Treatment Planning for Conformal Proton Radiation Therapy

Clinical results from various trials have demonstrated the viability of protons in radiation therapy and radiosurgery. This has motivated a few large medical centers to design and build expensive hospital based proton facilities based proton facilities (current cost estimates for a proton facility is around $100 million). Until this development proton therapy was done using retrofitted equipment originally designed for nuclear experiments. There are presently only three active proton therapy centers in the United States, 22 worldwide. However, more centers are under construction and being proposed in the US and abroad. The important difference between proton and x-ray therapy is in the dose distribution. X-rays deposit most of their dose at shallow depths of a few centimeters with a gradual decay with depth in the patient. Protons deliver most of their dose in the Bragg peak, which can be delivered at most clinically required depths followed by a sharp fall-off. This sharp falloff makes protons sensitive to variations in treatment depths within patients. Treatment planning incorporates all the knowledge of protons into a process, which allows patients to be treated accurately and reliably. This process includes patient immobilization, imaging, targeting, and modeling of planned dose distributions. Although the principles are similar to x-ray therapy some significant differences exist in the planning process, which described in this paper. Target dose conformality has recently taken on much momentum with the advent of intensity modulated radiation therapy (IMRT) with photon beams. Proton treatments provide a viable alternative to IMRT because they are inherently conformal avoiding normal tissue while irradiating the intended targets. Proton therapy will soon bring conformality to a new high with the development of intensity modulated proton therapy (IMPT). Future challenges include keeping the cost down, increasing access to conventional proton therapy as well as the clinical implementation of IMPT. Computing advances are making Monte Carlo techniques more accessible to treatment planning for all modalities including proton therapy. This technique will allow complex delivery configurations to be properly modeled in a clinical setting.