While CT is essential for planning radiation　therapy, MRI is used for imaging brain tumors for　greater soft tissue contrast and more accurate depiction　of tumors, particularly in cases involving　stereotactic radiosurgery（SRS）. However, MRI is characterized by greater image distortion than CT,　making accurate localization of the target tumor difficult.　This study evaluated the effects of such distortion　on SRS planning. CT and MRI incorporating　SRS planning parameters were performed on a brain phantom, and the images were then fused for　comparison. We compared treatment parameters obtained　from CT data alone with those obtained from　the fused images. A maximum linear distortion of　3.3 mm was observed on coronal MRI. When SRS　planning incorporated the coronal MRI data, treatment　parameters derived from CT data alone were less accurate than those obtained from the fused　images.