Cancer begins when cells grow and divide uncontrollably, forming a mass known as a primary tumor. This original site determines the cancer type, such as breast or prostate cancer. A secondary cancer arises when malignant cells from this initial tumor travel to a distant part of the body and begin to grow there. Understanding this distinction between where a cancer starts and where it spreads is crucial for diagnosis and management.
Defining Secondary Cancer
A secondary cancer is a tumor that has developed from cells that broke away from the original, or primary, tumor. This condition is also known as metastatic cancer, and the new growths are called metastases. A secondary cancer is not a new type of disease; it is a new manifestation of the original cancer.
For example, if prostate cancer cells travel to the bones and form a tumor, the diagnosis remains metastatic prostate cancer, not primary bone cancer. The cancer cells in the bone possess the specific characteristics and genetic markers of the prostate cells from which they originated. Common primary cancers that frequently result in secondary tumors include those of the breast, prostate, colon, and lung, often spreading to sites like the bones, liver, or lungs.
How Cancer Cells Travel
The spread of cancer cells from the primary site to distant organs is a multi-step process known as metastasis. This journey begins with local invasion, where malignant cells detach from the primary tumor and penetrate the surrounding tissue. To achieve this, the cells must break down the extracellular matrix, the structural support separating the tumor from adjoining tissues.
Once free, the cells enter the circulation by moving through the walls of nearby blood vessels or lymphatic vessels, a process called intravasation. They travel through the bloodstream or the lymphatic system as circulating tumor cells. Most of these traveling cells die due to the harsh environment or immune system attacks, making metastasis an inefficient process.
For the spread to be successful, surviving cancer cells must stop at a distant site, exit the vessel walls (extravasation), and colonize the new organ. Colonization involves the cancer cells adapting to the new microenvironment and often stimulating the growth of new blood vessels (angiogenesis) to ensure a blood supply. The location where the cancer establishes itself is not random, as certain primary tumors, such as colon cancer, show a higher propensity to spread to specific organs, like the liver.
Identifying Secondary Cancer for Staging
The detection of a secondary cancer is a major factor in determining the overall stage of the disease, which impacts prognosis and treatment planning. The presence of metastasis to a distant organ typically signifies Stage IV cancer, the most advanced classification. This staging is confirmed using the TNM system, where the ‘M’ component indicates whether distant metastasis has occurred.
To confirm a secondary cancer, doctors use various diagnostic tools, including imaging tests like CT scans, PET scans, and MRIs, to locate new tumor growths. A biopsy of the suspected secondary tumor is performed, and the tissue sample is examined under a microscope. This examination confirms that the cells in the new tumor match the cell type of the original primary cancer.
Treatment Strategies Based on Origin
The principle for treating secondary cancer is to target the disease based on its site of origin, not the location of the metastasis. Breast cancer that has spread to the liver, for instance, is treated with therapies effective against breast cancer, not primary liver cancer treatments.
Treatment for secondary cancer is generally systemic, meaning it uses therapies that circulate throughout the entire body to reach cancer cells wherever they may have spread. These systemic approaches include:
- Chemotherapy, which uses drugs to kill rapidly dividing cells.
- Targeted therapy, which focuses on specific molecular features of the cancer cells.
- Immunotherapy, which harnesses the body’s own immune system to recognize and attack the malignant cells.
Localized treatments, such as surgery or radiation, may still be used to manage symptoms or treat individual metastatic sites. However, the overall strategy focuses on controlling the systemic spread of the original cancer.