Real-time monitoring and verification of in vivo high dose rate brachytherapy using a pinhole camera

We investigated a pinhole imaging system for independent in vivo monitoring and verification of high dose rate (HDR) brachytherapy treatment. The syst em consists of a high-resolution pinhole collimator, an x-ray fluoroscope, and a standard radiographic screen-film combination. Autofluoroscopy provid es real-time images of the in vivo Ir-192 HDR source for monitoring the sou rce location and movement, whereas autoradiography generates a permanent re cord of source positions on film. Dual-pinhole autoradiographs render stere o-shifted source images that can be used to reconstruct the source dwell po sitions in three dimensions. The dynamic range and spatial resolution of th e system were studied with a polystyrene phantom using a range of source st rengths and dwell times. For the range of source activity used in HDR brach ytherapy, a 0.5 mm diameter pinhole produced sharp fluoroscopic images of t he source within the dynamic range of the fluoroscope. With a source-to-fil m distance of 35 cm and a 400 speed screen-film combination, the same pinho le yielded well recognizable images of a 281.2 GBq (7.60 Ci) Ir-192 source for dwell times in the typical clinical range of 2 to 400 s. This 0.5 mm di ameter pinhole could clearly resolve source positions separated by lateral displacements as small as 1 mm. Using a simple reconstruction algorithm, dw ell positions in a phantom were derived from stereo-shifted dual-pinhole im ages and compared to the known positions. The agreement was better than 1 m m. A preliminary study of a patient undergoing HDR treatment for cervical c ancer suggests that the imaging method is clinically feasible. Based on the se studies we believe that the pinhole imaging method is capable of providi ng independent and reliable real-time monitoring and verification for HDR b rachytherapy. (C) 2001 American Association of Physicists in Medicine.