Optimization of treatment technique for endorectal brachytherapy and feasibility of a newly designed applicator for vaginal vault recurrence patients

Abstract

Brachytherapy has long history of use in radiotherapy. Development in metal artifact reduced (MAR) computed tomography (CT), dose calculation for inhomogeneous medium and three-dimensional (3D) printing can further improve the treatment techniques in brachytherapy. These studies focused on optimizing the treatment techniques in endorectal and vaginal vault recurrence brachytherapy. For endorectal brachytherapy with shielded applicator, studies were performed to determine the optimal scanning parameters of CT to remove metal artifact on images and see if the dose calculated using MAR and non-MAR CT are the same. Moreover, study was performed with and without accounting for inhomogeneous medium in shielded applicator to see any dose difference. For vaginal vault recurrence brachytherapy, the applicator currently used to treat vaginal vault recurrence gives high dose to organs at risk (OAR), so the feasibility of the newly designed 3D printed applicator was evaluated. A tailor-made phantom with shielded applicator inside was scanned in CT using different kV and mAs. Same region of interest was drawn on all CT images and analyzed to see which combination of kV and mAs achieved the best metal artifact reduction. The best scanning parameters were applied to obtain planning CT for endorectal brachytherapy in ten patients. The CT scans were analyzed using two different dose calculation algorithms with and without accounting for inhomogeneous medium. Furthermore, the ten CT images were reconstructed to MAR CT. Dose calculated on both MAR and non-MAR CT were compared to analyze the dose difference in OAR and tumor. The 3D printed applicator was compared with two commercially available applicators. Each applicator was placed inside a female pelvic phantom and scanned by CT. Tumor and OAR were created in CT. By keeping the same prescription, the dose to OAR and treatment time were compared for all three applicators in separate plans. To test out the applicators in real patient anatomy, same study was repeated using three patients’ CT. The results showed the maximum kV and mAs available gave the best effect for metal artifact reduction. The difference in dose calculated on MAR and non-MAR CT were minimal. The newly designed applicator achieved lower dose to OAR with less treatment time over other applicators. To conclude, the studies suggested that CT scanned with maximum kV and mAs should be adopted to give the best effect of metal artifact reduction. Metal artifact reduction should be applied to CT images, as dose calculated on MAR and non-MAR CT were the same, so MAR CT allows oncologists to have a clearer image for contouring of tumor and OAR without dose deviation. Dose should be calculated using inhomogeneity correction algorithm to achieve a more accurate result for shielded applicator, as there was large dose difference between two algorithms, although the time spent on calculation are longer and the cost of this calculation option is high. It is feasible to use the newly designed applicator to treat vaginal vault recurrence patients as the dose to OAR and treatment time were reduced when compared with two other commercially available applicators.