C79.9
BillableSecondary malignant neoplasm of unspecified site
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C79.9 an HCC code?
Yes. C79.9 maps to Metastatic Cancer to Peritoneum and Other Specified Sites under the CMS-HCC V28 risk adjustment model (and Metastatic Cancer and Acute Leukemia 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 C79.9
For C79.9 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 C79.9 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C79.9 is the ICD-10-CM diagnosis code for secondary malignant neoplasm of unspecified site. Cancer that has spread to an unknown or unspecified location in the body from a primary cancer elsewhere. C79.9 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, C79.9 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, C79.9 maps to Metastatic Cancer and Acute Leukemia (HCC 8) with a community, non-dual, aged base RAF weight of 2.484. 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 is a non-specific code; use only when the secondary cancer site cannot be determined or is truly unspecified. Because C79.9 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 C79.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This is a non-specific code; use only when the secondary cancer site cannot be determined or is truly unspecified.
- •Query the provider if possible to identify the specific site of metastasis for more accurate coding.
Clinical Significance
Secondary malignant neoplasm of unspecified site indicates the patient has known metastatic cancer but the metastatic site cannot be identified or localized. This is the least specific secondary neoplasm code and should only be used when all attempts to identify the metastatic site have been exhausted. It may be appropriate when labs or tumor markers indicate metastatic disease but imaging has not yet localized the secondary site.
Documentation Requirements
- ✓Primary cancer site documented and coded separately, if known
- ✓Documentation of why the metastatic site cannot be determined
- ✓Evidence supporting the diagnosis of metastatic disease — rising tumor markers, clinical deterioration
- ✓Imaging studies performed and their findings
- ✓Provider query documenting inability to specify the metastatic site