Dosimetry assessment of patient-specific 3D printable materials for HDR surface brachytherapy.

PURPOSE: 3D printable material water equivalence was investigated within the range of Iridium-192 source energies. The aim is to compare the dose calculated by our treatment planning system (TPS) with the dose measured in the presence of printed materials. The purpose of this investigation is to assess the feasibility of using 3D materials for brachytherapy surface applicators. METHODS AND MATERIALS: Cheetah was examined both in a water tank and with the CIRS anthropomorphic phantom. Calibrated Gafchromic EBT3-V3 film was used and the measurements compared with TG-43 calculations on Oncentra®Brachy. A 3D-printed slab 5 mm thick was created to position the source and two films were irradiated at 5 mm and 15 mm of distance. A curved mould with 7 trajectories was created and coupled with CIRS phantom. A set of CT images of phantom and mould was acquired and imported on TPS, where a target was defined and a dose plan was created. Plan was delivered with two films positioned between two different slabs of phantom, at reciprocal distance of 2 cm, orientated perpendicularly to the source axis. RESULTS: All PDDs show a maximum difference of 4.7% (average 2.2%). At 5 mm and at 15 mm, the gamma pass rate is 100% with tolerance 2%/1 mm DTA. Results of films placed intra-slabs show a high pass rate (>99%) with tolerances of 2% dose and 1 mm DTA. CONCLUSION: 3D material investigated is water equivalent at Ir-192 energies and agreed with Oncentra®Brachy dose calculations which suggest that it is a suitable material for superficial brachytherapy.

Determination of MLC model parameters for Monaco using commercial diode arrays.

Multileaf collimators (MLCs) need to be characterized accurately in treatment planning systems to facilitate accurate intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). The aim of this study was to examine the use of MapCHECK 2 and ArcCHECK diode arrays for optimizing MLC parameters in Monaco X-ray voxel Monte Carlo (XVMC) dose calculation algorithm. A series of radiation test beams designed to evaluate MLC model parameters were delivered to MapCHECK 2, ArcCHECK, and EBT3 Gafchromic film for comparison. Initial comparison of the calculated and ArcCHECK-measured dose distributions revealed it was unclear how to change the MLC parameters to gain agreement. This ambiguity arose due to an insufficient sampling of the test field dose distributions and unexpected discrepancies in the open parts of some test fields. Consequently, the XVMC MLC parameters were optimized based on MapCHECK 2 measurements. Gafchromic EBT3 film was used to verify the accuracy of MapCHECK 2 measured dose distributions. It was found that adjustment of the MLC parameters from their default values resulted in improved global gamma analysis pass rates for MapCHECK 2 measurements versus calculated dose. The lowest pass rate of any MLC-modulated test beam improved from 68.5% to 93.5% with 3% and 2 mm gamma criteria. Given the close agreement of the optimized model to both MapCHECK 2 and film, the optimized model was used as a benchmark to highlight the relatively large discrepancies in some of the test field dose distributions found with ArcCHECK. Comparison between the optimized model-calculated dose and ArcCHECK-measured dose resulted in global gamma pass rates which ranged from 70.0%-97.9% for gamma criteria of 3% and 2 mm. The simple square fields yielded high pass rates. The lower gamma pass rates were attributed to the ArcCHECK overestimating the dose in-field for the rectangular test fields whose long axis was parallel to the long axis of the ArcCHECK. Considering ArcCHECK measurement issues and the lower gamma pass rates for the MLC-modulated test beams, it was concluded that MapCHECK 2 was a more suitable detector than ArcCHECK for the optimization process.

Impact of delineation uncertainties on dose to organs at risk in CT-guided intracavitary brachytherapy.

PURPOSE: This study quantifies the inter- and intraobserver variations in contouring the organs at risk (OARs) in CT-guided brachytherapy (BT) for the treatment of cervical carcinoma. The dosimetric consequences are reported in accordance with the current Gynecological Groupe Européen de Curiethérapie/European Society for Therapeutic Radiology and Oncology guidelines. METHODS AND MATERIALS: A CT planning study of 8 consecutive patients undergoing image-guided BT was conducted. The bladder, rectum, and sigmoid were contoured by five blinded observers on two identical anonymized scans of each patient. This provided 80 data sets for analysis. Dosimetric parameters analyzed were D0.1 cc, D1 cc, and D2 cc. The mean volume of each OAR was calculated. These endpoints were compared between and within the observers. The CT image sets from all patients were evaluated qualitatively. RESULTS: The interobserver coefficient of variation for reported D2 cc was 13.2% for the bladder, 9% for the rectum, and 19.9% for the sigmoid colon. Unlike the variation seen in bladder and rectal contours, which differed largely in localization of the organ walls on individual slices, sigmoid colon contours demonstrated large differences in anatomic interpretation. CONCLUSIONS: Variation in recorded D2 cc to the bladder and rectum is comparable with the previous published results. Inter- and intraphysician variations in reported D2 cc is high for the sigmoid colon, reflecting varying interpretation of sigmoid colon anatomy. Variation in delineation of the OARs may influence treatment optimization and is a potential source of uncertainty in the image-guided BT planning and delivery process.