Tumor lysis syndrome

Tumor lysis syndrome is a complication of cancer treatment, particularly chemotherapy for lymphoma and leukemia. However, it can also occur when the body's own immune system fights cancer cells.
When cancerous cells die, they often release potassium, phosphates, calcium, lactic acid, and uric acid into the bloodstream. In most cases, these chemicals are removed by the liver and kidneys. However, if the treatment is particularly effective, the increase in these chemical levels overwhelms the body's ability to remove them before they reach toxic levels. The patient starts suffering symptoms typical to the raised chemical level. In severe cases, kidney failure is common.
Frequent blood tests are required for anyone undergoing chemotherapy to determine if these chemicals are within limits, and in most cases patients will be given preventative treatment. Any high level combined with any of the associated symptoms (such as high potassium with muscle weakness) requires immediate treatment without needing to confirm.
Tumor lysis syndrome at Wikipedia