ICD-10 Code C78.6 – Secondary malignant neoplasm of retroperitoneum and peritoneum (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code C78.6 – Secondary malignant neoplasm of retroperitoneum and peritoneum
What it is
This code identifies metastatic cancer that has spread to the retroperitoneum or peritoneum. Use it when documentation states secondary malignant involvement of these abdominal spaces.
Clinical signs
Clinical features vary; refer to documentation. Common findings may include abdominal distention, pain, ascites, bowel changes, or imaging showing peritoneal or retroperitoneal implants.
When to use this code
Use C78.6 when the provider documents secondary malignant neoplasm in the retroperitoneum or peritoneum, including peritoneal carcinomatosis. It may be supported by pathology, imaging, or operative findings. Code the primary malignancy separately when known and still active.
Do not use for
Do not use this code for primary peritoneal cancer or benign peritoneal disease. Check documentation if the record only notes abdominal metastasis without specifying retroperitoneal or peritoneal involvement.
Coding tip
Confirm the metastatic site is explicitly documented; vague terms like “abdominal spread” are not enough.