The ICD-10-CM code for lung cancer is C34, which covers all malignant neoplasms of the bronchus and lung. This parent code branches into more specific subcodes based on where in the lung the tumor is located and which side it’s on. A secondary (metastatic) lung cancer that started somewhere else in the body uses a different code entirely: C78.0.
C34 Subcodes by Tumor Location
ICD-10-CM doesn’t just assign one code for “lung cancer.” It requires specificity about the anatomical site within the lung. Here’s how the C34 category breaks down:
- C34.0: Main bronchus
- C34.1: Upper lobe, bronchus or lung
- C34.2: Middle lobe, bronchus or lung (right lung only, since the left lung has no middle lobe)
- C34.3: Lower lobe, bronchus or lung
- C34.8: Overlapping sites of bronchus and lung (tumor spans more than one area)
- C34.9: Unspecified part of bronchus or lung
C34.9 is often used when documentation doesn’t specify the exact location of the tumor. It’s the most general option, but payers and registries prefer the most specific code supported by the medical record.
How Laterality Works
Each of those site codes adds a final digit to indicate which lung is affected. The pattern is consistent across the C34 family:
- 0: Unspecified side
- 1: Right lung
- 2: Left lung
So a malignant tumor in the upper lobe of the right lung would be coded C34.11. A lower lobe cancer on the left side would be C34.32. If the medical record doesn’t document which side, the code defaults to the unspecified option (ending in 0). CMS coding guidelines are clear on this: coders should not assume laterality when it isn’t documented.
Cancer Type Is Not Reflected in the Code
One thing that surprises many people is that ICD-10-CM does not distinguish between small cell lung cancer and non-small cell lung cancer through the diagnosis code. Whether the tumor is adenocarcinoma, squamous cell carcinoma, large cell carcinoma, or small cell carcinoma, the C34 code is the same. The distinction between histological types is captured separately through cancer registry systems and morphology codes used by tumor registrars, not through the ICD-10-CM billing code itself. The ICD-10-CM system is strictly anatomical for lung cancer: it cares about where the tumor is, not what cell type it is.
Primary vs. Metastatic Lung Cancer
The C34 codes are only for cancers that originated in the lung. If cancer started elsewhere (breast, colon, kidney) and spread to the lung, the correct code is C78.0, “secondary malignant neoplasm of lung.” This distinction matters significantly for treatment planning and insurance claims.
C78.0 follows the same laterality structure:
- C78.00: Secondary malignant neoplasm of unspecified lung
- C78.01: Secondary malignant neoplasm of right lung
- C78.02: Secondary malignant neoplasm of left lung
When coding metastatic disease, both the primary cancer site and the secondary site are typically reported. The sequencing depends on the focus of the encounter. If a patient with colon cancer is being treated specifically for lung metastases, C78.01 or C78.02 may be listed as the principal diagnosis, with the colon cancer code listed as an additional diagnosis.
Coding for Complications
Lung cancer frequently causes complications that need their own codes alongside C34. The most common is malignant pleural effusion, which is fluid buildup around the lungs caused directly by the cancer. This gets its own code: J91.0. It’s reported as an additional diagnosis alongside the C34 code for the underlying cancer.
Secondary spread to nearby structures also has dedicated codes. C78.2 covers metastasis to the pleura (the lining around the lungs), while C78.1 covers metastasis to the mediastinum, the space between the lungs that contains the heart and major blood vessels. Lymph node involvement is coded separately under C77.0 rather than under the lung cancer codes.
Quick Reference Table
For easy lookup, here are the most commonly used codes:
- C34.90: Lung cancer, unspecified location and side (most general)
- C34.11: Upper lobe, right lung
- C34.12: Upper lobe, left lung
- C34.2: Middle lobe (right lung only)
- C34.31: Lower lobe, right lung
- C34.32: Lower lobe, left lung
- C78.00: Metastatic cancer to lung, unspecified side
- J91.0: Malignant pleural effusion
These codes reflect the 2026 ICD-10-CM code set. Code definitions can shift with annual updates, so verifying against the current fiscal year’s tabular list is always worthwhile when accuracy matters for billing or documentation.