What is ICD-10 code C84.04?

C84.04 is the ICD-10-CM diagnosis code for Mycosis fungoides, lymph nodes of axilla and upper limb. A rare type of skin cancer (mycosis fungoides) that has spread to the lymph nodes in the armpits and upper arms. It belongs to the ICD-10-CM chapter for neoplasms (c00-d49). C84.04 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. C84.04 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. C84.04 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. C84.04 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 C84.04?

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 C84.04, 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 C84.04?

Document axillary and upper limb lymph node involvement through physical exam or imaging The '4' designator specifically indicates axillary and upper extremity lymph nodes Because C84.04 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 C84.04: Mycosis fungoides, lymph

ICD-10-CM Code View

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Code lookupC84.04Effective periodFY2026 Apr Update (Apr 1-Sep 30)

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

C84.04

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

Mycosis fungoides, lymph nodes of axilla and upper limb

A rare type of skin cancer (mycosis fungoides) that has spread to the lymph nodes in the armpits and upper arms.

Buddy Insight

Mycosis fungoides with involvement of axillary and upper limb lymph nodes indicates disease progression beyond the skin.

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 19

Lymphoma and Other Cancers

RAF 1.949

Code Book Path

Official

C84Mature T/NK-cell lymphomas

C84.0Mycosis fungoides

C84.04Mycosis fungoides, lymph nodes of axilla and upper limb

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for C84.04 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for C84.04 in this effective period.

Related Child Codes

Official

C84.00Mycosis fungoides, unspecified site

C84.01Mycosis fungoides, lymph nodes of head, face, and neck

C84.02Mycosis fungoides, intrathoracic lymph nodes

C84.03Mycosis fungoides, intra-abdominal lymph nodes

C84.05Mycosis fungoides, lymph nodes of inguinal region and lower limb

Includes

Official

ICD-10-CM does not list Includes notes for C84.04 in this effective period.

Excludes 1

Official

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

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for C84.04 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for C84.04 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for C84.04 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Skin biopsy confirming mycosis fungoides

Axillary lymph node biopsy or imaging confirming involvement

Distinction between dermatopathic lymphadenopathy and lymphomatous involvement

TNMB staging

MEAT Support

HCC Buddy guidance

Skin biopsy confirming mycosis fungoides

Axillary lymph node biopsy or imaging confirming involvement

Distinction between dermatopathic lymphadenopathy and lymphomatous involvement

TNMB staging

Audit Caution

HCC Buddy guidance

Coding reactive dermatopathic axillary lymphadenopathy as lymphomatous involvement without pathologic confirmation

Using unspecified site when axillary node involvement is documented

Confusing mycosis fungoides nodal involvement with metastatic disease from other malignancies

Not recognizing that the axillary/upper limb site code refers to lymph node involvement, not skin lesion distribution on the arms

Common Mistakes

HCC Buddy guidance

C84.00 — Mycosis fungoides, unspecified site: Use C84.04 when axillary/upper limb lymph node involvement is confirmed

C84.14 — Sezary disease, axilla/upper limb: Distinct disease entity requiring circulating malignant T-cells

C77.3 — Secondary malignant neoplasm, axillary lymph nodes: Do not use for primary lymphoma

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 C84.04 an HCC code?

Yes. C84.04 (Mycosis fungoides, lymph nodes of axilla and upper limb) 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: C84.04 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: C84.04
Description: Mycosis fungoides, lymph nodes of axilla and upper limb
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 19, Lymphoma and Other Cancers

1.949

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 C84.04 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for C84.04

For C84.04 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 C84.04 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

C84.04 is the ICD-10-CM diagnosis code for mycosis fungoides, lymph nodes of axilla and upper limb. A rare type of skin cancer (mycosis fungoides) that has spread to the lymph nodes in the armpits and upper arms. C84.04 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, C84.04 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, C84.04 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.

Document axillary and upper limb lymph node involvement through physical exam or imaging. Because C84.04 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 C84.04 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Document axillary and upper limb lymph node involvement through physical exam or imaging
•The '4' designator specifically indicates axillary and upper extremity lymph nodes

Clinical Significance

Mycosis fungoides with involvement of axillary and upper limb lymph nodes indicates disease progression beyond the skin. Axillary lymphadenopathy in mycosis fungoides patients may be reactive (dermatopathic) or represent true lymphomatous involvement, and pathologic distinction is important for accurate staging. True lymph node involvement indicates Stage III-IV disease requiring systemic treatment.

Documentation Requirements

✓Skin biopsy confirming mycosis fungoides
✓Axillary lymph node biopsy or imaging confirming involvement
✓Distinction between dermatopathic lymphadenopathy and lymphomatous involvement
✓TNMB staging
✓Treatment plan reflecting nodal disease

Commonly Confused Codes

•C84.00: Mycosis fungoides, unspecified site: Use C84.04 when axillary/upper limb lymph node involvement is confirmed
•C84.14: Sezary disease, axilla/upper limb: Distinct disease entity requiring circulating malignant T-cells
•C77.3: Secondary malignant neoplasm, axillary lymph nodes: Do not use for primary lymphoma