Particle beam radiation therapy in prostate cancer: is there an advantage?

Hadron therapy uses heavy particles to deliver therapeutic ionizing energy. Each particle's inherent attributes determine the pattern of energy deposited by its beam, expressed in macro (conformability to a three-dimensional target volume) and micro (radiobiologic properties) distributions. Mass and charge regulate the inherent properties; beam energy provides a controllable, variable characteristic. Generally, heavy charged particles provide superior macrodosimetric properties; heavy particles (charged or not) have microdosimetric characteristics that produce high linear energy transfer (LET). Neutron macrodosimetry is similar to that of photons. Protons and helium ions possess superior macrodosimetric properties, plus microdosimetric characteristics resulting in low LET, yielding beam characteristics that approach the ideal for clinical radiotherapy. Hadron therapy for prostate cancer has been limited by the availability of appropriate treatment facilities. Nonetheless, encouraging results have been obtained. Neutron therapy demonstrated improved overall survival in a multi-institutional randomized trial, and improved local disease control in a subsequent trial. Proton radiation forms the boost component of several conformal dose-escalation studies. A Loma Linda University study demonstrated low treatment-related morbidity despite a prostate dose of 75 CGE; late-morbidity data were superior to published reports from multi-field, conformal photon therapy. A Phase III dose-escalation study of protons for early prostate cancer is proceeding.