C31.1
BillableMalignant neoplasm of ethmoidal sinus
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C31.1 an HCC code?
Yes. C31.1 maps to Breast, Prostate, Colorectal and Other Cancers and Tumors under the CMS-HCC V28 risk adjustment model (and Colorectal, Bladder, and Other Cancers under V24).
HCC Category Mapping
RAF weights shown are the community, non-dual, aged base weights from the CMS risk adjustment model file. 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.
MEAT Criteria for C31.1
For C31.1 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 C31.1 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C31.1 is the ICD-10-CM diagnosis code for malignant neoplasm of ethmoidal sinus. This code represents cancer that starts in the ethmoid sinus, which is a small air-filled space in the bones between the nose and eyes. It is a type of malignant (cancerous) tumor in this specific location. C31.1 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, C31.1 maps to Breast, Prostate, Colorectal and Other Cancers and Tumors (HCC 21) with a community, non-dual, aged base RAF weight of 0.545. Under the older CMS-HCC V24 model, C31.1 maps to Colorectal, Bladder, and Other Cancers (HCC 11) with a community, non-dual, aged base RAF weight of 0.306. 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.
Verify the specific site of the malignancy is documented as the ethmoid sinus, not other paranasal sinuses (maxillary, sphenoid, or frontal), as each has a distinct code. Because C31.1 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 C31.1 sourced directly from the CMS-HCC risk adjustment model files and the CMS ICD-10-CM code set.
Coding Tips
- •Verify the specific site of the malignancy is documented as the ethmoid sinus, not other paranasal sinuses (maxillary, sphenoid, or frontal), as each has a distinct code
- •Ensure the histological type and stage of the cancer are documented separately, as C31.1 only specifies the anatomical location; additional codes may be needed for grade, stage, and laterality
Clinical Significance
Ethmoidal sinus cancer is relatively rare but clinically significant because of the sinus's proximity to the cribriform plate, orbit, and anterior cranial fossa. Esthesioneuroblastoma (olfactory neuroblastoma) is a unique histological type that arises here. Early skull base invasion is common, making these tumors high-resource cases requiring multidisciplinary management.
Documentation Requirements
- ✓Pathology confirmation with histological type (squamous cell, adenocarcinoma, esthesioneuroblastoma)
- ✓Imaging (CT and MRI) showing extent of disease and relationship to cribriform plate and orbit
- ✓Documentation of intracranial extension if present
- ✓Clear identification of ethmoidal sinus as the primary site
- ✓TNM staging and treatment plan