C31.8
BillableMalignant neoplasm of overlapping sites of accessory sinuses
Last updated: FY2026 ICD-10-CM (Oct 1, 2025 – Sep 30, 2026) | CMS-HCC V28 (100% phase-in, PY2026)
Is C31.8 an HCC code?
Yes. C31.8 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 C31.8
For C31.8 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 C31.8 during that encounter — not just copy-forwarded from a problem list.
What This Code Means
C31.8 is the ICD-10-CM diagnosis code for malignant neoplasm of overlapping sites of accessory sinuses. Cancer that involves multiple accessory sinuses (the small air-filled spaces around the nose and eyes) at the same time. C31.8 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, C31.8 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, C31.8 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.
Use this code only when the tumor clearly involves two or more different accessory sinus sites. Because C31.8 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 C31.8 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 tumor clearly involves two or more different accessory sinus sites
- •If a single sinus site is involved, use the specific site code instead of the overlapping sites code
Clinical Significance
This code captures paranasal sinus cancers that span two or more sinus cavities, indicating locally advanced disease. Overlapping sinus involvement typically requires more aggressive surgery and radiation, resulting in greater functional impairment and healthcare resource utilization. Accurate coding reflects the true extent of disease.
Documentation Requirements
- ✓Imaging demonstrating tumor involvement of two or more distinct sinus cavities
- ✓Pathology confirmation of malignancy
- ✓Documentation specifying which sinuses are involved
- ✓TNM staging reflecting the extent of multi-sinus disease
- ✓Surgical and treatment planning documentation