Cancer spreads by the process of metastasis, where malignant cells break off from tumors in one part of the body and travel to other parts of the body via the blood stream. It's been thought that when a rather large clump of cells breaks off a tumor that its progress through narrow blood vessels would be not be possible due to size. Simply put, clumps of cells that were too large to fit could not spread. Much like a square peg not fitting through a round hole, it was believed that lumpy clusters of cells could not access tiny capillaries.
Researchers have now found that's not always the case. A team at Harvard Medical School has published research in the Proceedings of the National Academy of Science that shows clusters of cells actually break apart and change shape to fit through small spaces. Individual cells from these clusters transform into oval shapes and almost march, one by one, through vessels as small as 10 micrometers in size. Once there, they can reform into clusters and continue to spread disease. The work at Harvard was done using live zebra fish implanted with clusters of cancer cells.