2027 ICD-10-CM Diagnosis Code C79.9Secondary malignant neoplasm of unspecified site
ICD-10-CM Codes›C00–D49›C76-C80›C79
- Billable — Valid for Submission
- CC — Complication or Comorbidity
- Risk Adjusts — HCC 18
- Chronic Condition
C79.9 is a billable ICD-10-CM diagnosis code for secondary malignant neoplasm of unspecified site. It is valid on HIPAA claims for fiscal year 2027 (October 1, 2026 through September 30, 2027) and groups to MS-DRG 826 through 830, 843 through 845. As a secondary diagnosis, it counts as a complication or comorbidity (CC) and moves an inpatient stay to a higher severity level within its MS-DRG family. It does not count, however, when the principal diagnosis is one of 19 closely related codes. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Secondary malignancies.
For Medicare Advantage risk adjustment, C79.9 maps to CMS-HCC Category 18 (Cancer Metastatic to Bone, Other and Unspecified Metastatic Cancer; Acute Leukemia Except Myeloid) under the V28 model, adding a risk factor of about 2.341 for a community, non-dual, aged beneficiary in payment year 2027.
Code Identity
Code Classification
Medicare Risk Adjustment (HCC)Billing
C79.9 maps to a payment category in the CMS-HCC model used to risk-adjust Medicare Advantage payments. Weights are the published community factors for payment year 2027.
Source: CMS Payment Year 2027 risk adjustment mappings and model software. Weights are relative factors, not dollar amounts; a beneficiary's total RAF also includes demographics and interactions. Browse all CMS-HCC categories.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Anaplastic thyroid carcinoma
- Balloon cell malignant melanoma
- Complex mixed and stromal malignant neoplasm of endometrium of corpus uteri
- Distant metastasis present
- Embryonal rhabdomyosarcoma
- Endometrial stromal tumor
- Fibrosarcoma
- Ganglioneuroblastoma
- Germline BRCA-mutated, HER2-negative metastatic breast cancer
- Human epidermal growth factor 2 negative carcinoma of breast
- Infantile fibrosarcoma
- Intergroup rhabdomyosarcoma study post-surgical clinical group finding
- Intergroup rhabdomyosarcoma study post-surgical clinical group IV: Any size primary tumor, with or without regional lymph node involvement, with distant metastases, without respect to surgical approach to primary tumor
- Lentigo maligna melanoma
- Lymphedema due to malignant infiltration
- Malignant meningioma
- Malignant neoplasm of unknown origin
- Malignant tumor involving an organ by direct extension from bladder
- Malignant tumor involving an organ by direct extension from endometrium
- Malignant tumor involving an organ by direct extension from fallopian tube
- Malignant tumor involving an organ by direct extension from ovary
- Malignant tumor involving an organ by direct extension from prostate
- Malignant tumor involving an organ by direct extension from uterine cervix
- Malignant tumor involving an organ by direct extension from uterus
- Malignant tumor involving an organ by direct extension from vagina
- Metastatic acantholytic squamous cell carcinoma
- Metastatic adenocarcinoma
- Metastatic adenocarcinoma of unknown origin
- Metastatic adenosquamous carcinoma
- Metastatic anaplastic thyroid carcinoma
- Metastatic apocrine adenocarcinoma
- Metastatic balloon cell malignant melanoma
- Metastatic carcinoma to breast
- Metastatic cholangiocarcinoma
- Metastatic clear cell renal cell carcinoma
- Metastatic desmoplastic malignant melanoma
- Metastatic eccrine porocarcinoma
- Metastatic embryonal rhabdomyosarcoma
- Metastatic endometrial stromal sarcoma
- Metastatic extraskeletal osteosarcoma
- Metastatic fibrosarcoma
- Metastatic ganglioneuroblastoma
- Metastatic germinoma
- Metastatic glassy cell carcinoma
- Metastatic hepatocellular carcinoma
- Metastatic infantile fibrosarcoma
- Metastatic inflammatory carcinoma
- Metastatic lentigo maligna melanoma
- Metastatic lobular carcinoma
- Metastatic lymphoepithelial carcinoma
- Metastatic malignant glioma
- Metastatic malignant melanoma
- Metastatic malignant melanoma to skin
- Metastatic malignant melanoma with diffuse hypermelanosis
- Metastatic malignant meningioma
- Metastatic malignant neoplasm
- Metastatic malignant neoplasm to thyroid gland
- Metastatic malignant neoplasm to unknown site
- Metastatic myxofibrosarcoma
- Metastatic neuroblastoma
- Metastatic oxyphilic adenocarcinoma
- Metastatic papillary squamous cell carcinoma
- Metastatic pilomatrix carcinoma
- Metastatic pleomorphic rhabdomyosarcoma
- Metastatic renal cell carcinoma
- Metastatic rhabdomyosarcoma
- Metastatic sebaceous adenocarcinoma
- Metastatic small cell neuroendocrine carcinoma
- Metastatic spindle cell melanoma
- Metastatic spindle cell rhabdomyosarcoma
- Metastatic squamous cell carcinoma
- Metastatic superficial spreading melanoma
- Metastatic synovial sarcoma
- Metastatic urothelial carcinoma
- Metastatic well-differentiated neuroendocrine tumor
- Oligometastatic malignant neoplasm
- Oxyphilic adenocarcinoma
- Pachymeningitis
- Pachymeningitis due to secondary malignant neoplastic disease
- Pathologic fracture of bone at site of metastatic neoplasm
- Pleomorphic rhabdomyosarcoma
- Spindle cell malignant melanoma
- Spindle cell rhabdomyosarcoma
- Stage III: Extraglandular extension of tumor without other organ involvement
- Stage III: Macroscopic invasion of neighboring organs
- Stage IV: Distant metastasis or extension into other organs
- Stage IVb: Hematogenous or lymphatic dissemination
- Synovial sarcoma
- Thymic epithelial neoplasm stage finding
- Tumor invades beyond pancreatic capsule to adjacent structures AND/OR organs
- Tumor invasion by direct extension from organ of origin to adjacent organ
- Tumor invasion by direct extension to other structures
- Tumor invasion into adjacent tissues
- Tumor of unknown origin
Instructional NotesGuidance
Instructions from the official ICD-10-CM Tabular List that apply to C79.9: its own notes plus those printed at C79 and Chapter 2. A note printed at a category, block or chapter applies to every code under it.
