What is ICD-10 code C31.9?

C31.9 is the ICD-10-CM diagnosis code for Malignant neoplasm of accessory sinus, unspecified. Cancer of the accessory sinuses when the specific sinus location cannot be determined or is not documented. It belongs to the ICD-10-CM chapter for neoplasms (c00-d49). C31.9 is a billable code, meaning it can be reported on a claim as a primary or secondary diagnosis when supported by documentation. C31.9 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. C31.9 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. C31.9 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.

Which HCC does C31.9 map to under V28?

C31.9 maps to CMS-HCC V28 21 (Lymphoma and Other Cancers). The category number and label come from the CMS-HCC V28 model file for payment year 2026.

What does C31.9 pay in RAF?

C31.9 carries a community, non-dual, aged (CNA) RAF weight of 0.671 under CMS-HCC V28 for payment year 2026. The weight comes from the CMS V28 relative-factor file, not a hand-typed table.

What is the V28 vs V24 difference for C31.9?

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 C31.9, V28 maps to Lymphoma and Other Cancers at RAF 0.671 while V24 mapped to Colorectal, Bladder, and Other Cancers at RAF 0.307. Both models can still appear in legacy data and in payer-specific lookback periods.

What documentation supports C31.9?

This is a default code used when documentation does not specify which accessory sinus is affected Query the provider if possible to obtain more specific anatomical location for better clinical specificity Because C31.9 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. See the MEAT criteria checklist at /meat-criteria for what Monitor, Evaluate, Assess, or Treat looks like in a note. MEAT is generic documentation evidence, not a payer-specific rule.

2026 ICD-10-CM C31.9: Malignant neoplasm

C31.9 maps to CMS-HCC V28 21 (RAF 0.671). Documentation must support MEAT. MEAT criteria · RAF calculator · HCC Buddy coding tools

ICD-10-CM Code View

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Code lookupC31.9Effective periodFY2026 Apr Update (Apr 1-Sep 30)

FY 2026 Apr update / Neoplasms (C00-D49) / Malignant neoplasms of respiratory and intrathoracic organs (C30-C39)

C31.9

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

Malignant neoplasm of accessory sinus, unspecified

Cancer of the accessory sinuses when the specific sinus location cannot be determined or is not documented.

Buddy Insight

Unspecified accessory sinus cancer is used when paranasal sinus malignancy is confirmed but the specific sinus (maxillary, ethmoidal, frontal, sphenoid) cannot be determined.

CMS-HCC V28

Mapped

HCC 21

Lymphoma and Other Cancers

RAF 0.671

CMS-HCC V24

Mapped

HCC 11

Colorectal, Bladder, and Othe...

RAF 0.307

ACA/HHS

Mapped

HCC 11

HHS-HCC 11

Varies by metal level

ESRD/PACE

Mapped

HCC 11

Colorectal, Bladder, and Othe...

RAF 0.059

RXHCC

Mapped

HCC 22

Cancer, Other Specified Sites

RAF 0.124

Code Book Path

Official

C31Malignant neoplasm of accessory sinuses

C31.9Malignant neoplasm of accessory sinus, unspecified

Inclusion Terms

Official

ICD-10-CM does not list inclusion terms for C31.9 in this effective period.

Excludes 2

Official

ICD-10-CM does not list Excludes 2 notes for C31.9 in this effective period.

Related Child Codes

Official

C31.0Malignant neoplasm of maxillary sinus

C31.1Malignant neoplasm of ethmoidal sinus

C31.2Malignant neoplasm of frontal sinus

C31.3Malignant neoplasm of sphenoid sinus

C31.8Malignant neoplasm of overlapping sites of accessory sinuses

Includes

Official

ICD-10-CM does not list Includes notes for C31.9 in this effective period.

Excludes 1

Official

ICD-10-CM does not list Excludes 1 notes for C31.9 in this effective period.

Code First

Official

ICD-10-CM does not list Code First sequencing instructions for C31.9 in this effective period.

Use Additional

Official

ICD-10-CM does not list Use Additional Code instructions for C31.9 in this effective period.

Code Also

Official

ICD-10-CM does not list Code Also instructions for C31.9 in this effective period.

