There is limited evidence of its effectiveness at slowing the growth of tumors (in animal studies) accompanied by serious side effects.
"Studies conducted in several experimental tumor models in the 1980s found that the use of cesium or cesium chloride led to decreased tumor growth and fewer deaths in certain mice with cancerous tumors, such as those with sarcoma or breast cancer. In animal studies, giving cesium over the long term caused serious blood and neuromuscular side effects and even death.
http://www.cancer.org/Treatment/TreatmentsandSideEffects/ComplementaryandAlternativeMedicine/HerbsVitaminsandMinerals/cesium-chloride
....Several medical case reports have described serious side effects in people with cancer who have taken cesium chloride, including life-threatening problems with heart rhythm, seizures, loss of consciousness, and electrolyte imbalances. Cesium appears to take the place of potassium, a common element in the body, which can cause many of these problems.
Newer reports have suggested that it can take weeks or even months for cesium to cause some of the more severe heart problems. A 2009 report described a woman who had been taking 3 grams of cesium each day for a few months. Even though she also took electrolytes and potassium, she started losing consciousness due to interruptions in heart rhythm. Medical treatment included electrical shocks to the heart and extra potassium, but even after 6 days without cesium her potassium level remained low. With a half-life over 6 weeks, cesium takes a long time to get out of the body."*
* half-life in this context refers to metabolic, not radioactive, half-life!