Quick summary

Think of platelets in the same way you'd think of cornstarch or flour when you mix it in a sauce or stew, and it's easy to see why they're so important: They're the clotting factors that make blood thicken and harden. Without them, bleeding becomes unstoppable and suddenly every minor cut or injury becomes dangerous.

Why does cancer make platelet counts drop?

In most cases, it's not the cancer itself but the treatment that causes low platelets. Chemotherapy can make platelet counts fall because it targets cells that grow rapidly. Tumor cells grow and divide quickly, but so do the cells in the bone marrow that produce red blood cells and platelets. Radiation can also cause a low platelet count (also called thrombocytopenia), especially radiation to the pelvic area, because there's a higher proportion of bone marrow in the pelvic bones. But some cancers, particularly lymphoma and leukemia, can lower platelet production if they spread to the bone marrow.