What are primary and metastatic tumors?

The primary tumor is the original site where the tumor was first discovered or originated. This is like having a single mass that's in the lung. This would be the primary source of lung cancer. A metastatic tumor is a tumor that's spread to another part of the body as a result of the original tumor. This would be like having a mass in the liver or the adrenal gland that came or was metastatic from the primary lung tumor.
'Primary' tumors are tumors that originate from that organ, for instance a cancer in the breast is primary if it originated there. But 'metastatic' implies it came from another place, another organ. For instance, a cancer in the liver may be a 'metastatic' breast cancer, i.e. it started in the breast and traveled the blood stream or lymph channels and lodged in the liver to grow there. 
Shelby A. Terstriep, MD
A primary tumor is where and what type of cell the cancer originated from. Metastatic tumors are cells from the area and type of cell that have spread outside of that organ or site.
For example: If there are cancerous tumors in the liver when biopsied and evaluated under the microscope they look like breast cancer it is considered "metastatic breast cancer"
Juliet Wilkinson
Oncology Nursing
Primary and metastatic tumors are two locational terms used to describe cancer and its' spread. Your primary tumor (lung, liver, colon, etc.) is the original tumor. Metastatic tumors travel from the primary tumor to other organs (brain, skin, prostate). For example, if you are diagnosed with breast cancer and found to have another tumor in your lungs, the lung tumor would be metastatic (traveling) while the breast cancer would be the primary tumor.
Ajay K. Sahajpal, MD
Transplant Surgery
The term primary tumor refers to where the tumor originated, i.e. colon or breast. The term metastatic refers to a tumor that originated elsewhere and spread to a location such as metastatic colon cancer to the liver.

Primary tumors of the CNS are growths that begin in the brain or spinal cord. They can be either malignant or benign and are identified by the types of cells they contain, their location, or both. Most primary CNS tumors occur in adults.

Metastatic, or secondary, tumors in the CNS are caused by cancer cells that break away from the primary tumor that developed in a non-CNS part of the body. These tumors are named after the type of cancer that causes them. Metastastic tumors (also called metastases) to the brain occur in about one-fourth of all cancers that develop in other parts of the body, such as cancer of the lung, breast, or kidneys; or melanoma, a form of skin cancer. They are more common than primary tumors and occur more often in adults than in children.

Metastatic spine tumors usually form within the bony covering of the spinal column but may also invade the spinal canal from the chest or abdomen.

While cancers elsewhere in the body can easily cause tumors inside the brain and spinal cord, CNS tumors rarely spread outside the nervous system.

This answer is based on source information from the National Institute of Neurological Disorders and Stroke.

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