PROSTATE VOLUMES DEFINED BY MAGNETIC-RESONANCE-IMAGING AND COMPUTERIZED TOMOGRAPHIC SCANS FOR 3-DIMENSIONAL CONFORMAL RADIOTHERAPY

Purpose: To compare the prostate volumes defined on magnetic resonance imaging (MRI), and noncontrast computerized tomographic (CT) scans us ed for three-dimensional (3D) treatment planning. Methods and Material s: Ten patients were simulated for treatment using immobilization and a retrograde urethrogram, 3D images were used to compare prostate volu mes defined by MRI (4-6 mm thick slices) and CT images (5 mm thick sli ces). Prostate volumes were calculated in cm(3) using the Scanditronix 3D planning system. MRI/CT images were merged using bony anatomy to d efine the regions of discrepancy in prostate volumes. Results: The mea n prostate volume was 32% larger (range -5-63%) when defined by noncon trast CT compared to MRI, The areas of nonagreement tended to occur in four distinct regions of discrepancy: (a) the posterior portion of th e prostate, (b) the posterior-inferior-apical portion of the prostate, (c) the apex due to disagreement between a urethrogram based definiti on and the location defined by MRI, (d) regions corresponding to the n eurovascular bundle. Conclusion: There is a tendency to overestimate t he prostate volume by noncontrast CT compared to MRI. Awareness of thi s tendency should allow us to be to more accurately define the prostat e during 3-D treatment planning.