Comparison of MRI with CT for the radiotherapy planning of prostate cancer: a feasibility study

This feasibility study was performed to evaluate the suitability of MRI in defining appropriate pelvic radiotherapy treatment volumes, and to compare MRI sequences with CT for prostate cancer radiotherapy. Five patients with localized prostate cancer, imaged with four MRI sequences (spin echo (SE) T -1, turbo SE (TSE) T-2, high resolution TSE (HR) T-2, and FLASH 3D (F3D)), compared with their corresponding CT planning scans. Segmentation ability o f the following pelvic structures: prostatic apex (PA), prostate, rectum, b ladder and seminal vesicles (SV), were evaluated by three independent obser vers. They used a five point grading scale based on the anatomical definiti on of the organ boundary, tissue contrast and multiplanar display. Results were averaged for the group and for each sequence. Then was no significant interobserver variation in the assessed scores (p>0.1). The average scores (+/-1 SD) for all pelvic structures assessed by each imaging sequence were CT 1.3 +/- 0.6; SE T-1 2.4 +/- 0.9; TSE T-2 2.4 +/- 0.7; HR T-2 2.2 +/- 0.7 and F3D 3.4 +/- 0.6. Compared with CT, the average MR score for each asses sed pelvic structure was higher with a trend for all transaxial MR sequence s to provide improved segmentation of the PA and rectum. The F3D sequence s cored highest as it provided multiplanar views and avoided the problem of p artial volume averaging. MRI, compared with CT, appears to provide improved definition of pelvic treatment volumes but further work is required to con firm this and to address the issues of MRI associated distortion and dosime try before MRI can be used routinely for pelvic radiotherapy planning.