Tumor angiogenesis is the process by which a growing cancer forms new blood vessels to feed itself with oxygen and nutrients. Without this blood supply, a tumor can only grow to about 1 to 2 millimeters in diameter, roughly the size of a pinhead. Beyond that limit, cells in the tumor’s core can’t get what they need through simple diffusion alone, so the cancer must recruit its own network of vessels to keep expanding. This process is also what gives cancer cells a direct route into the bloodstream, enabling spread to distant organs.
The Angiogenic Switch
Early-stage tumors can sit dormant for months or years, too small to cause harm. The turning point comes when the tumor flips what researchers call the “angiogenic switch,” a shift from a quiet, contained mass to one that actively builds blood vessels. This switch is triggered by a combination of factors: the tumor’s own genetic mutations (activation of cancer-promoting genes or loss of tumor-suppressing ones), inflammatory signals from surrounding tissue, and most importantly, low oxygen levels inside the growing mass.
When oxygen drops, tumor cells begin pumping out a cocktail of signaling proteins that call nearby blood vessels to sprout new branches toward the tumor. The most powerful of these signals belongs to a family of proteins that stimulate the cells lining blood vessels to multiply, migrate, and form tubes. Once the switch flips, the tumor transitions from a harmless dot to a mass capable of rapid, sustained growth.
How Tumors Signal for New Vessels
The dominant driver of tumor angiogenesis is a protein called VEGF-A, part of a five-member family of vascular growth factors. VEGF-A exists in several forms that vary in size and tissue distribution, but they all do essentially the same thing: they form pairs that latch onto receptors on the surface of blood vessel cells and trigger those cells to grow and move. One receptor in particular, VEGFR-2, is the main mediator of these effects, including vessel cell proliferation and migration toward the tumor.
VEGF-A isn’t the only signal involved. Tumors also release fibroblast growth factor, inflammatory molecules, and other proteins that amplify the process. This redundancy matters, because it means blocking a single signal doesn’t always shut down vessel growth entirely. Other members of the VEGF family play distinct roles: VEGF-C and VEGF-D primarily drive the formation of new lymphatic vessels, which creates yet another potential route for cancer cells to spread.
Why Tumor Blood Vessels Are Abnormal
The blood vessels that tumors build are not like healthy ones. Normal vessels form in an orderly pattern with tight junctions between cells. Tumor vessels are irregular in shape and size, with gaps between the cells that line them and abnormal pores that leak blood and plasma into surrounding tissue. This leakiness is a direct consequence of the excessive VEGF signaling that drives their formation, which pushes vessel growth so aggressively that the vessels never fully mature.
These structural defects have two important consequences. First, blood flow through the tumor is chaotic and uneven, which paradoxically leaves some areas of the tumor still starved of oxygen even after new vessels form. Second, the gaps between vessel cells make it easier for cancer cells to squeeze into the bloodstream, a process called intravasation. Loose junctions between vessel-lining cells are one of the key mechanisms that allow tumor cells to enter the circulation, travel through the body, and establish new tumors in distant organs like the liver, lungs, or bones.
Angiogenesis and Cancer Spread
Tumor blood vessels don’t just feed the cancer. They serve as highways for metastasis. Cancer cells invade through the surrounding tissue, push through the leaky walls of tumor vessels into the bloodstream, circulate through the body, then exit the blood supply at a distant site and begin growing there. Every step of this journey depends on the blood vessel network the tumor has built. This is why tumors with denser blood vessel networks tend to be more dangerous.
A measurement called microvessel density, which counts the number of small blood vessels in a tumor sample under a microscope, has been studied extensively as a predictor of outcomes. In breast cancer, a large meta-analysis of over 6,500 patients found that women with high microvessel density had roughly 1.5 times the risk of cancer recurrence compared to those with lower counts. Among lymph node-negative breast cancer patients specifically, high vessel density nearly doubled the risk of recurrence. The density of a tumor’s blood vessel network, in other words, is a meaningful indicator of how aggressively the cancer may behave.
Anti-Angiogenic Treatments
Because tumors depend so heavily on new blood vessels, cutting off that supply has become a major treatment strategy. The FDA has approved multiple drugs that target angiogenesis across a range of cancers. Bevacizumab, the first widely used anti-angiogenic drug, works by directly neutralizing VEGF-A before it can reach vessel cells. It was approved for recurrent brain tumors (glioblastoma) in 2009 and is used in several other cancers as well.
A second class of drugs blocks the receptors that VEGF binds to, effectively shutting down the signal from the receiving end. These multi-target inhibitors include sunitinib (approved in 2006 for kidney cancer and gastrointestinal stromal tumors), sorafenib (approved for kidney cancer, liver cancer, and thyroid cancer), axitinib (approved in 2012 for advanced kidney cancer), pazopanib (approved in 2009 for kidney cancer), and lenvatinib (approved in 2015 for radiation-resistant thyroid cancer). Collectively, these drugs have improved survival in advanced stages of kidney cancer, thyroid cancer, liver cancer, colorectal cancer, gastrointestinal stromal tumors, and non-small cell lung cancer.
These treatments come with characteristic side effects. Because VEGF plays a role in maintaining normal blood vessels throughout the body, blocking it commonly raises blood pressure. Hypertension occurs in 21 to 62% of patients receiving bevacizumab, and protein in the urine (a sign of kidney stress from vessel changes) appears in 3 to 43% of patients, depending on the study.
How Tumors Resist Anti-Angiogenic Therapy
Anti-angiogenic drugs can slow tumor growth, but cancers frequently find workarounds. One major escape route is called vessel co-option: instead of building new vessels, tumor cells latch onto and grow along blood vessels that already exist in the surrounding tissue. This strategy requires no growth factor signaling at all, which means it is inherently resistant to drugs that block VEGF. Studies in patients with colorectal cancer that had spread to the liver found that vessel co-option was a key reason some tumors didn’t respond to bevacizumab. In brain tumor patients treated with anti-VEGF therapy, vessel co-option actually increased after treatment began, suggesting the cancer shifted strategies in response to the drug.
A second workaround is vasculogenic mimicry, in which tumor cells essentially disguise themselves as blood vessel cells and form their own vessel-like channels without involving actual blood vessels at all. The tumor cells take on characteristics of vessel-lining cells, assembling into tube structures that can carry blood. Several preclinical studies have shown that this process ramps up after anti-angiogenic treatment or when oxygen levels drop, making it another escape mechanism that kicks in precisely when conventional vessel-blocking therapies are applied.
The redundancy of signaling molecules also plays a role. Even when VEGF is effectively blocked, tumors can switch to alternative growth factors to stimulate vessel formation through different pathways. This built-in flexibility is one reason anti-angiogenic drugs are typically combined with chemotherapy or immunotherapy rather than used alone.
Measuring Tumor Blood Supply
Doctors can assess how well-supplied a tumor is with blood vessels using specialized imaging. The most established method is dynamic contrast-enhanced MRI, which tracks the movement of a contrast dye through tumor tissue over time. As the dye flows through and leaks out of blood vessels, the MRI captures how quickly it arrives (reflecting blood flow), how much accumulates (reflecting vessel density), and how fast it escapes into surrounding tissue (reflecting vessel leakiness). These measurements help characterize a tumor’s vascularity without surgery and can be used to monitor whether anti-angiogenic treatment is working by tracking changes in blood flow over time.