C79.81
BillableSecondary malignant neoplasm of breast
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C79.81 an HCC code?
Yes. C79.81 maps to Metastatic Cancer to Peritoneum and Other Specified Sites 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 C79.81
For C79.81 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 C79.81 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C79.81 is the ICD-10-CM diagnosis code for secondary malignant neoplasm of breast. Cancer that has spread to the breast tissue from a cancer that originated elsewhere in the body. C79.81 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering malignant neoplasms of ill-defined, other secondary and unspecified sites (c76-c80).
Under the CMS-HCC V28 risk adjustment model, C79.81 maps to Metastatic Cancer to Peritoneum and Other Specified Sites (HCC 18) with a community, non-dual, aged base RAF weight of 0.368. Under the older CMS-HCC V24 model, C79.81 maps to Lymphoma and Other Cancers (HCC 10) with a community, non-dual, aged base RAF weight of 0.675. 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.
This code represents metastatic disease to the breast; do not confuse with primary breast cancer (C50 codes). Because C79.81 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 C79.81 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code represents metastatic disease to the breast; do not confuse with primary breast cancer (C50 codes).
- •Always code the primary malignancy site first before this secondary cancer code.
Clinical Significance
Secondary malignant neoplasm of the breast indicates cancer has metastasized to the breast tissue from a primary cancer elsewhere. This is an uncommon scenario since the vast majority of breast malignancies are primary. Metastatic disease to the breast most often comes from contralateral breast cancer, lymphoma, lung cancer, or melanoma. Accurate distinction is critical since treatment for metastatic disease to the breast differs fundamentally from primary breast cancer management.
Documentation Requirements
- ✓Primary cancer site documented and coded separately
- ✓Pathological confirmation that the breast lesion represents metastatic disease, not a new primary
- ✓Immunohistochemistry panel comparing histology to the known primary site
- ✓Imaging — mammography, ultrasound, or MRI showing the breast lesion
- ✓Laterality and location within the breast
Commonly Confused Codes
- •C50.011-C50.929 — Primary malignant neoplasm of breast; primary breast cancer is overwhelmingly more common than metastatic disease TO the breast
- •C79.82 — Secondary malignant neoplasm of genital organs; breast is not a genital organ — use C79.81
- •C79.89 — Secondary malignant neoplasm of other specified sites; breast has its own specific secondary code
- •D05.00-D05.92 — Carcinoma in situ of breast; in situ disease is not metastatic