What is ICD-10 code C96.20?

C96.20 is the ICD-10-CM diagnosis code for Malignant mast cell neoplasm, unspecified. A malignant cancer arising from mast cells (immune cells) where the specific type or extent of disease is not specified. It belongs to the ICD-10-CM chapter for neoplasms (c00-d49). C96.20 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. C96.20 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. C96.20 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. C96.20 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 C96.20?

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 C96.20, V28 maps to Lymphoma and Other Cancers at RAF 0.671 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 C96.20?

Use this code only when the specific type of mast cell neoplasm cannot be determined from documentation Review records for more specific information to assign C96.21, C96.22, or C96.29 if available Because C96.20 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 C96.20: Malignant mast cell

ICD-10-CM Code View

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Code lookupC96.20Effective periodFY2026 Apr Update (Apr 1-Sep 30)

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

C96.20

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

Malignant mast cell neoplasm, unspecified

A malignant cancer arising from mast cells (immune cells) where the specific type or extent of disease is not specified.

Buddy Insight

Malignant mast cell neoplasm, unspecified, is used when a mast cell malignancy is confirmed but the specific subtype (aggressive systemic mastocytosis, mast cell sarcoma, or mast cell leukemia) cannot be determined from available documentation.

CMS-HCC V28

Mapped

HCC 21

Lymphoma and Other Cancers

RAF 0.671

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

C96Other and unspecified malignant neoplasms of lymphoid, hematopoietic and related tissue

C96.2Malignant mast cell neoplasm

C96.20Malignant mast cell neoplasm, unspecified

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for C96.20 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for C96.20 in this effective period.

Related Child Codes

Official

C96.21Aggressive systemic mastocytosis

C96.22Mast cell sarcoma

C96.29Other malignant mast cell neoplasm

Includes

Official

ICD-10-CM does not list Includes notes for C96.20 in this effective period.

Excludes 1

Official

indolent mastocytosis (D47.02)
mast cell leukemia (C94.30)
mastocytosis (congenital) (cutaneous) (Q82.2)

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for C96.20 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for C96.20 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for C96.20 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Documentation should confirm the malignant nature of the mast cell proliferation and explain why further subclassification is not possible.

Bone marrow biopsy findings, serum tryptase levels, KIT mutation status, and the presence or absence of organ damage should be recorded to enable more specific classification.

MEAT Support

HCC Buddy guidance

Documentation should confirm the malignant nature of the mast cell proliferation and explain why further subclassification is not possible.

Bone marrow biopsy findings, serum tryptase levels, KIT mutation status, and the presence or absence of organ damage should be recorded to enable more specific classification.

Audit Caution

HCC Buddy guidance

Query the provider for additional diagnostic information to assign a more specific code.

Unspecified mast cell neoplasm codes may not adequately capture disease severity.

Distinguish malignant from indolent mastocytosis (D47.02), as this distinction significantly affects management and prognosis.

Common Mistakes

HCC Buddy guidance

C96.21 (aggressive systemic mastocytosis), C96.22 (mast cell sarcoma), and C94.3

(mast cell leukemia) are more specific alternatives. D47.02 (systemic mastocytosis) is for non-aggressive forms. C96.29 (other malignant mast cell neoplasm) is used for known but non-standard malignant variants.

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 C96.20 an HCC code?

Yes. C96.20 (Malignant mast cell neoplasm, unspecified) maps to Lymphoma and Other Cancers under the CMS-HCC V28 risk adjustment model (and Lymphoma and Other Cancers under V24), with a community non-dual aged RAF of 0.671. It is billable for payment year 2026.

Coder answer: C96.20 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: C96.20
Description: Malignant mast cell neoplasm, unspecified
HCC (V28): HCC 21 — Lymphoma and Other Cancers
RAF: 0.671
Billable: Yes
Payment year: 2026

HCC Category Mapping

V28HCC 21, Lymphoma and Other Cancers

0.671

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 C96.20 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for C96.20

For C96.20 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 C96.20 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

C96.20 is the ICD-10-CM diagnosis code for malignant mast cell neoplasm, unspecified. A malignant cancer arising from mast cells (immune cells) where the specific type or extent of disease is not specified. C96.20 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, C96.20 maps to Lymphoma and Other Cancers (HCC 21) with a community, non-dual, aged base RAF weight of 0.671. Under the older V24 model, C96.20 mapped to the same category but with a base RAF weight of 0.675, V28 recalibrated weights across the entire model. 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 mast cell neoplasm cannot be determined from documentation. Because C96.20 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 C96.20 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 mast cell neoplasm cannot be determined from documentation
•Review records for more specific information to assign C96.21, C96.22, or C96.29 if available

Clinical Significance

Documentation Requirements

✓Documentation should confirm the malignant nature of the mast cell proliferation and explain why further subclassification is not possible.
✓Bone marrow biopsy findings, serum tryptase levels, KIT mutation status, and the presence or absence of organ damage should be recorded to enable more specific classification.

Commonly Confused Codes

•C96.21 (aggressive systemic mastocytosis), C96.22 (mast cell sarcoma), and C94.3
•(mast cell leukemia) are more specific alternatives. D47.02 (systemic mastocytosis) is for non-aggressive forms. C96.29 (other malignant mast cell neoplasm) is used for known but non-standard malignant variants.