Colorectal Cancer ICD 10 Code
Refresher: What are ICD-10 codes?
ICD-10 stands for the International Classification of Diseases, 10th revision. This coding system classifies diseases, symptoms, complaints, procedures, and abnormal findings to accurately and precisely define a patient’s health condition and for epidemiological research. 
First developed in the 1850s, this coding system introduced a form of standardization to data in medical records to improve statistical analysis. Through the decades, it has been updated multiple times, with the development of ICD 10 starting in 1983 and first published in 1994. The World Health Organization (WHO) manages the official base list.
Various countries have adopted and modified this original ICD-10 list per their healthcare system requirements. For example, in the United States, two variations are used: the ICD-10-CM (clinical modification) and ICD-10-PCS (procedure coding system).
ICD-10-CM is used for disease diagnosis, while ICD-10-PCS is used for inpatient medical procedures. There are approximately 68,000 ICD-10CM codes and 70,000 ICD-10-PCS codes, which are used to record diseases and medical conditions, track patient care, and as references in medical billings and insurance claims. 
For active cancer cases under treatment, ICD-10 codes C00 – D49 are used, which define malignant neoplasms. It is important to note that while the term cancer and neoplasm are often used interchangeably, these two medical conditions are different. Neoplasms can be noncancerous or cancerous. Furthermore, ICD-10 codes are different for primary tumors (“malignant neoplasms”) and secondary or metastasized tumors (“secondary malignant neoplasms”). 
What is the ICD 10 code for colorectal cancer?
Typically growths that originate in the colon are called colon cancers, and growths that originate in the rectum are called rectal cancer. However, since these two cancers share much in common, the umbrella term colorectal cancer describes both. Therefore, more than one ICD-10 code falls under this type of cancer.
The ICD 10 CM codes for primary colorectal cancers are C18, C19, and C20.   
For secondary colorectal cancer, i.e., metastasized from elsewhere, the ICD 10 code is C78.5 which represents secondary malignant neoplasm of the large intestine and rectum.
Note that cancers that originate in the anus or anal cancer are not considered colorectal cancer because different cell types are responsible for cancerous growths in this region.
These specifications are described in further detail below.
Primary colorectal cancer ICD 10 code: C18 – C20
Primary colorectal cancer ICD 10 code C18 – C20 lookup
The table below lists the ICD-10 colorectal cancer codes for primary colorectal cancers classified under C18 – C20.   
C18 – C20 : Malignant neoplasm of the colorectal  
Types of colorectal cancer included under C18 – C20
The ICD 10 codes C18 – C20 cover a wide range of primary sites for colorectal cancers but not the exact subtype. Therefore depending on the anatomical location, C18 – C20 codes can be applied to various types of primary colorectal cancers, including:
- Mucinous adenocarcinoma
- Signet ring cell adenocarcinoma
- Primary colorectal lymphomas
- Colorectal leiomyosarcomas
- Gastrointestinal stromal tumors
In other words, the adenocarcinoma ICD 10, colorectal lymphomas ICD 10, colorectal leiomyosarcomas ICD 10, and gastrointestinal stromal tumors ICD 10 codes can all be classified within C18 – C20 if they are diagnosed as the primary malignant neoplasms in the colon or rectum.
For example, suppose one patient has stage II adenocarcinoma in the descending colon, and another has stage I primary colorectal lymphoma in the same location. In that case, both will be given the CD 10 code C18.6.
Colorectal cancers excluded from ICD-10 C18 – C20
Malignant carcinoid tumors of the colon or rectum are excluded from this ICD code range because these cancerous growths start in neuroendocrine cells found throughout the digestive tract.
Primary malignant neuroendocrine cancers like colon or rectum carcinoid tumors are labeled with ICD 10 code C7A.2. 
Secondary and metastatic colorectal cancer ICD 10 code: C78.5
Secondary or metastatic colorectal cancer ICD 10 code 78.5 lookup
The ICD 10 colorectal cancer codes for secondary colorectal cancers, or cancers that originated in the colon or rectum and have metastasized, spreading out to other locations in the body, are coded as 78.5, Secondary malignant neoplasm of large intestine and rectum and includes the following: 
- Secondary malignant neoplasm of the large intestine
- Secondary malignant neoplasm of the rectosigmoid junction
- Secondary malignant neoplasm of the rectum
The colorectal cancer ICD-10 code depends on several factors, including whether the cancerous cells are a primary or secondary malignant neoplasm and the specific location of the malignancy as summarized below:
- Primary colorectal cancer that starts in the colon is coded ICD 10 C18
- Primary colorectal cancer that starts in the rectosigmoid junction is coded ICD 10 C19
- Primary colorectal cancer that starts in the rectum is coded ICD 10 C20
Secondary colorectal cancers in the large intestine and rectum are coded C78.5.
However, the ICD-10 code does not specify the exact subtype of colorectal cancer, meaning different subtypes can have the same code if located in the same region of the colon or rectum.
Therefore, it is necessary to clearly understand the specific case of colorectal cancer to accurately code the malignant neoplastic condition. This includes locating the anatomical site of origin, cell behavior, and cell type of the malignant tumor, along with any treatments, complications, and pre-existing and co-existing conditions the patient may have.