Evaluating dose distributions of high dose rate 60Co brachytherapy in an asymmetric tumor: A comparison of different designs of vaginal cylindrical applicators.

The aim of this study is to evaluate organ-at-risk (OAR) doses obtained from different designs of intracavitary cylinder applicators during high-dose-rate (HDR) brachytherapy of an asymmetric vaginal tumor. Dose distributions around five cylinder applicators were obtained using the Geant4 toolkit. Three of these applicators are commercially available: single-channel, multi-channel, and shielded. Additionally, two proposed sub-configurations were evaluated: (I) a combination of the multichannel applicator with a central shield and (II) a dynamically modulated cylinder (DMC). The dose distributions of the proposed applicators were compared to those of the single-channel cylinder applicator. The results showed that using a cylindrical applicator with a higher degree of freedom in plan optimization can improve OAR, sparing up to 60% for the rectum and 20% for the bladder. In conclusion, this work suggests using a new design of a cylindrical applicator to enhance the therapeutic ratio of brachytherapy for asymmetric and irregularly shaped vaginal tumors.

The geometric and dosimetric accuracy of kilovoltage cone beam computed tomography images for adaptive treatment: a systematic review.

Objectives: To provide an overview and meta-analysis of different techniques adopted to accomplish kVCBCT for dose calculation and automated segmentation. Methods: A systematic review and meta-analysis were performed on eligible studies demonstrating kVCBCT-based dose calculation and automated contouring of different tumor features. Meta-analysis of the performance was accomplished on the reported γ analysis and dice similarity coefficient (DSC) score of both collected results as three subgroups (head and neck, chest, and abdomen). Results: After the literature scrutinization (n = 1008), 52 papers were recognized for the systematic review. Nine studies of dosimtric studies and eleven studies of geometric analysis were suitable for inclusion in meta-analysis. Using kVCBCT for treatment replanning depends on a method used. Deformable Image Registration (DIR) methods yielded small dosimetric error (≤2%), γ pass rate (≥90%) and DSC (≥0.8). Hounsfield Unit (HU) override and calibration curve-based methods also achieved satisfactory yielded small dosimetric error (≤2%) and γ pass rate ((≥90%), but they are prone to error due to their sensitivity to a vendor-specific variation in kVCBCT image quality. Conclusions: Large cohorts of patients ought to be undertaken to validate methods achieving low levels of dosimetric and geometric errors. Quality guidelines should be established when reporting on kVCBCT, which include agreed metrics for reporting on the quality of corrected kVCBCT and defines protocols of new site-specific standardized imaging used when obtaining kVCBCT images for adaptive radiotherapy. Advances in knowledge: This review gives useful knowledge about methods making kVCBCT feasible for kVCBCT-based adaptive radiotherapy, simplifying patient pathway and reducing concomitant imaging dose to the patient.

Dosimetric comparison between microSelectron iridium-192 and flexi cobalt-60 sources in high-dose-rate brachytherapy using Geant4 Monte Carlo code.

Purpose: Manufacturing of miniaturized high activity iridium-192 (192Ir) sources have been made a market preference in modern brachytherapy. Smaller dimensions of the sources are flexible for smaller diameter of the applicators, and it is also suitable for interstitial implants. Presently, cobalt-60 (60Co) sources have been commercialized as an alternative to 192Ir sources for high-dose-rate (HDR) brachytherapy, since 60Co source have an advantage of longer half-life comparing with 192Ir source. One of them is the HDR 60Co Flexisource manufactured by Elekta. The purpose of this study was to compare the TG-43 dosimetric parameters of HDR flexi 60Co and HDR microSelectron 192Ir sources. Material and methods: Monte Carlo simulation code of Geant4 (v.11.0) was applied. Following the recommendations of AAPM TG-43 formalism report, Monte Carlo code of HDR flexi 60Co and HDR microSelectron 192Ir was validated by calculating radial dose function, anisotropy function, and dose-rate constants in a water phantom. Finally, results of both radionuclide sources were compared. Results: The calculated dose-rate constants per unit air-kerma strength in water medium were 1.108 cGy h-1U-1 for HDR microSelectron 192Ir, and 1.097 cGy h-1U-1 for HDR flexi 60Co source, with the percentage uncertainty of 1.1% and 0.2%, respectively. The values of radial dose function for distances above 22 cm for HDR flexi 60Co source were higher than that of the other source. The anisotropic values sharply increased to the longitudinal sides of HDR flexi 60Co source, and the rise was comparatively sharper to that of the other source. Conclusions: The primary photons from the lower-energy HDR microSelectron 192Ir source have a limited range and are partially attenuated when considering the results of radial and anisotropic dose distribution functions. This implies that a HDR flexi 60Co radionuclide could be used to treat tumors beyond the source compared with a HDR microSelectron 192Ir source, despite the fact that 192Ir has a lower exit dose than HDR flexi 60Co radionuclide source.