What is ICD-10 code C84.99?

C84.99 is the ICD-10-CM diagnosis code for Mature T/NK-cell lymphomas, unspecified, extranodal and solid organ sites. A type of blood cancer affecting T cells or natural killer cells that has spread to organs outside the lymph node system, but the specific subtype cannot be determined. C84.99 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. C84.99 is part of the FY2026 ICD-10-CM code set published by the CMS National Center for Health Statistics and used on Medicare, Medicaid, and commercial claims in the United States.

Yes. C84.99 is a billable ICD-10-CM code that can be reported on professional and institutional claims as a primary or secondary diagnosis when the provider documentation supports the specific condition described. Billable codes in ICD-10-CM are coded to the highest level of specificity available for the documented diagnosis. C84.99 is current for fiscal year 2026 and remains in the active CMS ICD-10-CM code set published by the National Center for Health Statistics.

What is the V28 vs V24 difference for C84.99?

CMS phased in the V28 risk adjustment model over payment years 2024 (33%), 2025 (67%), and 2026 (100%), replacing V24. V28 consolidates HCC categories, removes some lower-acuity conditions, and recalibrates RAF weights across the entire model. For C84.99, V28 maps to Lung and Other Severe Cancers at RAF 1.136 while V24 mapped to Lymphoma and Other Cancers at RAF 0.675. Both models can still appear in legacy data and in payer-specific lookback periods.

What should coders know when reporting C84.99?

Use this code only when the specific type of mature T/NK-cell lymphoma cannot be identified The '99' indicates extranodal and solid organ involvement; verify documentation specifies organ sites affected Because C84.99 maps to a payment HCC, the documentation must also satisfy MEAT criteria (Monitor, Evaluate, Assess, or Treat) for the encounter to count toward the patient's risk adjustment score.

2026 ICD-10-CM C84.99: Mature T/NK-cell lymphomas

ICD-10-CM Code View

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Code lookupC84.99Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Neoplasms (C00-D49) / Malignant neoplasms of lymphoid, hematopoietic and related tissue (C81-C96)

C84.99

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

Mature T/NK-cell lymphomas, unspecified, extranodal and solid organ sites

A type of blood cancer affecting T cells or natural killer cells that has spread to organs outside the lymph node system, but the specific subtype cannot be determined.

Buddy Insight

Mature T/NK-cell lymphomas, unspecified, involving extranodal and solid organ sites indicates unclassified T/NK-cell lymphoma affecting organs outside the lymphatic system.

CMS-HCC V28

Mapped

HCC 20

Lung and Other Severe Cancers

RAF 1.136

CMS-HCC V24

Mapped

HCC 10

Lymphoma and Other Cancers

RAF 0.675

ACA/HHS

Mapped

HCC 10

HHS-HCC 10

Varies by metal level

ESRD/PACE

Mapped

HCC 10

Lymphoma and Other Cancers

RAF 0.111

RXHCC

Mapped

HCC 21

Hodgkin Lymphoma and Other Ca...

RAF 0.410

Code Book Path

Official

C84Mature T/NK-cell lymphomas

C84.9Mature T/NK-cell lymphomas, unspecified

C84.99Mature T/NK-cell lymphomas, unspecified, extranodal and solid organ sites

Inclusion Terms

Official

NK/T cell lymphoma NOS

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for C84.99 in this effective period.

Related Child Codes

Official

C84.90Mature T/NK-cell lymphomas, unspecified, unspecified site

C84.91Mature T/NK-cell lymphomas, unspecified, lymph nodes of head, face, and neck

C84.92Mature T/NK-cell lymphomas, unspecified, intrathoracic lymph nodes

C84.93Mature T/NK-cell lymphomas, unspecified, intra-abdominal lymph nodes

C84.94Mature T/NK-cell lymphomas, unspecified, lymph nodes of axilla and upper limb

Includes

Official

ICD-10-CM does not list Includes notes for C84.99 in this effective period.

