What is it you are confused about? It sounds like you have a very good handle on what CUP is. Cancer can begin in any organ or tissue of the body. The primary, or original, tumor is usually named for the part of the body or the type of tissue in which the cancer begins. The disease may spread (metastasize) from the primary tumor and form metastatic tumors in other parts of the body. For example, breast cancer cells can metastasize to the lungs and cause the growth of a new tumor. When this happens, the tumor in the lung is called metastatic breast cancer because the breast cancer cells have traveled via the lymphatic or circulatory systems to the lung.
However, when the primary (i.e. original) site of the cancer cannot be identified, cancer of unknown primary (CUP) is the resulting diagnosis. Most often, the metastatic cancer is first found in the lymph nodes, liver, lung, or bone.
In any case, treatment depends on where the cancer is found, the microscopic appearance of the cancer cells, the biochemical characterization of the cells and the patient’s age and overall physical condition. No method is standard, but chemotherapy, radiation therapy, hormone therapy, and surgery may be used alone or in combination to treat patients who have CUP, just as would be the case for those with cancer with an identified primary location.