ICD-10 Code C18.1 – Malignant neoplasm of appendix (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code C18.1 – Malignant neoplasm of appendix
What it is
C18.1 identifies a malignant tumor arising in the appendix. Use it when documentation states primary cancer of the appendix, including appendiceal carcinoma or other malignant neoplasm of that site.
Clinical signs
Clinical features vary; refer to documentation. Patients may present with right lower quadrant pain, an appendiceal mass, or findings discovered during surgery or pathology after appendectomy.
When to use this code
Use C18.1 when the provider documents a primary malignant neoplasm of the appendix. It is also appropriate when pathology confirms appendiceal cancer and the record does not specify a different primary site.
Code this diagnosis from operative reports, oncology notes, or pathology results. If the record only says appendicitis, appendiceal mucocele, or secondary spread to the appendix, check documentation before assigning C18.1.
Do not use for
Do not use C18.1 for benign appendiceal lesions, acute appendicitis, or metastatic disease involving the appendix from another primary site. Check documentation if the primary origin is unclear.
Coding tip
Confirm the appendix is the primary cancer site, and assign additional codes for any documented spread or associated conditions as directed by the record.