Excludes 1

Official

mature T-cell lymphoma, not elsewhere classified (C84.4-)

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for C84.99 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for C84.99 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for C84.99 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Pathology confirming mature T-cell or NK-cell lymphoma in extranodal tissue

Specific organ or extranodal site identified

Biopsy of extranodal site with immunophenotyping

Documentation excluding specific extranodal T-cell lymphoma subtypes

MEAT Support

HCC Buddy guidance

Pathology confirming mature T-cell or NK-cell lymphoma in extranodal tissue

Specific organ or extranodal site identified

Biopsy of extranodal site with immunophenotyping

Documentation excluding specific extranodal T-cell lymphoma subtypes

Audit Caution

HCC Buddy guidance

Using this unspecified code when a specific extranodal T-cell lymphoma subtype is identifiable

Coding splenic involvement as extranodal when C84.97 exists

Not considering specific extranodal entities (C86.0-C86.3) that have distinct codes

Failing to document the specific extranodal site involved

Common Mistakes

HCC Buddy guidance

C86.0 — Extranodal NK/T-cell lymphoma, nasal type: Specific EBV-associated entity affecting nasal/midface region

C86.1 — Hepatosplenic T-cell lymphoma: Specific entity affecting liver and spleen

C86.2 — Enteropathy-associated T-cell lymphoma: Specific GI tract entity associated with celiac disease

C86.3 — Subcutaneous panniculitis-like T-cell lymphoma: Specific entity involving subcutaneous fat

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 C84.99 an HCC code?

Yes. C84.99 (Mature T/NK-cell lymphomas, unspecified, extranodal and solid organ sites) maps to Lung and Other Severe Cancers under the CMS-HCC V28 risk adjustment model (and Lymphoma and Other Cancers under V24), with a community non-dual aged RAF of 1.136. It is billable for payment year 2026.

Coder answer: C84.99 is billable and maps to V28 HCC 20, Lung and Other Severe Cancers. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.

Code: C84.99
Description: Mature T/NK-cell lymphomas, unspecified, extranodal and solid organ sites
HCC (V28): HCC 20 — Lung and Other Severe Cancers
RAF: 1.136
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 20, Lung and Other Severe Cancers

1.136

V24HCC 10, Lymphoma and Other Cancers

0.675

ESRDHCC 10, Lymphoma and Other Cancers

0.111

RxHCCHCC 21, Hodgkin Lymphoma and Other Cancers

0.410

Each model's RAF is its CMS base weight for that model's standard population, so 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 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.

Work C84.99 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for C84.99

For C84.99 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 C84.99 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

C84.99 is the ICD-10-CM diagnosis code for mature t/nk-cell lymphomas, unspecified, extranodal and solid organ sites. A type of blood cancer affecting T cells or natural killer cells that has spread to organs outside the lymph node system, but the specific subtype cannot be determined. C84.99 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering malignant neoplasms of lymphoid, hematopoietic and related tissue (c81-c96).

Under the CMS-HCC V28 risk adjustment model, C84.99 maps to Lung and Other Severe Cancers (HCC 20) with a community, non-dual, aged base RAF weight of 1.136. Under the older CMS-HCC V24 model, C84.99 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.

Use this code only when the specific type of mature T/NK-cell lymphoma cannot be identified. Because C84.99 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 C84.99 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Use this code only when the specific type of mature T/NK-cell lymphoma cannot be identified
•The '99' indicates extranodal and solid organ involvement; verify documentation specifies organ sites affected

Clinical Significance

Mature T/NK-cell lymphomas, unspecified, involving extranodal and solid organ sites indicates unclassified T/NK-cell lymphoma affecting organs outside the lymphatic system. Extranodal T-cell lymphomas include several specific entities (enteropathy-associated, hepatosplenic, subcutaneous panniculitis-like) that should be excluded before using this unspecified code. Specific subtyping is critical for treatment selection.

Documentation Requirements

✓Pathology confirming mature T-cell or NK-cell lymphoma in extranodal tissue
✓Specific organ or extranodal site identified
✓Biopsy of extranodal site with immunophenotyping
✓Documentation excluding specific extranodal T-cell lymphoma subtypes
✓Clinical staging

Commonly Confused Codes

•C86.0: Extranodal NK/T-cell lymphoma, nasal type: Specific EBV-associated entity affecting nasal/midface region
•C86.1: Hepatosplenic T-cell lymphoma: Specific entity affecting liver and spleen
•C86.2: Enteropathy-associated T-cell lymphoma: Specific GI tract entity associated with celiac disease
•C86.3: Subcutaneous panniculitis-like T-cell lymphoma: Specific entity involving subcutaneous fat
•C84.97: Mature T/NK-cell lymphoma, spleen: Spleen has its own code