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C79.81 ICD-10-CM Code: Secondary malignant neoplasm of breast

C79.81 maps to CMS-HCC V28 18. A source-labeled RAF reference is available. Documentation must support MEAT. MEAT criteria · RAF Calculator · HCC Buddy coding tools

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

C79.81

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

Secondary malignant neoplasm of breast

Cancer that has spread to the breast tissue from a cancer that originated elsewhere in the body.

Buddy the Bee presenting code insight

Buddy Insight

Secondary malignant neoplasm of the breast indicates cancer has metastasized to the breast tissue from a primary cancer elsewhere.

CMS-HCC V28

HCC 18

Code-level coefficient reference

CMS-HCC V24

Historical

Historical

Not used for CY2026 payment

ACA/HHS

HCC 008

Code-level coefficient reference

ESRD/PACE

HCC 10

Code-level coefficient reference

RXHCC

HCC 18

Code-level coefficient reference

Code Book Path

Official
C79Secondary malignant neoplasm of other and unspecified sites
C79.8Secondary malignant neoplasm of other specified sites
C79.81Secondary malignant neoplasm of breast

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for C79.81 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for C79.81 in this effective period.

Related Child Codes

Official
C79.82Secondary malignant neoplasm of genital organs
C79.89Secondary malignant neoplasm of other specified sites

Includes

Official

ICD-10-CM does not list Includes notes for C79.81 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for C79.81 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for C79.81 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for C79.81 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for C79.81 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance
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

MEAT Support

HCC Buddy guidance
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

Audit Caution

HCC Buddy guidance
Assuming all breast malignancies are primary — secondary breast cancer does occur and requires different management
Not obtaining immunohistochemistry to distinguish a new primary breast cancer from metastatic deposit
Coding contralateral breast cancer recurrence as secondary when it may actually be a new primary
Using this code for breast cancer that has metastasized to other sites — C79.81 is for cancer spreading TO the breast, not FROM it

Common Mistakes

HCC Buddy guidance
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

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 C79.81 an HCC code?

Yes. C79.81 (Secondary malignant neoplasm of breast) 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: C79.81 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
C79.81
Description
Secondary malignant neoplasm of breast
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 10, Lymphoma and Other Cancers
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 C79.81 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

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.

Coder workflow notes

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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 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. No V24 mapping is shown for C79.81; use the applicable model and payment year when reviewing the V28 mapping. 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. 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.

Secondary malignant neoplasm of breast, metastatic disease to the breast from a non-breast primary (or contralateral breast spread). Pair with the primary-site C code. Because C79.81 maps to an HCC category, the provider's documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) before the diagnosis is used for risk adjustment. 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 C79.81 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

  • •Secondary malignant neoplasm of breast, metastatic disease to the breast from a non-breast primary (or contralateral breast spread). Pair with the primary-site C code.
  • •Do not confuse with C50.* (primary breast). Metastatic cancer carries a different HCC, HCC 18 vs HCC 23.

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

Child Codes

C79.81Secondary malignant neoplasm of breast
HCC 18
C79.82Secondary malignant neoplasm of genital organs
HCC 18
C79.89Secondary malignant neoplasm of other specified sites
HCC 18

Code Hierarchy

└C79Secondary malignant neoplasm of other and unspecified sites└C79.8Secondary malignant neoplasm of other specified sites└C79.81Secondary malignant neoplasm of breast
└C79.81Secondary malignant neoplasm of breast

Because C79.81 maps to an HCC category, the documentation must satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) before the diagnosis is used for risk adjustment.

C79.81 maps to CMS-HCC V28 category 18, Cancer Metastatic to Bone, Other and Unspecified Metastatic Cancer; Acute Leukemia Except Myeloid. See the ICD-10 to HCC mapping hub for how the V28 crosswalk works. The mapping identifies a payment HCC category for C79.81. Review its source-labeled HCC coefficient above, check the RAF Calculator with complete member context for CMS-HCC V28 PY2026, and confirm the documentation the chart needs before the code is submitted. HCC Buddy shows no RAF score unless every required source and calculation check passes.

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