Magnetic resonance imaging-based treatment planning for prostate brachytherapy.

PURPOSE: Transrectal ultrasound (TRUS) is the standard imaging modality for planning prostate brachytherapy. However, magnetic resonance imaging (MRI) provides greater anatomic detail than TRUS. We compared treatment plans generated using TRUS, endorectal coil MRI (erMRI), and standard body array coil MRI (sMRI). METHODS AND MATERIALS: Treatment plans were used from patients treated with permanent, stranded-seed (125)I brachytherapy in a prospective trial. All men underwent pretreatment planning based on TRUS, and all underwent erMRI before treatment and sMRI 30 days after the implant. Treatments for 20 consecutive patients were replanned on sMRI and erMRI images by investigators blinded to TRUS-based plans. Prostate volume/dimensions, radioactivity-to-prostate-volume ratio, and dosimetric parameters were compared.
RESULTS: Compared with TRUS, mean prostate volume measured by erMRI was smaller, medial-lateral diameter was larger, and anterior-posterior diameter was smaller, suggesting that the endorectal coil produced anatomic distortions. Craniocaudal prostate length was smaller on both types of MRI than on TRUS, suggesting that TRUS overestimates prostate length. Activity per volume was 7.5% lower for plans based on sMRI than on TRUS (0.901 vs. 0.974mCi/cm(3), p<0.001). sMRI plans had similar coverage of the planning target volume (PTV) (dose to 90% of the prostate [D(90)] 116.6% sMRI vs. 117.5% TRUS, p=0.526) and improved dose homogeneity (percentage of PTV receiving 150% of the prescription dose [V(150)] 47.4% sMRI vs. 53.8% TRUS, p=0.001 and percentage of PTV receiving 200% of the prescription dose [V(200)] 16.6% sMRI vs. 19.2% TRUS, p<0.001).
CONCLUSIONS: Staging erMRI should not be routinely used for treatment planning because it produces anatomic distortion. sMRI may have treatment planning advantages over TRUS because of superior soft-tissue delineation of the prostate and adjacent normal tissue structures.