ICD-10 Code C31.8 – Malignant neoplasm of overlapping sites of accessory sinuses (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code C31.8 – Malignant neoplasm of overlapping sites of accessory sinuses
What it is
This code identifies a malignant tumor arising in accessory sinuses that spans overlapping sites, so no single sinus site is clearly primary. Use it when the documentation states overlapping involvement of these structures.
Clinical signs
Clinical features vary; refer to documentation. Commonly recorded findings may include persistent sinonasal obstruction, facial pain or pressure, epistaxis, or mass effect, but the diagnosis should be supported by imaging, endoscopy, or pathology.
When to use this code
Use C31.8 when the provider documents a primary malignant neoplasm involving more than one accessory sinus site, or specifically names overlapping sites. It is appropriate when the record does not support assignment to one exact sinus location. Check documentation for the anatomic extent and whether the lesion is primary or metastatic.
Do not use for
Do not use this code for a tumor confined to one named sinus, such as maxillary or ethmoidal sinus. Also avoid it when the documentation describes benign disease, uncertain behavior, or metastatic disease from another primary site.
Coding tip
Verify the exact sinus involvement in the pathology or operative note before selecting C31.8.