Buddy Documentation Tip

HCC Buddy guidance

Pathology confirmation of sinus malignancy

Evidence that provider was queried for sinus specificity

Imaging reviewed for anatomical detail

Documentation explaining why specific sinus cannot be identified

MEAT Support

HCC Buddy guidance

Pathology confirmation of sinus malignancy

Evidence that provider was queried for sinus specificity

Imaging reviewed for anatomical detail

Documentation explaining why specific sinus cannot be identified

Audit Caution

HCC Buddy guidance

Defaulting to this code without reviewing imaging that almost always identifies the specific sinus

Not querying the provider for sinus specificity when surgical or pathology reports are available

Using this code when documentation actually supports a specific sinus location

Failing to recapture this code annually if the malignancy remains active

Common Mistakes

HCC Buddy guidance

C31.0 — Maxillary sinus: Most common sinus cancer site; review imaging before defaulting to unspecified

C31.8 — Overlapping sinus sites: Use when multiple sinuses are clearly involved

C30.0 — Nasal cavity: Ensure the tumor is in a sinus, not the nasal cavity proper

C76.0 — Malignant neoplasm of head, face, and neck: Even less specific; use C31.9 when sinus origin is confirmed

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 C31.9 an HCC code?

Yes. C31.9 (Malignant neoplasm of accessory sinus, unspecified) maps to Lymphoma and Other Cancers under the CMS-HCC V28 risk adjustment model (and Colorectal, Bladder, and Other Cancers under V24), with a community non-dual aged RAF of 0.671. It is billable for payment year 2026.

Coder answer: C31.9 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: C31.9
Description: Malignant neoplasm of accessory sinus, 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 11, Colorectal, Bladder, and Other Cancers

0.307

ESRDHCC 11, Colorectal, Bladder, and Other Cancers

0.059

RxHCCHCC 22, Cancer, Other Specified Sites

0.124

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 C31.9 in the Code Book — tabular path, V28 RAF, and MEAT checklist →

MEAT Criteria for C31.9

For C31.9 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 C31.9 during that encounter, not just copy-forwarded from a problem list.

Coder workflow notes

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What This Code Means

C31.9 is the ICD-10-CM diagnosis code for malignant neoplasm of accessory sinus, unspecified. Cancer of the accessory sinuses when the specific sinus location cannot be determined or is not documented. C31.9 sits in the ICD-10-CM chapter for neoplasms (c00-d49), within the section covering malignant neoplasms of respiratory and intrathoracic organs (c30-c39).

Under the CMS-HCC V28 risk adjustment model, C31.9 maps to Lymphoma and Other Cancers (HCC 21) with a community, non-dual, aged base RAF weight of 0.671. Under the older CMS-HCC V24 model, C31.9 maps to Colorectal, Bladder, and Other Cancers (HCC 11) with a community, non-dual, aged base RAF weight of 0.307. 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.

This is a default code used when documentation does not specify which accessory sinus is affected. Because C31.9 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 C31.9 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.

Coding Tips

•This is a default code used when documentation does not specify which accessory sinus is affected
•Query the provider if possible to obtain more specific anatomical location for better clinical specificity

Clinical Significance

Unspecified accessory sinus cancer is used when paranasal sinus malignancy is confirmed but the specific sinus (maxillary, ethmoidal, frontal, sphenoid) cannot be determined. This code represents a documentation gap that should trigger a provider query, as treatment planning and prognosis vary by sinus location.

Documentation Requirements

✓Pathology confirmation of sinus malignancy
✓Evidence that provider was queried for sinus specificity
✓Imaging reviewed for anatomical detail
✓Documentation explaining why specific sinus cannot be identified
✓Treatment status and disease stage

Commonly Confused Codes

•C31.0: Maxillary sinus: Most common sinus cancer site; review imaging before defaulting to unspecified
•C31.8: Overlapping sinus sites: Use when multiple sinuses are clearly involved
•C30.0: Nasal cavity: Ensure the tumor is in a sinus, not the nasal cavity proper
•C76.0: Malignant neoplasm of head, face, and neck: Even less specific; use C31.9 when sinus origin is confirmed