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2017 Rik Westendorp

C-arm conebeam CT guided 125I prostate brachytherapy : Dynamic dose calculation and implant dynamics

 

The aim of 125I prostate brachytherapy is to cover the prostate with an adequate dose to control local disease and, at the same time, limit dose to healthy structures to minimize toxicity of the treatment. The AAPM, ABS and GEC/ESTRO recommend to perform postimplant dosimetry approximately a month after implantation. Underdosages often are unnoticed until postimplant dosimetry. In most cases, it is practically not feasible to correct underdosages. We introduced an efficient dynamic brachytherapy technique to correct underdosages already during the implantation procedure. After finishing the implant, we acquired a transrectal ultrasound (TRUS) scan on which the prostate was contoured by the physician. On TRUS images, implanted seeds cannot be visualized accurately. Therefore, cone-beam CT (CBCT) images were obtained using an isocentric C-arm. On these images 125I seeds are clearly visible; anatomy however is poorly visualized on this modality. Dose was calculated using the seed positions obtained with CBCT. Three fiducial gold markers, being visible on both TRUS and CBCT, were used as reference points to register TRUS and CBCT imaging datasets. Following registration, the contours and the dose distribution were merged providing dosimetry during the implantation procedure. If the physician noticed underdosages at critical locations, an adaptive plan was made and remedial seeds were placed to resolve the underdosage. Furthermore, the TRUS and CBCT datasets allowed to study implant dynamics. We developed a method to track seeds over time to study edema of the implant and the displacement patterns of seeds. This algorithm proved to be very fast and accurate in linking seeds. With a cylindrical and spherical model we looked at the time trends of edema and found that edema changes the geometry of the implant only to a limited extent, we found values that were relatively small compared to literature. Being limited, edema is the main cause of differences between intraoperative and postimplant dosimetry. With a simple equation this effect can be compensated. Seed displacements are another cause of differences in intraoperative and postimplant dosimetry. This effect cannot be compensated, making accurate individual predictions of postimplant dosimetry at Day 30 impossible, as was concluded after studying a group of 699 implants. In a study we showed that TRUS-based dosimetry tends to overestimate Day 30 dosimetry whereas CBCT-based dosimetry underestimates Day 30 dosimetry. We studied the dosimetric consequences of our dynamic brachytherapy technique and found that the technique improved dosimetry considerably, during the implantation procedure as well as at (Day 30) postimplant dosimetry as was concluded after a study with 1266 patients. The study also showed that our dynamic technique allows for sparse implants, limiting the high dose volume, thereby potentially reducing toxicity of the treatment. With this study we show that the dynamic technique is efficient and effective in identifying implants that need adaptation. Finally we studied the inter- and intraobserver variability contouring and registration variability of our procedure. The added dosimetrical uncertainty resulting from these variabilities was small compared to the improvements in dosimetry made by adapting implants.

 

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