C4A.70
BillableMerkel cell carcinoma of unspecified lower limb, including hip
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C4A.70 an HCC code?
Yes. C4A.70 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.70
For C4A.70to 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.70 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C4A.70 is the ICD-10-CM diagnosis code for merkel cell carcinoma of unspecified lower limb, including hip. Merkel cell carcinoma is a rare and aggressive skin cancer that develops from neuroendocrine cells in the skin, occurring somewhere on the lower leg or hip area without a more specific location documented. This type of cancer typically appears as a firm, painless nodule and tends to grow quickly. C4A.70 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.70 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.70 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 whether the provider has documented a more specific anatomical site (such as thigh, knee, ankle, or foot) before defaulting to 'unspecified'; if available, use the more specific code for better clinical documentation. Because C4A.70 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.70 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Verify whether the provider has documented a more specific anatomical site (such as thigh, knee, ankle, or foot) before defaulting to 'unspecified'; if available, use the more specific code for better clinical documentation
- •Ensure you have confirmed this is Merkel cell carcinoma (C4A codes) and not other skin cancers like melanoma (C43) or non-melanoma skin cancers, as the C4A category is specific to Merkel cell carcinoma
Clinical Significance
Merkel cell carcinoma of the unspecified lower limb, including hip is a rare aggressive neuroendocrine cutaneous malignancy. Lower extremity lesions drain to inguinal and pelvic lymph nodes, making inguinal sentinel lymph node biopsy essential for staging. Lower limb Merkel cell carcinoma may present at a larger size than head/neck lesions due to delayed detection in areas often covered by clothing.
Documentation Requirements
- ✓Histopathologic confirmation of Merkel cell carcinoma with immunohistochemistry
- ✓Laterality must be queried — right or left lower limb
- ✓Specific lower limb subsite (hip, thigh, knee, lower leg, ankle, foot, toes)
- ✓Tumor dimensions and depth of invasion
- ✓Inguinal sentinel lymph node biopsy status and results
- ✓Staging per American Joint Committee on Cancer criteria
- ✓Treatment plan documentation
Commonly Confused Codes
- •C4A.71 — Merkel cell carcinoma of right lower limb; use when right side is documented
- •C4A.72 — Merkel cell carcinoma of left lower limb; use when left side is documented
- •C44.70x — Other malignant neoplasm of unspecified lower limb skin; for non-Merkel cell skin cancers
- •C43.70 — Melanoma of unspecified lower limb; different tumor histology
- •C4A.59 — Merkel cell carcinoma of other trunk; hip area can overlap with trunk