SU‐E‐T‐356: Intensity Modification of Small Diameter Proton Beam for Proton Radiosurgery

Purpose: The aim of this study is to improve the dose uniformity in the degraded SOBP (Spread‐Out Bragg Peak) of small diameter (less than 1.5 cm) proton beam by modifying beam intensity data in BCM (Beam Current Modulation). Methods: A diode detector with 0.5 mm of effective radius and a 3D‐moving water phantom have been used to measure the small field dosimetric data. The IBA proton therapy facility installed at National Cancer Center in Korea modulates proton beam using the range modulator with 10Hz‐rotating wheels and the Beam Current Modulation in the rotation phases. In a small diameter proton beam, however, the depth‐dose distribution is degraded as inverse proportional because of multiple Coulomb scattering effects. To compensate for it, we modified the proton beam intensity in BCM data of small field. The distal and lateral profiles have been measured for the original fields and the modified. Simple linear correction of BCM was applied.Results: The reduction rate of the degraded SOBP has the quadratic relation with the slope of linear correction. The Strong correlation between the SOBP degradation and beam range was found. Lateral profile was not affected by the BCM correction. Conclusions: By the modification of beam intensity data in BCM files, the dose uniformity in the degraded SOBP of small proton field could be improved. Then, the change of lateral beam shape was negligible. However the strong correlation of beam range was shown. The intensity modification of small diameter proton beam suggests the possibility of proton radiosugery for very small tumor size with more degree of freedom.