Fiducial marker for prostate radiotherapy: comparison of 0.35- and 0.5-mm-diameter computed tomography and magnetic resonance images

Purpose When performing intensity-modulated radiotherapy for prostate cancer, a marker is inserted into the prostate to enable the recognition of its position using cone–beam computed tomography (CT). However, it is difficult to recognize the prostatic outline using CT alone. Magnetic resonance imaging (MRI) can depict the prostatic outline better than CT. In treatment plans using CT and MRI registration, various markers are used in institutions; however, the selection of an optimal marker size is difficult. Comparison of a different fiducial marker study was conducted using phantom, but no study in vivo was found. Therefore, we prospectively investigated the effects of different marker diameter sizes using CT and MR images.