Prostate volumes defined by magnetic resonance imaging and computerized tomographic scans for three-dimensional conformal radiotherapy

Purpose: To compare the prostate volumes defined on magnetic resonance imaging (MRI), and noncontrast computerized tomographic (CT) scans used for three-dimensional (3D) treatment planning. Methods and Materials: Ten patients were simulated for treatment using immobilization and a retrograde urethrogram. 3D images were used to compare prostate volumes defined by MRI (4-6 mm thick slices) and CT images (5 mm thick slices). Prostate volumes were calculated in cm³ using the Scanditronix 3D planning system. MRI/CT images were merged using bony anatomy to define the regions of discrepancy in prostate volumes. Results: The mean prostate volume was 32% larger (range - 5-63%) when defined by noncontrast CT compared to MRI. The areas of nonagreement tended to occur in four distinct regions of discrepancy: (a) the posterior portion of the prostate, (b) the posterior-inferior-apical portion of the prostate, (c) the apex due to disagreement between a urethrogram based definition and the location defined by MRI, (d) regions corresponding to the neurovascular bundle. Conclusion: There is a tendency to overestimate the prostate volume by noncontrast CT compared to MRI. Awareness of this tendency should allow us to be to more accurately define the prostate during 3-D treatment planning.