Metastatic Lung Cancer ICD-10 Codes: Secondary Sites Explained

If a lung tumour has spread from somewhere else in the body, its ICD-10-CM code is not the primary lung cancer code. Metastatic lung cancer, meaning a malignancy that started elsewhere and has reached the lung, is classified to category C78.0, Secondary malignant neoplasm of respiratory and digestive organs, with a laterality character at the end. The primary-versus-secondary distinction decides which code family a claim lands in, and mixing the two changes the story the record tells about the patient.

This guide sets out the metastatic lung cancer ICD-10 codes, how to choose laterality, which code is sequenced first when the encounter concerns the lung, and what changes once the original cancer has been removed. The rules come from the ICD-10-CM Official Guidelines for Coding and Reporting, FY 2027. Check them against the release in force for your date of service before you report.

Primary and secondary lung cancer use different codes

Primary malignant neoplasm of the bronchus and lung is in category C34, with site subcategories for the main bronchus, lobes and other parts. A cancer that began in the lung is coded there even when it has spread widely. A malignancy that began in another organ and has reached the lung is a secondary neoplasm, and the lung is coded in C78.0-.

The guidelines make the rule explicit: any mention of extension, invasion or metastasis to another site is coded as a secondary malignant neoplasm to that site (Section I.C.2.d). Documentation that says a cancer is metastatic to the lungs is therefore a prompt to establish where the disease started, not a reason to default to C34. Coders who want a broader view of how codes are structured can start with the ICD-10-CM code guide.

Choosing laterality: C78.00, C78.01 or C78.02

The final character of C78.0- records laterality. Use C78.01 for documented right lung disease and C78.02 for documented left lung disease. C78.00 is the fallback for an unspecified lung and belongs only where the record does not say which lung is involved. If the imaging report or the provider's assessment names a side, the more specific code is correct, and the unspecified code should not be used to avoid a judgement about the record. A billable code check confirms that the final character you have chosen is a valid, reportable leaf.

Sequencing: which code goes first

Sequencing depends on why the patient is in front of the clinician, not on which tumour is largest. Section I.C.2.l and Section I.C.2.f of the guidelines give the main cases:

• Treatment is for the primary malignancy. Assign the primary code first, followed by any metastatic sites.
• Treatment is directed only at the metastatic site. The metastatic site becomes the principal or first-listed diagnosis, and the primary malignancy is coded as an additional code.
• The encounter is to determine the extent of the malignancy. The primary or the appropriate metastatic site is designated first, even though chemotherapy or radiotherapy is given.

The second case is the one most often missed. A lung metastasis that is being treated in its own right, while a colon or breast primary is only historical or quiescent, takes the lung code first. The primary still goes on the claim, but as a secondary code.

When the original cancer has been removed

Once a primary malignancy has been excised, no further treatment is directed to that site and no evidence of the cancer remains there, a code from category Z85, Personal history of malignant neoplasm, indicates the former site (Section I.C.2.d and Section I.C.2.m). Z85 applies to the former site of a primary malignancy, not to the site of a secondary. The one exception is Z85.89, which the guidelines allow for the former site of either a primary or a secondary malignancy.

This matters for lung metastases. A patient whose breast cancer was excised and who now has active disease in the lung has the lung coded as current disease in C78.0-, not as a Z85 history. The breast can carry a history code, if the rules above are met for that site. The two decisions are independent, and each has to be made on its own documentation.

Unknown primary: when C80 applies

Sometimes the lung is the only site named and the origin of the disease has not been established. Section I.C.2.j says C80.0, Disseminated malignant neoplasm, unspecified, is for advanced metastatic disease where no primary or secondary sites are specified, and that it should not be used in place of coding the known secondary sites. C80.1, Malignant (primary) neoplasm, unspecified, is for cases where no determination can be made about the primary site. The guidelines say it should rarely be used in the inpatient setting (Section I.C.2.k). If the lung is documented, code it in C78.0- and let the workup decide what else is added later.

A worked example

Consider a record that documents a metastasis in the right lung from a colon adenocarcinoma. The colon primary was resected, and the current encounter is chemotherapy directed at the lung metastasis only. Under Section I.C.2.l, the metastatic site is the principal diagnosis, so C78.01 is sequenced first. The colon primary is no longer under current treatment and has been removed, so it is reported with a Z85 history code for the former large intestine site rather than as a current malignancy. The sequence would change if the encounter were for the colon itself. This is an illustration of the rules, not a substitute for reading the encounter note.

Checks before you report

• Confirm the origin of the tumour from the pathology or oncology note, not from the lung finding alone.
• Check laterality in the same documentation as the diagnosis, and use C78.00 only when the side is genuinely unspecified.
• Decide whether the treatment is aimed at the lung, the original primary or both, because that decides sequencing.
• Confirm whether the original primary is still present or has been eradicated before choosing between a current code and Z85.
• Verify the code and descriptor in the tabular list for the fiscal year of the date of service.

Common pitfalls

• Coding metastatic lung disease to C34 because the lung is where the abnormality was seen.
• Using C78.00 out of habit when the record names the side.
• Applying a Z85 code to the lung when the lung disease is current.
• Using C80.0 when a secondary site is already documented.

For related guidance, the small cell lung cancer coding guide covers primary lung histology, and the prostate cancer code guide shows how metastatic disease is handled in another site.