Feasibility study of in situ imaging of Ir-192 source during HDR brachytherapy procedure using a small gamma imager based on a Hamamatsu R3292 PSPMT

Preliminary results obtained utilizing a compact gamma camera fitted with a pinhole collimator to track the position of a high activity 5.3 Ci pellet of Ir-192 used in high dose rate (HDR) brachytherapy are presented. HDR brachytherapy procedures depend on delivering fractionated therapy doses to designated target volumes by timed insertion of a small (2-4 mm long) seed source of Ir-192 in body cavities or tumors. The Ir-192 source is attached to a wire and pulled/pushed in fine catheters that are operatively inserted in body cavities or tumor volumes. Presently, HDR procedures rely on a computer controlled mechanical delivery system to track the location of the source in the body. The objective of this study is to provide real-time images of the Ir-192 source in-vivo to monitor that the source is delivered to predefined locations for specified dwell times. The compact gamma camera is based on a 5" Hamamatsu R3292 position sensitive photomultiplier tube with a spatial resolution of about 1 mm. Several scintillator sensors were tested based on thin CsI(Na) crystals and a plastic scintillator (such as Bicron BC 400). The low efficiency of the plastic scintillation detector provided acceptable images of the 5.3 Ci Ir-192 source viewed with a pinhole collimator (dia 0.5 and 1.0 mm) located 16 cm from the source. This gamma camera approach provides a unique solution to the problem of verifying HDR brachytherapy treatments in the clinic