A side effect of the rapid destruction and breakdown of a tumor, as with chemotherapy or radiation therapy, that can have dangerous, life- threatening effects. Certain bulky and fast-growing tumors that are very sensitive to treatment can shrink rapidly. The products of the breakdown of the tumor cells are rapidly released into the person’s circulation, which can result in high levels of potassium that can affect the electrical rhythm of the heart or high levels of uric acid that can damage the kidneys. The syndrome can be anticipated in certain types of tumors and managed in advance to avoid harm to the person.