Can I have a metastatic tumor without a primary cancer?

Dennis L. Citrin, MD
Hematology & Oncology
Yes, it's unusual but occasionally we'll see a patient with metastatic disease, and we don't find a primary site.
The word 'metastatic' implies that the cancer has spread from its 'primary' site to a distant one. There are instances when the primary site is difficult to elucidate, or simply regresses as is the case sometimes with malignant melanoma. There are also unclassified malignancies due to difficulty in finding primary sites, called 'cancer of unknown primary'. These tend to have poorer prognoses in general, likely due to our inability to give appropriate treatments because we do not know their origin.
Yes. There are a small percentage of cancers that appear in locations for which the proper classification cannot be made. Despite the best capabilities of our current state of medical technology, these cancers are termed "Unknown Primary" malignancies. It presents a treatment dilemma as FDA approved chemotherapy treatments are based primarily upon identifying the site of disease origination. When dealing with these unknown primary cancers it is absolutely imperative to narrow the possible sites of disease origination down to a minimum. Based on the results of certain lab tests performed, a best estimation as to the site of origin may sometimes be made so as to permit initiation of treatment. Sometimes the original site is never found - which is rare; but treatment may still be possible in those instances.
Shelby A. Terstriep, MD
There are times when a primary tumor cannot clearly be identified but it is generally felt that there was once an abnormal cell that started the cancer process. Sometimes for example in melanoma the primary tumor can regress/disappear (most likely from immune system trying to take care of the problem).
In the setting of metastatic disease it can be difficult at times to know where it originated. The pathologist can run several tests that can give us clues but sometimes it is just not clear and we have to treat with chemo that is useful for a variety of tumors.
Dr Camille

No. The very definition of "metastatic" implies that it originates somewhere else. However, it is quite possible (and happens sometimes) that the primary cancer is so small, that it cannot be located or seen on any of the regular Radiographic evaluation (MRI, PET, CT, etc.). These studies are often limited by size (example, PET scan requires ~0.7cm size tumor to give a reasonable diagnosis). This phenomena may also be referred to as "Unknown Primary" as in the case of Head and Neck Cancers.

Steven G. Eisenberg, DO
Hematology & Oncology

This is an unfortunate dilemma in oncology. By definition, "metastatic" infers that the cancer cells traveled from the primary lesion to a distant (metastatic) site. However, there are times when the primary source of the cancer is not discovered, despite our best efforts. This is frustrating not only to the human being living with cancer, but to the oncologist. Why?  Because it is imperative to know the primary source in order to design the best treatment.

Each type of cancer has a unique spectrum of chemotherapy drugs that may work. So knowing the primary site helps guide the treatment. When no primary is found, we treat using the best evidence we have and the most likely source of the primary lesion. Performing a biopsy of the metastasis can sometimes give us the answer with help from our friendly neighborhood pathologist. They will perform all types of tests on the tissue to try and determine the origin of the cancer cells.

It has been estimated that between 2 and 5 percent of all new cancer cases involve metastatic tumors for which no primary tumor can be identified. In some of these cases, a primary tumor is actually present, but is too small to be detected. In other cases, elimination of the primary tumor is thought to have occurred, either by the patient's own immune system or by a previous biopsy that is either unrecalled by the patient or that was misdiagnosed as a benign lesion.
Robert A. Wascher, MD, FACS
Author, "A Cancer Prevention Guide for the Human Race"
Ajay K. Sahajpal, MD
Transplant Surgery
Yes and No. People can present with the metastatic tumor before a known primary. This usually leads to investigations and numerous tests to find the primary. Usually it can be found. Sometimes the primary is never found either because it is small or involuted. If no primary is found, we call in "metastatic cancer with unknown primary".
No. A metastatic tumor always starts from cancer cells in another part of the body. In most cases, when a metastatic tumor is found first, the primary tumor can be found. The search for the primary tumor may involve lab tests, x-rays, and other procedures. However, in a small number of cases, a metastatic tumor is diagnosed but the primary tumor cannot be found, in spite of extensive tests. The pathologist knows the tumor is metastatic because the cells are not like those in the organ or tissue in which the tumor is found. Doctors refer to the primary tumor as unknown or occult (hidden), and the patient is said to have cancer of unknown primary origin (CUP). Because diagnostic techniques are constantly improving, the number of cases of CUP is going down.

This answer is based on source information from the National Cancer Institute.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.