The contribution of magnetic resonance imaging to the three-dimensional treatment planning of localized prostate cancer

Purpose: To investigate whether the use of transaxial and coronal MR imagin g improves the ability to localize the apex of the prostate and the anterio r part of the rectum compared to the use of transaxial CT alone, and whethe r the incorporation of MR could improve the coverage of the prostate by the radiotherapy field and change the volume of rectum irradiated. Methods and Materials: Ten consecutive patients with localized prostate car cinoma underwent a CT and an axial and coronal MR scan in treatment positio n. The CT and MR images were mathematically aligned, and three observers we re asked to contour independently the prostate and the rectum on CT and on MR. The interobserver variability of the prostatic apex location and of the delineation of the anterior rectal wall were assessed for each image modal ity. A dosimetry study was performed to evaluate the dose to the rectum whe n MR was used in addition to CT to localize the pelvic organs. Results: The interobserver variation of the prostatic apex location was lar gest on CT ranging from 0.54 to 1.07 cm, and smallest on coronal MR ranging from 0.17 to 0.25 cm. The interobserver variation of the delineation of th e anterior rectum on MR was small and constant along the whole length of th e prostate (0.09 +/- 0.02 cm), while for CT it was comparable to that for t he MR delineation at the base of the prostate, but it increased gradually t owards the apex, where the variation reached 0.39 cm. The volume of MR rect um receiving more than 80% of the prescribed dose was on average reduced by 23.8 +/- 11.2% from the CT to the MR treatment plan. Conclusion: It can be concluded that the additional use of axial and corona l MR scans, in designing the treatment plan for localized prostate carcinom a, improves substantially the localization accuracy of the prostatic apex a nd the anterior aspect of the rectum, resulting in a better coverage of the prostate and a potential to reduce the volume of the rectum irradiated to a high dose. (C) 1999 Elsevier Science Inc.