Applicable To
Conditions this code is used for: synonyms of the title or, for "other specified" codes, the conditions assigned to it. The list is not exhaustive.
- Metastatic cancer NOS
- Metastatic disease NOS
Excludes1
Not coded here: the excluded code is never reported together with this one, unless the two conditions are unrelated.
Notes
General instructions on how these codes are used.
- Functional activity
- All neoplasms are classified in this chapter, whether they are functionally active or not. An additional code from Chapter 4 may be used, to identify functional activity associated with any neoplasm.
- Morphology [Histology]
- Chapter 2 classifies neoplasms primarily by site (topography), with broad groupings for behavior, malignant, in situ, benign, etc. The Table of Neoplasms should be used to identify the correct topography code. In a few cases, such as for malignant melanoma and certain neuroendocrine tumors, the morphology (histologic type) is included in the category and codes.
- Primary malignant neoplasms overlapping site boundaries
- A primary malignant neoplasm that overlaps two or more contiguous (next to each other) sites should be classified to the subcategory/code .8 ('overlapping lesion'), unless the combination is specifically indexed elsewhere. For multiple neoplasms of the same site that are not contiguous, such as tumors in different quadrants of the same breast, codes for each site should be assigned.
- Malignant neoplasm of ectopic tissue
- Malignant neoplasms of ectopic tissue are to be coded to the site mentioned, e.g., ectopic pancreatic malignant neoplasms are coded to pancreas, unspecified (C25.9).
Source: CMS ICD-10-CM Tabular List. How to read instructional notes.
Referenced in Other NotesGuidance
Instructional notes printed at other codes that name C79.9, its category, or a range that includes it.
Applicable To 2
These notes mention C79.9.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
Disease, diseased See Also: Syndrome;
metastatic See Also: Neoplasm, secondary, by site; C79.9
disease See Also: Neoplasm, secondary, by site; C79.9
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Secondary Malignant Neoplasm
a malignant neoplasm that arises from a pre-existing lower grade lesion, or as a result of a primary lesion that has spread to secondary sites, or due to a complication of a cancer treatment.
Table of NeoplasmsClinical
Anatomical sites in the Table of Neoplasms that reference this code family.
Patient EducationClinical
Cancer
Cancer is not just one disease but many diseases. Cancer begins in your cells, which are the building blocks of your body. Usually, your body forms new cells as needed, replacing old cells that die. Sometimes this process goes wrong. New cells grow even when you don't need them, and old cells don't die when they should.
The full article covers:
- What is cancer?
- What are the types of cancer?
- How does cancer develop?
- What are the symptoms of cancer?
- How is cancer diagnosed?
- What are the treatments for cancer?
- Can cancer be prevented?
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert C79.9 to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About C79.9Overview
What is the ICD-10 code for secondary malignant neoplasm of unspecified site?
The ICD-10-CM code for secondary malignant neoplasm of unspecified site is C79.9 (sometimes written as C799). It is billable on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
Is C79.9 (Secondary malignant neoplasm of other and unspecified sites) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report secondary malignant neoplasm of unspecified site on HIPAA-covered claims from October 1, 2026 through September 30, 2027.
What MS-DRG does C79.9 group to?
When secondary malignant neoplasm of unspecified site is the principal diagnosis on an inpatient stay, it groups to MS-DRG 844, 827, 829, 843, 828, 826, 830, 845, with relative weights from 0.8443 to 4.7560 depending on complications. Higher weights mean higher Medicare reimbursement.
Is C79.9 a CC or MCC?
CMS lists C79.9 as a CC (complication or comorbidity) for FY 2027. Reported as a secondary diagnosis, it moves the inpatient stay to a higher-weighted DRG within its severity family. It does not count when the principal diagnosis is one of the 19 closely related codes in its exclusion list.
Can C79.9 be reported with C80.0?
Not as a rule. The Excludes1 note on C79.9 lists "carcinomatosis NOS (C80.0)". An Excludes1 note means the excluded code is never reported together with this one; the only exception is when the two conditions are unrelated to each other (Official Guidelines, Section I.A.12.a).
What HCC is C79.9?
C79.9 (secondary malignant neoplasm of unspecified site) maps to CMS-HCC Category 18 (Cancer Metastatic to Bone, Other and Unspecified Metastatic Cancer; Acute Leukemia Except Myeloid), commonly written as HCC 18, in the CMS-HCC V28 model used for Medicare Advantage risk adjustment in payment year 2027. It mapped to HCC 8 under the retired V24 model. It also maps in the PACE (CMS-HCC V22), ESRD (V21), and ESRD (V24) models. In the Part D prescription drug model it maps to RxHCC 18.
Does C79.9 risk-adjust for Medicare Advantage payment?
Yes. When documented and reported on a Medicare Advantage encounter, C79.9 adds a risk adjustment factor of about 2.341 to the beneficiary's RAF score for a community, non-dual, aged enrollee (published V28 weights range from 1.110 to 2.537 depending on the payment segment). A more severe related category (HCC 17) supersedes it when both are reported. See the full factor table on the HCC 18 category page.
