How is a “primary cancer” defined?

A “primary cancer” is defined as the original cancer site where the malignancy first developed. This is a critical concept in cancer registration and staging because it helps in understanding the origins of the disease and its progression. The primary cancer is distinguished from secondary or metastatic cancers, which arise when cancer cells spread from the initial tumor to other parts of the body. Recognizing the primary site allows for accurate treatment planning and facilitates data collection for research and epidemiology.

The other options represent concepts that are related to cancer but do not accurately define what primary cancer is. For instance, the site where cancer metastasizes refers to secondary tumors, while the treatment method, such as chemotherapy, does not determine the cancer's primary status. Additionally, a cancer that does not spread beyond its original site may describe localized cancer but does not encompass the broader definition of primary cancer, which focuses on the origin of the malignancy.