The theory behind IPT includes the idea that at least some types of cancer cells have far more insulin receptors than normal cells, which in theory allows the use of insulin to trick the cancer cells into rapidly absorbing chemotherapy from the blood. As a result, lower doses of chemotherapy agents are used, with significantly fewer toxic effects to the patient. The IPT also induces a temporary lowering of the patient's blood sugar, which is thought to also aid the chemotherapy uptake.
Physicians using this method have claimed dramatic results with few or no side effects. This type of treatment is not experimental in the usual sense, but is an "off-label" use of existing agents. Rigorous clinical studies need to be performed to prove the effectiveness of this method.
The principal proponent of this technique is Dr. Donato Perez Garcia, who claims that IPT is especially effective with breast cancer. He suggests that it is also effective with a number of other cancers, including small-cell lung cancer and prostate cancer. Essentially, if a cancer can be significantly affected by existing chemotherapy, then insulin potentiation therapy may be effective.
A critical perspective: http://www.quackwatch.org/01QuackeryRelatedTopics/Cancer/ipt.html