Cancer of<unk><unk>  source is not a single type of cancer, although researchers have attempted to find a common characteristic that explains why a cancer might spread very early without causing symptoms at the site of<unk> . It is generally accepted that cancer of<unk><unk>  site exists because the<unk>  tumor is not identified due to clinical or technological inefficiencies, or because the<unk>  tumor regresses or stays dormant after spreading the cancer cells that generate the metastases. This view is widely accepted in oncology; in fact, these cancers are also referred to as occult<unk>  tumors. Because the site of the<unk>  tumor usually dictates the treatment and expected outcome, this view may help create uncertainty and anxiety among health professionals and patients, who may feel that their evaluation has been incomplete. Because stem cells have a natural ability to migrate and also play a key role in cancer development, it has been proposed that a cancer of<unk><unk>  site may form when deregulated, premalignant or cancerous stem cells migrate away from their natural tissue and give rise to a cancer in the new site before or without generating a tumor in their<unk> al tissue. Forming a tumor in a tissue is not a prerequisite for stem cells to move away from that tissue. Stem cells can migrate from their natural tissue and initiate a cancer in the new site before generating a detectable tumor in their natural tissue. In this case, the<unk>  tumor could be identified after some time. However, stem cells can also migrate away from their natural tissue without generating a cancer there. In this case, the<unk>  tumor” would never exist (see Figure). This can explain why the<unk>  site is not identified even on postmortem examination in many patients with these cancers. This view of CUP may provide relief to health professionals and patients. After a detailed imaging and pathologic analysis, oncologists can tell their patients with CUP that metastasis does not always involve the formation of a<unk>  tumor, and that their diagnostic evaluation is complete.
Cancer of unknown primary origin
Metastatic breast cancer
Metastatic carcinoma
Non-Hodgkin lymphoma
Hodgkin lymphoma
Large cell neuroendocrine carcinoma of the lung
Radiation-induced cancer
Atypical pulmonary carcinoid tumour