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Question: A patient had a lumbar puncture that showed metastatic breast cancer to the cerebrospinal fluid (CSF). What diagnosis code would you use for the secondary CSF?

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Answer: You would code breast cancer metastatic to CSF to C79.49 (Secondary malignant neoplasm of other parts of nervous system).

Here’s why: You won’t find CSF or spinal fluid in the ICD-10-CM table of neoplasms; but as the type of cancer has been identified as secondary or metastatic, you would begin your search for the correct code by going to the malignant neoplasms of ill-defined, other secondary, and unspecified sites (C76-C80) group of codes. There, you will find three code groups for secondary cancers, but as C77.- (Secondary and unspecified malignant neoplasm of lymph nodes) and C78.- (Secondary malignant neoplasm of respiratory and digestive organs) point to places that are not the location of CST, you are left to choose a code from C79.- (Secondary malignant neoplasm of respiratory and digestive organs).

There, as the metastatic cancer site has been specified, but the site is not one of the ones specified by the other codes in the group, you will have to look to one of the two C79.4- (Secondary malignant neoplasm of other and unspecified parts of nervous system) codes to find the correct choice. As C79.40 (Secondary malignant neoplasm of unspecified part of nervous system) would be incorrect because the site is specified in the documentation, you are left with C79.49.

Remember: You will also need to provide a code from C50.- (Malignant neoplasm of breast) to identify the specific location of the breast cancer. And, per ICD-10-CM Guideline C.2.l.2, you will sequence the two codes dependent on the malignancy being treated at the encounter, with the principal, or first-listed, diagnosis being the metastatic, or secondary, site if that is the condition being evaluated or managed during the visit.

Bruce Pegg, BA, MA, CPC, CFPC, Managing Editor, AAPC