Cancer in the bloodstream refers to the presence of cancerous cells within the body’s circulatory system. This can occur in two main ways: cancers that originate directly from blood-forming tissues or immune cells, or solid tumors that have spread from their initial location to the blood.
Primary Cancers of the Blood
Cancers that begin directly in the blood, bone marrow, or lymphatic system are known as hematologic cancers. These conditions involve the uncontrolled growth of abnormal blood cells. The three main categories are leukemia, lymphoma, and multiple myeloma, each affecting different types of blood cells or their precursors.
Leukemia
Leukemia is a cancer of the blood and bone marrow, characterized by the overproduction of abnormal white blood cells. These abnormal cells, often immature, multiply rapidly and interfere with the bone marrow’s ability to produce healthy red blood cells and platelets. Leukemia cells can then circulate widely throughout the blood.
Lymphoma
Lymphoma originates in the lymphatic system, a network of vessels and organs that are part of the body’s immune system. This cancer involves abnormal lymphocytes, a type of white blood cell, which can collect in lymph nodes, the spleen, and other tissues. While lymphoma often forms solid masses, cancerous lymphocytes can also circulate in the blood.
Multiple Myeloma
Multiple myeloma is a cancer of plasma cells, specialized white blood cells found primarily in the bone marrow. These abnormal plasma cells multiply uncontrollably, crowding out healthy blood-producing cells. Their products, and sometimes the cells themselves, can be detected within the bloodstream.
Solid Tumors Spreading Through the Bloodstream
Cancer originating in solid organs, such as the breast, lung, or colon, can spread to other parts of the body through a process called metastasis. This occurs when cancer cells detach from the primary tumor and travel through the circulatory system. The spread through the bloodstream is known as hematogenous dissemination.
Cancer cells first invade nearby tissues and then penetrate the walls of blood vessels to enter the bloodstream. Once inside, these cells can travel to distant organs, where they might exit the vessels and form new tumors. This new tumor, even in a different organ, is still composed of cells from the original primary cancer and is named after the original site.
Many common solid tumors are known to metastasize via the blood. For example, breast cancer frequently spreads to bones, lungs, liver, and brain. Colorectal cancer often metastasizes to the liver and lungs, while prostate cancer commonly spreads to the bones. The ability of these cells to survive the journey and establish new growths determines the extent of metastatic disease.
How Cancer Cells Navigate the Bloodstream
The journey of cancer cells through the bloodstream from a solid tumor to a distant site is complex. Cancer cells must first enter the blood vessels, a process known as intravasation, by invading through the vessel walls. This step is challenging, and many cells do not survive it.
Once in the circulation, these circulating tumor cells (CTCs) face a hostile environment. They are subjected to significant mechanical forces, such as the shear stress from blood flow, which can damage them. The immune system also poses a threat, as immune cells can recognize and attack the foreign cancer cells.
Despite these challenges, some cancer cells manage to survive the journey, often by interacting with other blood components like platelets, which can offer protection from immune attacks and shear forces. These cells adhere to the inner lining of blood vessels at a distant site. They then exit the bloodstream, a process called extravasation, by moving through the vessel walls into the surrounding tissue. Once in the new tissue, these cells can begin to divide and form a new tumor, completing the metastatic cascade.
Detecting Cancer in the Blood
Detecting cancer in the bloodstream involves various methods that look for abnormal blood cells, specific proteins, or genetic material released by tumors. A complete blood count (CBC) is a common test that can indicate certain blood cancers like leukemia or lymphoma by revealing abnormal levels of different blood cells. While a CBC can suggest the presence of a blood cancer, it does not detect solid organ cancers.
Tumor marker tests measure specific substances, often proteins, produced by cancer cells or by the body in response to cancer. Examples include Prostate-Specific Antigen (PSA) for prostate cancer or Cancer Antigen 125 (CA-125) for ovarian cancer. High levels of these markers can signal cancer, but they can also be elevated due to non-cancerous conditions, so they are typically used for monitoring or alongside other diagnostic tools.
More advanced techniques include liquid biopsies, which analyze blood for components shed by tumors. These non-invasive tests can detect circulating tumor cells (CTCs) that have entered the bloodstream. Liquid biopsies can also identify circulating tumor DNA (ctDNA), fragments of genetic material released by dying cancer cells into the blood. These methods are valuable for monitoring disease progression, assessing treatment effectiveness, and sometimes for early detection, offering insights into the tumor’s genetic makeup.