C40.21
BillableMalignant neoplasm of long bones of right lower limb
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C40.21 an HCC code?
Yes. C40.21 maps to Lymphoma and Other Cancers under the CMS-HCC V28 risk adjustment model (and Lymphoma and Other Cancers 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 C40.21
For C40.21to 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 C40.21 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C40.21 is the ICD-10-CM diagnosis code for malignant neoplasm of long bones of right lower limb. Cancer that starts in the long bones of the right leg, including the thighbone, shinbone, or smaller bone of the lower leg. C40.21 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering malignant neoplasms of bone and articular cartilage (c40-c41).
Under the CMS-HCC V28 risk adjustment model, C40.21 maps to Lymphoma and Other Cancers (HCC 21) with a community, non-dual, aged base RAF weight of 0.671. Under the older V24 model, C40.21 mapped to the same category but with a base RAF weight of 0.675 — V28 recalibrated weights across the entire model. 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.
Confirm right side laterality in documentation; this code is specific to right lower limb. Because C40.21 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 C40.21 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Confirm right side laterality in documentation; this code is specific to right lower limb
- •Long bones include femur, tibia, and fibula; document the specific bone involved when possible
Clinical Significance
Primary malignant neoplasm of the long bones (femur, tibia, fibula) of the right lower limb. The distal femur and proximal tibia are the most common sites for osteosarcoma, particularly in adolescents and young adults. In older adults, chondrosarcoma is more prevalent. These cancers require aggressive multimodal treatment and carry significant risk adjustment weight.
Documentation Requirements
- ✓Right-side laterality explicitly documented
- ✓Specific long bone involved (femur, tibia, or fibula)
- ✓Histologic type and tumor grade from surgical pathology
- ✓Primary vs. metastatic origin confirmed by pathology
- ✓TNM staging or Enneking staging
- ✓Current treatment modality and disease status
Commonly Confused Codes
- •C40.22 — Long bones of LEFT lower limb; laterality differentiator
- •C40.20 — Long bones of UNSPECIFIED lower limb; avoid when right is documented
- •C40.31 — Short bones of right lower limb; different anatomic axis (tarsal/metatarsal)
- •C79.51 — Secondary malignant neoplasm of bone; common in metastatic prostate and breast cancer