ICD-10 Code C39 – Malignant neoplasm of other and ill-defined sites in the respiratory system and intrathoracic organs (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code C39 – Malignant neoplasm of other and ill-defined sites in the respiratory system and intrathoracic organs
What it is
C39 identifies a malignant neoplasm in an ill-defined or other specified site within the respiratory system or intrathoracic organs. Use it when the record names a primary cancer in this region but does not support a more specific site code.
Clinical signs
Clinical features vary; refer to documentation. Typical findings may include a documented mass, abnormal imaging, or biopsy-proven malignancy involving a respiratory or intrathoracic structure.
When to use this code
Use C39 when the provider documents a primary malignant tumor in the respiratory system or intrathoracic organs, but the exact anatomic site is vague or not further specified. It may also fit when the chart says “other” or “ill-defined” site and no more precise code is supported. Check documentation if the site could be coded elsewhere.
Do not use for
Do not use C39 for secondary or metastatic disease, or when a more specific site code is documented. If the record describes a benign, in situ, or uncertain neoplasm, choose the appropriate different category.
Coding tip
Confirm whether the malignancy is primary and whether the exact site is documented before assigning C39.