ICD-10 Code C78.00 – Secondary malignant neoplasm of unspecified lung (2026): Diagnosis, Symptoms & Billing Guide
2026 ICD-10-CM Diagnosis Code C78.00 – Secondary malignant neoplasm of unspecified lung
What it is
This code identifies metastatic cancer involving the lung when the primary site is known to be elsewhere, but the affected lung is not further specified. Use it for secondary malignant disease in an unspecified lung.
Clinical signs
Clinical features vary; refer to documentation. Common findings may include imaging evidence of pulmonary metastasis, such as lung nodules or masses, with pathology or clinical notes indicating secondary malignant involvement.
When to use this code
Use C78.00 when the record documents metastatic cancer in the lung and does not identify right or left lung. It is appropriate when the clinician states secondary malignant neoplasm of lung, pulmonary metastasis, or similar wording without laterality. Check documentation if the primary cancer site is unclear.
Do not use for
Do not use this code for primary lung cancer or for benign lung nodules. If the chart specifies a right or left lung metastasis, use the more specific laterality code instead.
Coding tip
Assign the code only when the lung involvement is documented as secondary malignant disease, and code the primary malignancy separately when supported by the record.