Prostate deformation from inflatable rectal probe cover and dosimetric effects in prostate seed implant brachytherapy

Purpose: Prostate brachytherapy is an important treatment technique for patients with localized prostate cancer. An inflatable rectal ultrasound probe cover is frequently utilized during the procedure to adjust for unfavorable prostate position relative to the implant grid. However, the inflated cover causes prostate deformation, which is not accounted for during dosimetric planning. Most of the therapeutic dose is delivered after the procedure when the prostate and surrounding organs-at-risk are less deformed. The aim of this study is to quantify the potential dosimetry changes between the initial plan (prostate deformed) and the more realistic dosimetry when the prostate is less deformed without the cover. Methods: The authors prospectively collected the ultrasound images of the prostate immediately preceding and just after inflation of the rectal probe cover from thirty-four consecutive patients undergoing real-time planning of I-125 permanent seed implant. Manual segmentations of the deformed and undeformed images from each case were used as the input for model training to generate the initial transformation of a testing patient. During registration, the pixel-to-pixel transformation was further optimized to maximize the mutual information between the transferred deformed image and the undeformed images. The accuracy of image registration was evaluated by comparing the displacement of the urethra and calcification landmarks and by determining the Dice index between the registered and manual prostate contours. After registration, using the optimized transformation, the implanted seeds were mapped from the deformed prostate onto the undeformed prostate. The dose distribution of the undeformed anatomy, calculated using the VariSeed treatment planning system, was then analyzed and compared with that of the deformed prostate. Results: The accuracy of image registration was 1.5±1.0 mm when evaluated by the displacement of calcification landmarks, 1.9±1.1 mm when characterized by the displacement of the centroid of the urethra, and 0.86±0.05 from the determination of the Dice index of prostate contours. The magnitude of dosimetric changes was associated with the degree of prostate deformation. The prostate coverage V100% dropped from 96.6±1.7% on prostate-deformed plans to 92.6±3.8% (p < 0.01) on undeformed plans, and the rectum V100% decreased from 0.48±0.39 to 0.06±0.14 cm3 (p < 0.01). The dose to the urethra increased, with the V150% increasing from 0.02±0.06 to 0.11±0.10 cm3 (p < 0.01) and D1% changing from 203.5±22.7 to 239.5±25.6 Gy (p < 0.01). Conclusions: Prostate deformation from the inflation of an ultrasound rectal probe cover can significantly alter brachytherapy dosimetry. The authors have developed a deformable image registration method that allows for the characterization of dose with the undeformed anatomy. This may be used to more accurately reflect the dosimetry when the prostate is not deformed by the probe cover.