C79.9 maps to CMS-HCC V28 18. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · HCC Buddy coding tools

ICD-10-CM Code View

HCC Buddy Code Card

Digital ICD-10 code-book layout with official code detail, always-visible risk models, Code Trumping, and Buddy coding guidance.

Code lookupC79.9

Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Neoplasms (C00-D49) / Malignant neoplasms of ill-defined, other secondary and unspecified sites (C76-C80)

C79.9

Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidance

Secondary malignant neoplasm of unspecified site

Cancer that has spread to an unknown or unspecified location in the body from a primary cancer elsewhere.

Buddy Insight

Secondary malignant neoplasm of unspecified site indicates the patient has known metastatic cancer but the metastatic site cannot be identified or localized.

Overview Risk Models Code Book Documentation Audit Notes Edition comparison

CMS-HCC V28

Mapped

HCC 18

Cancer Metastatic to Bone, Ot...

Coefficient HCC 18: 2.341 (Community Non-Dual Aged (CNA))

CMS-HCC V24

Historical

Not used for standard CY2026 Medicare Advantage payment. Select a historical period to review V24.

Not used for CY2026 payment

ACA/HHS

Mapped

HCC 008

HHS-HCC 008 adult, RAF varies...

Code-level coefficient reference

ESRD/PACE

Mapped

HCC 8

Metastatic Cancer and Acute L...

Code-level coefficient reference

RXHCC

Mapped

HCC 18

Secondary Cancer of Lung, Liv...

Code-level coefficient reference

Code Book Path

Official

C79Secondary malignant neoplasm of other and unspecified sites C79.9Secondary malignant neoplasm of unspecified site

Inclusion Terms

Official

Metastatic cancer NOS
Metastatic disease NOS

Excludes 2

Official

No Excludes 2 notes are included in this display for C79.9. Check the code and parent instructions in the Code Book.

Related Codes

Official

C79.0Secondary malignant neoplasm of kidney and renal pelvis C79.1Secondary malignant neoplasm of bladder and other and unspecified urinary organs C79.2Secondary malignant neoplasm of skin C79.3Secondary malignant neoplasm of brain and cerebral meninges C79.4Secondary malignant neoplasm of other and unspecified parts of nervous system

Includes

Official

No Includes notes are included in this display for C79.9. Check the code and parent instructions in the Code Book.

Excludes 1

Official

carcinomatosis NOS (C80.0)
generalized cancer NOS (C80.0)
malignant (primary) neoplasm of unspecified site (C80.1)
secondary carcinoid tumors (C7B.-)Inherited from C79
secondary neuroendocrine tumors (C7B.-)Inherited from C79

Code First

Official

No Code First sequencing instructions are included in this display for C79.9. Check the code and parent instructions in the Code Book.

Use Additional

Official

No Use Additional Code instructions are included in this display for C79.9. Check the code and parent instructions in the Code Book.

Code Also

Official

No Code Also instructions are included in this display for C79.9. Check the code and parent instructions in the Code Book.

Buddy Documentation Tip

HCC Buddy guidance

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

MEAT Support

HCC Buddy guidance

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

Audit Caution

HCC Buddy guidance

Using C79.9 as a convenience code when imaging clearly identifies the metastatic site

Confusing unknown primary site (C80.1) with unknown secondary site (C79.9) — these are fundamentally different scenarios

Not reviewing all available imaging before defaulting to this unspecified code

Failing to query the provider for additional information about the metastatic site

Common Mistakes

HCC Buddy guidance

C79.89 — Secondary malignant neoplasm of other specified sites; use when the site IS known but has no specific code

C80.0 — Disseminated malignant neoplasm, unspecified; for widespread cancer without known primary or secondary sites

C80.1 — Malignant (primary) neoplasm, unspecified; this is for unknown PRIMARY site, not unknown secondary site

C76.0-C76.8 — Malignant neoplasm of other and ill-defined sites; these are primary ill-defined site codes

Current with CMS: FY2026 ICD-10-CM Apr 1 update (effective Apr 1 – Sep 30, 2026) · CMS-HCC V28, 100% phased in for payment year 2026. FY2027 code set already staged for October 1, 2026. How HCC Buddy stays current →

Is C79.9 an HCC code?

Yes. C79.9 (Secondary malignant neoplasm of unspecified site) maps to HCC 18, Cancer Metastatic to Bone, Other and Unspecified Metastatic Cancer; Acute Leukemia Except Myeloid under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 2.341. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes. It is billable for payment year 2026.

Coder answer: C79.9 is billable and maps to V28 HCC 18, Cancer Metastatic to Bone, Other and Unspecified Metastatic Cancer; Acute Leukemia Except Myeloid. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code: C79.9
Description: Secondary malignant neoplasm of unspecified site
HCC (V28): HCC 18 — Cancer Metastatic to Bone, Other and Unspecified Metastatic Cancer; Acute Leukemia Except Myeloid
RAF reference coefficient: 2.341
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 18, Cancer Metastatic to Bone, Other and Unspecified Metastatic Cancer; Acute Leukemia Except Myeloid

2.341

ESRDHCC 8, Metastatic Cancer and Acute Leukemia

Not separately weighted

RxHCCHCC 18, Secondary Cancer of Lung, Liver, Brain, and Other Sites

Not separately weighted

These are source-labeled model coefficients, not member totals. A category may still be removed by hierarchy or model cleanup rules. Weights are not directly comparable across models: CMS-HCC V28 and V24 use Community, Non-Dual, Aged; ESRD uses the dialysis continuing-enrollee model; RxHCC is the Part D continuing-enrollee, non-low-income, aged weight (a larger scale than CMS-HCC). ACA/HHS has no single weight — it varies by metal level. Actual per-patient RAF contribution depends on member context, hierarchy and cleanup rules, interactions, and the model year used by the payer. V28 is the CMS-HCC model phased in over payment years 2024–2026; V24 remains available for historical review.

Work C79.9 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT review for C79.9

For C79.9, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule.

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

Coder workflow notes

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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 Cancer Metastatic to Bone, Other and Unspecified Metastatic Cancer; Acute Leukemia Except Myeloid (HCC 18) with a source-labeled community, non-dual, aged reference coefficient of 2.341. For CY2026 non-PACE Medicare Advantage, CMS uses 100% of the 2024 CMS-HCC model (V28). PACE uses a separate model blend. Source-labeled code-level coefficients are references, not member totals. Actual contribution depends on complete member context, hierarchy and cleanup rules, interactions, and model year. HCC Buddy's RAF Calculator supports CMS-HCC V28 PY2026 and shows no score unless every required source and calculation check passes.

This is a non-specific code; use only when the secondary cancer site cannot be determined or is truly unspecified. For C79.9, confirm that the documentation supports the diagnosis and meets the applicable coding, encounter, program and payer requirements. MEAT (Monitor, Evaluate, Assess, or Treat) is a review mnemonic, not a universal CMS coding rule. 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, source-labeled coefficient references, 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

Excludes 1, Do NOT code together

carcinomatosis NOS (C80.0)
generalized cancer NOS (C80.0)
malignant (primary) neoplasm of unspecified site (C80.1)

Commonly Confused Codes

•C79.89: Secondary malignant neoplasm of other specified sites; use when the site IS known but has no specific code
•C80.0: Disseminated malignant neoplasm, unspecified; for widespread cancer without known primary or secondary sites
•C80.1: Malignant (primary) neoplasm, unspecified; this is for unknown PRIMARY site, not unknown secondary site
•C76.0-C76.8: Malignant neoplasm of other and ill-defined sites; these are primary ill-defined site codes