C78

C78 is a code category used in the ICD-10-CM medical classification system to designate secondary malignant neoplasms, i.e., cancers that have spread from a primary site to other parts of the body. The base code is used to indicate metastasis, with additional digits after the decimal point specifying the exact site of the metastatic lesion as documented by clinicians.

In practice, C78 is employed in medical records, billing, epidemiology, and cancer statistics to record that a patient’s cancer has spread. Because the exact site of metastasis can influence treatment decisions and prognosis, coders often use a more specific subcode within the C78 family to capture the organ or tissue affected.

The C78 category lies within ICD-10-CM Chapter 2, which covers neoplasms. Correct use requires clinical documentation of metastasis and the metastatic site, typically drawn from pathology reports and imaging studies. The full mapping of subcodes to specific sites is defined in the ICD-10-CM tabular lists maintained by the Centers for Medicare & Medicaid Services and the National Center for Health Statistics.

Relatedly, there are other categories within the ICD-10-CM system that address malignant neoplasms, including codes for metastases to different sites. The coding framework emphasizes accuracy in reflecting both the presence of metastatic disease and the exact locations involved. The transition from ICD-9-CM to ICD-10-CM in the United States occurred in 2015, and C78 has been used since then to support consistent documentation and reporting of metastatic cancer.