C4A.8
BillableMerkel cell carcinoma of overlapping sites
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C4A.8 an HCC code?
Yes. C4A.8 maps to Lymphoma and Other Cancers 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.8
For C4A.8to 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.
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