Abstract
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. Thirty-one consecutive patients were enrolled in this study. CT and MRI were performed 3weeks after marker placement. The 0.35-mm-diameter marker was placed on the left side of the prostate, and the 0.5-mm-diameter marker was placed on the right side. The length of each marker was 10mm. The better MRI image was selected between those obtained using T2*-two-dimensional weighted image (T2*2D) and T2*-three-dimensional weighted image (T2*3D). Two observers evaluated and scored the prostatic outline image quality as well as visualized the prostatic markers using CT and MRI. MRI was significantly superior to CT in depicting the prostatic outline. The CT artifacts were significantly lesser for the 0.35-mm-diameter marker than for the 0.5-mm-diameter marker. The degree of marker recognition using MRI was significantly better with the 0.5-mm-diameter marker. The 0.5-mm-diameter fiducial marker had significantly better visualization than the 0.35-mm-diameter marker. While CT artifacts were significantly worse with the 0.5-mm-diameter marker, the artifact level was tolerable for clinical practice. Therefore, we recommend the 0.5-mm-diameter diameter marker in terms of prostatic outline and marker visualization using MRI.
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