C96.6 ICD-10-CM Code: Unifocal Langerhans-cell histiocytosis
C96.6 maps to CMS-HCC V28 21. A source-labeled RAF reference is available. Confirm the documented diagnosis and applicable coding requirements. MEAT criteria · RAF Calculator · free HCC coding tools
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FY 2026 Apr update / Neoplasms (C00-D49) / Malignant neoplasms of lymphoid, hematopoietic and related tissue (C81-C96)
C96.6
Billable / SpecificICD-10-CMOfficial ICD-10-CMCodebook guidanceUnifocal Langerhans-cell histiocytosis
A rare cancer involving abnormal growth of Langerhans cells (immune cells) that is limited to a single site in the body.
Buddy Insight
Unifocal Langerhans-cell histiocytosis, previously known as eosinophilic granuloma, is a localized proliferative disorder of Langerhans cells confined to a single anatomical site, most commonly bone.
CMS-HCC V28
MappedHCC 21
Coefficient HCC 21: 0.671 (Community Non-Dual Aged (CNA))
CMS-HCC V24
HistoricalHistorical
Not used for CY2026 payment
ACA/HHS
MappedHCC 010
Code-level coefficient reference
ESRD/PACE
MappedHCC 10
Code-level coefficient reference
RXHCC
MappedHCC 21
Code-level coefficient reference
Code Book Path
Inclusion Terms
Official- Eosinophilic granuloma
- Histiocytosis X, unifocal
- Histiocytosis X NOS
- Langerhans-cell histiocytosis NOS
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
Includes
OfficialNo Includes notes are included in this display for C96.6. Check the code and parent instructions in the Code Book.
Excludes 1
Official- multifocal and multisysemic (disseminated) Langerhans-cell histiocytosis (C96.0)
- multifocal and unisystemic Langerhans-cell histiocytosis (C96.5)
- personal history of other malignant neoplasms of lymphoid, hematopoietic and related tissues (Z85.79)Inherited from C96
Code First
OfficialNo Code First sequencing instructions are included in this display for C96.6. Check the code and parent instructions in the Code Book.
Use Additional
OfficialNo Use Additional Code instructions are included in this display for C96.6. Check the code and parent instructions in the Code Book.
Code Also
OfficialNo Code Also instructions are included in this display for C96.6. Check the code and parent instructions in the Code Book.
Buddy Documentation Tip
MEAT Support
Audit Caution
Common Mistakes
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 C96.6 an HCC code?
Yes. C96.6 (Unifocal Langerhans-cell histiocytosis) maps to HCC 21, Lymphoma and Other Cancers under the CMS-HCC V28 risk adjustment model, with a source-labeled community non-dual aged reference coefficient of 0.671. 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: C96.6 is billable and maps to V28 HCC 21, Lymphoma and Other Cancers. Open it in the Code Book for the tabular path, RAF, and MEAT checklist.
- Code
- C96.6
- Description
- Unifocal Langerhans-cell histiocytosis
- HCC (V28)
- HCC 21 — Lymphoma and Other Cancers
- RAF reference coefficient
- 0.671
- Billable
- Yes
- Payment year
- 2026
HCC Category Mapping
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 C96.6 in the Code Book — tabular path, V28 RAF reference, and MEAT checklist →
MEAT review for C96.6
For C96.6, 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
C96.6 is the ICD-10-CM diagnosis code for unifocal langerhans-cell histiocytosis. A rare cancer involving abnormal growth of Langerhans cells (immune cells) that is limited to a single site in the body. C96.6 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, C96.6 maps to Lymphoma and Other Cancers (HCC 21) with a source-labeled community, non-dual, aged reference coefficient of 0.671. 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.
This code indicates disease confined to one location; confirm unifocal involvement is documented. For C96.6, 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 C96.6 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •This code indicates disease confined to one location; confirm unifocal involvement is documented
- •Distinguish from C96.0 (multifocal/disseminated) which affects multiple sites
Clinical Significance
Unifocal Langerhans-cell histiocytosis, previously known as eosinophilic granuloma, is a localized proliferative disorder of Langerhans cells confined to a single anatomical site, most commonly bone. It generally carries a favorable prognosis with high cure rates after local treatment such as curettage or intralesional steroid injection. However, it requires monitoring because a subset of cases can progress to multifocal or multisystem disease.
Documentation Requirements
- ✓Documentation must confirm the diagnosis is limited to a single anatomical site (unifocal) with pathologic confirmation of Langerhans-cell histiocytosis through biopsy showing CD1a or langerin positivity.
- ✓The specific site of involvement, imaging confirmation of the solitary lesion, and treatment plan should be recorded.
- ✓Follow-up documentation should note stability or any progression to multifocal disease.