New understanding from Cf brachytherapy trials and considerations for neutron therapy of bulky gyn carcinoma for future

PURPOSE: This study reviews the radiobiology of neutrons and the results and methods that have evolved in the Cf-252 neutron brachytherapy trials to postulate methods to explore for their potential applicability to fast neutron beam therapy. METHODS AND MATERIALS: The results of radiobiological experimental studies are reviewed for RBE for different fraction number and dose per fraction. RBE was always higher for fractionated neutrons based on comparison with isoeffective doses of photons. This is inherent in the basic radiobiological properties of photons. RBE was highest for low dose rate (LDR) Cf brachytherapy. RESULTS: Brachytherapy methods deliver radiation dose which is localized and conformal to the tumor. These methods have been used for Cf therapy and led to good tissue tolerance and local tumor control. The use of large dose/session (or fraction), small fraction number, short treatment times, followed by photon beam therapy has been the practice in Cf brachytherapy. It has been found that bulky or localized advanced tumors responded better if neutron treatment preceded the photon beam therapy. Therapeutic efficacy is dependent on the fraction size of neutrons and not the time duration of application. CONCLUSIONS: Available radiobiological data on neutron RBE with fraction number and size of dose and the favorable experience from Cf brachytherapy with up-front neutron applications suggest new avenues to explore for the neutron beam trials. The high efficacy noted for small volume dose, 10 to 20 Gy-eqs of dose per session, localized dose, conformal methods, small number of sessions, short treatment times and an up-front or early schedule in combining neutron and photon therapy suggests that similar schedules and methodology may also be effective for neutron beam therapy. Further clinical trials are indicated and bulky GYN tumors represents suitable tumors for study.