C4A.4
BillableMerkel cell carcinoma of scalp and neck
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C4A.4 an HCC code?
Yes. C4A.4 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.4
For C4A.4to 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.4 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C4A.4 is the ICD-10-CM diagnosis code for merkel cell carcinoma of scalp and neck. Merkel cell carcinoma is a rare and aggressive skin cancer that develops on the scalp or neck area. It arises from neuroendocrine cells in the skin and tends to grow quickly and spread to other parts of the body. C4A.4 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.4 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.4 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.
Verify the anatomical site is specifically the scalp and neck region; other body sites have different C4A codes (C4A.0-C4A.9 represent different anatomical locations). Because C4A.4 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.4 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Verify the anatomical site is specifically the scalp and neck region; other body sites have different C4A codes (C4A.0-C4A.9 represent different anatomical locations)
- •This code should be used with additional codes for staging, laterality if applicable, and any metastatic disease to provide complete clinical documentation
Clinical Significance
Merkel cell carcinoma of the scalp and neck represents an aggressive neuroendocrine cutaneous malignancy in a region with extensive lymphatic drainage to cervical, occipital, and supraclavicular nodes. The scalp and neck are among the most common sites for Merkel cell carcinoma due to chronic sun exposure, and these locations carry a particularly high risk of regional lymph node metastasis.
Documentation Requirements
- ✓Histopathologic confirmation of Merkel cell carcinoma (CK20+, synaptophysin+, TTF-1 negative)
- ✓Specific subsite: scalp (vertex, temporal, occipital, parietal) or neck (anterior, posterior, lateral)
- ✓Tumor size, thickness, and margin status
- ✓Sentinel lymph node biopsy results — particularly cervical chain
- ✓Staging per American Joint Committee on Cancer criteria
- ✓Treatment modality (wide local excision, radiation, immunotherapy with anti-PD-L1 agents)
- ✓History of immunosuppression (transplant recipients have 10-15x increased risk)
Commonly Confused Codes
- •C4A.39 — Merkel cell carcinoma of other parts of face; for facial skin, not scalp or neck
- •C4A.30 — Merkel cell carcinoma of unspecified face; scalp and neck are distinct from face
- •C43.4 — Melanoma of scalp and neck; different histology — melanocytic, not neuroendocrine
- •C44.40 — Other malignant neoplasm of scalp and neck skin; basal cell carcinoma or squamous cell carcinoma, not Merkel cell
- •C76.0 — Malignant neoplasm of head, face, and neck not otherwise specified; too nonspecific when Merkel cell carcinoma of scalp/neck is documented