ICD-10-CM Code for Hormone Receptor-Negative, HER2-Positive Metastatic Breast Cancer with Pulmonary Metastasis
The primary ICD-10-CM code is C50.9 (Malignant neoplasm of breast, unspecified site) combined with C78.00 (Secondary malignant neoplasm of unspecified lung) to capture both the primary breast cancer and the pulmonary metastasis.
Code Structure and Application
For accurate coding of this clinical scenario, you need a combination of codes that capture:
Primary Diagnosis Codes:
- C50.9 - Malignant neoplasm of breast, unspecified site (use more specific C50.x codes if the quadrant/location is known, such as C50.911 for right breast or C50.912 for left breast)
- C78.00 - Secondary malignant neoplasm of unspecified lung (or C78.01 for right lung, C78.02 for left lung if laterality is documented)
Essential Additional Codes:
- Z17.0 - Estrogen receptor negative status [ER-]
- Z17.1 - Progesterone receptor negative status [PR-]
- Z15.04 - Genetic susceptibility to malignant neoplasm of breast (if applicable for BRCA status per 1)
Note: There is currently no specific ICD-10-CM code for HER2-positive status in the coding system. The HER2-positive designation must be documented in the clinical notes and pathology reports but cannot be directly coded. This represents a significant limitation in the current ICD-10-CM system for breast cancer subtyping 2, 3.
Critical Coding Considerations
Receptor Status Documentation
The hormone receptor-negative status is crucial for treatment decisions, as these patients are candidates for HER2-targeted therapy without endocrine therapy 4, 5. Always code both ER-negative (Z17.0) and PR-negative (Z17.1) status when documented, as this defines the triple-negative-except-HER2 phenotype that has distinct treatment implications 2.
Metastatic Site Specificity
Use the most specific lung metastasis code available based on documentation:
- If bilateral pulmonary metastases: use both C78.01 and C78.02
- If multiple pulmonary nodules are documented, this still falls under C78.0x codes
- Document whether this represents initial metastatic presentation (de novo stage IV) versus recurrent disease, as this affects staging codes
Staging Code
- Add code for stage IV disease when applicable for registry purposes, though this may vary by institutional requirements
Common Pitfalls to Avoid
Do not use C79.81 (Secondary malignant neoplasm of breast) - this is for metastases TO the breast from other primaries, not FROM breast cancer
Do not omit the metastatic site code - coding only C50.x without C78.00 fails to capture the stage IV nature of disease, which impacts reimbursement and quality metrics
Verify HER2 status is in pathology reports - while you cannot code it, payers increasingly require documented HER2 testing results to authorize HER2-targeted therapies like trastuzumab, pertuzumab, or trastuzumab deruxtecan 3, 5, 6
Re-biopsy considerations - if this represents recurrent disease, guidelines recommend re-testing receptor status on metastatic lesions when feasible, as discordance rates between primary and metastatic disease range from 3.4% to 60% for hormone receptors 4, 7. Document if receptor status changed from primary tumor.
Clinical Context for Accurate Coding
This specific breast cancer subtype (HR-negative/HER2-positive with pulmonary metastases) represents approximately 5-8% of all metastatic breast cancer cases 2. The HER2-enriched subtype commonly metastasizes to lung, liver, and brain 2, making pulmonary involvement an expected pattern.
Ensure documentation includes:
- Method of HER2 testing (IHC 3+ or ISH amplified per 2018 ASCO/CAP guidelines) 4, 8
- Whether this is de novo stage IV or recurrent disease
- Prior treatments received, as this affects sequencing of HER2-targeted agents 9, 6
- Performance status and extent of disease for treatment planning
The absence of a specific HER2-positive ICD-10 code remains a significant gap in the coding system that limits database research on this important breast cancer subtype 10.