individually rather than using C4A.8

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.8 an HCC code?

Yes. C4A.8 (Merkel cell carcinoma of overlapping sites) maps to Lymphoma and Other Cancers under the CMS-HCC V28 risk adjustment model (and Breast, Prostate, and Other Cancers and Tumors under V24), with a community non-dual aged RAF of 0.671. It is billable for payment year 2026.

Coder answer: C4A.8 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.8
Description: Merkel cell carcinoma of overlapping sites
HCC (V28): HCC 21 — Lymphoma and Other Cancers
RAF: 0.671
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 21, Lymphoma and Other Cancers

0.671

V24HCC 12, Breast, Prostate, and Other Cancers and Tumors

0.150

ESRDHCC 12, Breast, Prostate, and Other Cancers and Tumors

0.045

RxHCCHCC 22, Cancer, Other Specified Sites

0.124

Each model's RAF is its CMS base weight for that model's standard population, so 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 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.

Work C4A.8 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for C4A.8

For C4A.8 to 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.8 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

C4A.8 is the ICD-10-CM diagnosis code for merkel cell carcinoma of overlapping sites. Merkel cell carcinoma is a rare and aggressive skin cancer that affects multiple overlapping areas of the body. This code is used when the cancer involves two or more adjacent or nearby skin regions. C4A.8 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.8 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.8 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.

When Merkel cell carcinoma involves multiple body sites, use C4A.8 only if the sites are overlapping or contiguous; if they are separate/non-contiguous sites, code each site separately. Because C4A.8 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.8 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•When Merkel cell carcinoma involves multiple body sites, use C4A.8 only if the sites are overlapping or contiguous; if they are separate/non-contiguous sites, code each site separately
•Always verify the pathology report confirms Merkel cell carcinoma diagnosis and document which specific overlapping anatomical sites are involved for complete medical record documentation

Clinical Significance

Merkel cell carcinoma of overlapping sites indicates a tumor that spans two or more contiguous anatomical regions that cannot be classified to a single C4A subsite. This presentation typically suggests a larger, more advanced tumor at diagnosis and may indicate locally aggressive disease. Overlapping-site tumors often require more extensive surgical intervention and carry a higher risk of incomplete excision.

Documentation Requirements

✓Histopathologic confirmation of Merkel cell carcinoma
✓Documentation of the specific overlapping anatomical sites involved (e.g., face extending to scalp, trunk crossing to limb)
✓Confirmation that the sites are contiguous/overlapping, not separate discrete lesions
✓Tumor dimensions across the overlapping regions
✓All involved lymph node basins assessed
✓Staging that reflects the full extent of disease

Commonly Confused Codes

•C4A.0-C4A.7 site-specific codes: if the tumor can be classified to a single site, use that specific code instead
•C4A.9: Merkel cell carcinoma, unspecified; use C4A.8 when sites ARE known but overlap, not when site is unknown
•C80.0: Disseminated malignant neoplasm, unspecified; for widely metastatic disease, not contiguous skin sites
•Multiple separate C4A codes: if lesions are non-contiguous (separate tumors), code each individually rather than using C4A.8