Treatment planning considerations and quality assurance for CT-guided transischiorectal implantation of the prostate

A CT-guided technique for prostate brachytherapy has been developed which c an be performed on patients with large prostates and which allows real time evaluation and modification of implant geometry. The patient is positioned prone and radioactive Pd 103 or I 125 seeds are implanted through the tran sischiorectal space. Needles are inserted through a template whose plane is oriented at a nominal angle of 26 degrees away from the horizontal to faci litate needle penetration through the ischiorectal space. The CT gantry is tilted 26 degrees, so that its plane is orthogonal to that of the template. The oblique geometry between the CT gantry and direction of couch translat ion must be considered during source position planning, implantation, and p ost-treatment evaluation. This consideration is presented along with discus sion of relevant quality assurance procedures and recommended tolerances. T he mechanical and radiological tolerances of the CT scanner must be consist ent with the high level of precision required in radiation therapy. Special emphasis was placed on gantry laser and image plane alignment, sensitivity and radiation profiles, and spatial accuracy of image reconstruction, tabl e translation, and gantry tilt. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)04109-7].