C84.05 maps to CMS-HCC V28 20. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · HCC coding software

ICD-10-CM Code View

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Code lookupC84.05

Effective periodFY2026 Apr Update (Apr 1-Sep 30)

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

C84.05

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

Mycosis fungoides, lymph nodes of inguinal region and lower limb

A rare type of skin cancer (mycosis fungoides) that has spread to the lymph nodes in the groin and lower leg areas.

Buddy Insight

Mycosis fungoides involving inguinal and lower limb lymph nodes indicates disease that has spread beyond the skin to the lymph nodes in the groin and lower extremities.

Overview Risk Models Code Book Documentation Audit Notes Edition comparison

CMS-HCC V28

Mapped

HCC 20

Lung and Other Severe Cancers

Coefficient HCC 20: 1.136 (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 010

HHS-HCC 010 adult, RAF varies...

Code-level coefficient reference

ESRD/PACE

Mapped

HCC 10

Lymphoma and Other Cancers

Code-level coefficient reference

RXHCC

Mapped

HCC 19

Leukemias and Other Hematolog...

Code-level coefficient reference

Code Book Path

Official

C84Mature T/NK-cell lymphomas C84.0Mycosis fungoides C84.05Mycosis fungoides, lymph nodes of inguinal region and lower limb

Inclusion Terms

Official

No inclusion terms are included in this display for C84.05. Check the code and parent instructions in the Code Book.

Excludes 2

Official

Kaposi's sarcoma of lymph nodes (C46.3)Inherited from C81-C96
secondary and unspecified neoplasm of lymph nodes (C77.-)Inherited from C81-C96
secondary neoplasm of bone marrow (C79.52)Inherited from C81-C96
secondary neoplasm of spleen (C78.89)Inherited from C81-C96

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

Includes

Official

No Includes notes are included in this display for C84.05. Check the code and parent instructions in the Code Book.

Excludes 1

Official

personal history of non-Hodgkin lymphoma (Z85.72)Inherited from C84, C84.0
peripheral T-cell lymphoma, not elsewhere classified (C84.4-)Inherited from C84, C84.0

Code First

Official

No Code First sequencing instructions are included in this display for C84.05. Check the code and parent instructions in the Code Book.

Use Additional

Official

No Use Additional Code instructions are included in this display for C84.05. Check the code and parent instructions in the Code Book.

Code Also

Official

No Code Also instructions are included in this display for C84.05. Check the code and parent instructions in the Code Book.

Buddy Documentation Tip

HCC Buddy guidance

Skin biopsy confirming mycosis fungoides

Inguinal lymph node biopsy or imaging confirming involvement

Distinction between reactive and lymphomatous nodal involvement

TNMB staging reflecting nodal disease

MEAT Support

HCC Buddy guidance

Skin biopsy confirming mycosis fungoides

Inguinal lymph node biopsy or imaging confirming involvement

Distinction between reactive and lymphomatous nodal involvement

TNMB staging reflecting nodal disease

Audit Caution

HCC Buddy guidance

Confusing inguinal lymph nodes with intrapelvic lymph nodes — different anatomic locations with different codes

Using unspecified site code when inguinal lymphadenopathy is clearly documented

Not distinguishing between reactive inguinal lymphadenopathy from lower extremity skin disease and true lymphomatous involvement

Coding skin lesion location on the legs as inguinal lymph node involvement — these are different concepts

Common Mistakes

HCC Buddy guidance

C84.06 — Mycosis fungoides, intrapelvic lymph nodes: Intrapelvic nodes are deep within the pelvis; inguinal nodes are in the groin — distinct anatomic locations

C84.00 — Mycosis fungoides, unspecified site: Use C84.05 when inguinal/lower limb involvement is confirmed

C77.4 — Secondary malignant neoplasm, inguinal: 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 C84.05 an HCC code?

Yes. C84.05 (Mycosis fungoides, lymph nodes of inguinal region and lower limb) maps to HCC 20, Lung and Other Severe Cancers under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 1.136. 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: C84.05 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: C84.05
Description: Mycosis fungoides, lymph nodes of inguinal region and lower limb
HCC (V28): HCC 20 — Lung and Other Severe Cancers
RAF reference coefficient: 1.136
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 20, Lung and Other Severe Cancers

1.136

ESRDHCC 10, Lymphoma and Other Cancers

Not separately weighted

RxHCCHCC 19, Leukemias and Other Hematologic Cancers

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 C84.05 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →

MEAT review for C84.05

For C84.05, 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

C84.05 is the ICD-10-CM diagnosis code for mycosis fungoides, lymph nodes of inguinal region and lower limb. A rare type of skin cancer (mycosis fungoides) that has spread to the lymph nodes in the groin and lower leg areas. C84.05 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, C84.05 maps to Lung and Other Severe Cancers (HCC 20) with a source-labeled community, non-dual, aged reference coefficient of 1.136. 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.

Verify inguinal and lower limb lymph node involvement through physical examination or imaging. For C84.05, 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 C84.05 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•Verify inguinal and lower limb lymph node involvement through physical examination or imaging
•The '5' designator indicates inguinal region and lower extremity lymph node sites

Clinical Significance

Mycosis fungoides involving inguinal and lower limb lymph nodes indicates disease that has spread beyond the skin to the lymph nodes in the groin and lower extremities. This advancement in staging requires evaluation for systemic therapy and carries prognostic significance. Inguinal nodes are commonly involved in mycosis fungoides patients with lower extremity skin disease.

Documentation Requirements

✓Skin biopsy confirming mycosis fungoides
✓Inguinal lymph node biopsy or imaging confirming involvement
✓Distinction between reactive and lymphomatous nodal involvement
✓TNMB staging reflecting nodal disease
✓Treatment plan for advanced-stage disease

Commonly Confused Codes

•C84.06: Mycosis fungoides, intrapelvic lymph nodes: Intrapelvic nodes are deep within the pelvis; inguinal nodes are in the groin: distinct anatomic locations
•C84.00: Mycosis fungoides, unspecified site: Use C84.05 when inguinal/lower limb involvement is confirmed
•C77.4: Secondary malignant neoplasm, inguinal: Do not use for primary lymphoma