Kilo-voltage cone beam computed tomography (kV-CBCT) integrated with a linac can produce online volumetric and anatomical images for patient set-up and dosimetric analysis in adaptive radiotherapy. However CBCT is prone to motion artifacts. This study investigates the impact of target motion in CBCT imaging. To simulate respiratory movement, a dynamic phantom was moved in three-dimensions with a period of 4 s and two different amplitudes (PA1 and PA2). The targets of well defined geometries were made using wax. A reference image of the static target was achieved with fan beam CT. Using CBCT, the targets in static and dynamic modes were imaged under full-fan beam conditions. The length of average HU spread was reduced in range from 19.35 to 44.44% along the cranio-caudal direction of targets. The percentage volume loss of dynamic targets imaged using CBCT (for Hounsfield Units with window width -500 to 0) ranged from 14.35 to 30.95% for PA1 and 21.29 to 43.80% for PA2 in comparison with static targets imaged with fan beam CT. A significant loss of volumetric information may result for non-gated CBCT imaging of moving targets and may result in a systematic error in re-contouring when CBCT images are used for radiotherapy re-planning.
Purpose: To perform a Quality Assurance test based on gamma analysis of fluence test patterns for HDR brachytherapy. Method and Materials: A phantom using physiotherapy wax material was made to carryout the study. Five stainless steel interstitial needles of 20 cm length with beveled point end and 1.5mm outer diameter were implanted. A computed tomography image of the phantom with 1mm slice thickness was obtained. Patient specific treatment plans were generated and the fluence maps at the reference plane for all plans were generated. All the treatment plans were transferred to the MICROSELECTRON HDR machine and the fluence was delivered. Gafchromic and EDR2 films were placed at the reference plane i.e., 5 mm from the center of the implant for measuring the fluence. The calculated and measured fluences were then compared and analyzed using commercial software (PTW VERISOFT) based on the gamma index method. Results: As there are no specific guidelines for the DD and DTA values to be used for brachytherapy, various combinations of DD and DTA values were tried and analyzed. The results for both EDR2 and Gafchromic films were analyzed. The response of Gafchromic film at various times after irradiation was also included in the analysis. Conclusion: The results indicate that 2 mm DTA and 10 % DD is optimum for gamma analysis of fluence test patterns for QA in HDR brachytherapy. Radiochromic films are a better choice than EDR2 films. Like patient specific QA procedures practiced widely in IMRT based on gamma analysis, a similar approach in HDR brachytherapy can be adopted. Conflict of Interest: Nil
In CT anatomy‐based inverse treatment planning of interstitial high‐dose rate (HDR) brachytherapy planning, the planning target volume (PTV) delineated by the radiation oncologist includes the applicator volume. The applicator volume can be eliminated with the help of two methods: one based on Boolean operations and the other using the erasing option of the application software. Both methods are compared, and the results are discussed. Elimination of the applicator volume results in the proper estimation of the PTV and the exclusion of the clinically insignificant hot volume from the PTV in the dose‐volume histogram (DVH) analysis. Five cases of prostate cancer are considered for analysis. The PTV, applicator volume, maximum, mean, modal, and minimum dose, and the percentage volume of the PTV structure receiving the percent dose for both cases, that is, with and without applicator volume, are tabulated and analyzed. The elimination of the applicator volume results in the proper volume estimation of the PTV structure and leads to better DVH analysis of interstitial HDR prostate implants. The procedure would have little relevance in routine planning but improves dose reporting. It is too early to conclude its clinical significance or insignificance.PACS numbers: 57.53.Jw, 87.53.Tf
Purpose: To investigate the effect of internal anatomical changes on dose delivered by IMRT and RapidArc (RA) for the patients with carcinoma of uterine cervix using kV‐CBCT. Method and Materials: Five patients were taken for this study. In CT images, target (CTV minus nodes), PTV, bladder, rectum and femoral head were contoured. IMRT with seven fields of equal gantry spacing and dual arc RapidArc with gantry angles: 181–179 and 179–181 degree were done to deliver 50.4Gy to PTV in 28 fractions. Pre‐fraction CBCT were acquired weekly and target and OAR's were delineated. The use of CBCT for dose calculation was validated prior to the study. From the CT plan, verification plans were created on CBCT images. The dose variation in target and OAR's between the CT and periodical CBCT (pCBCT) based IMRT and RA plans were analyzed. Results: The mean (±SD) of pCBCT based IMRT plan's target D98% and D2% doses were 98.33%±0.92% and 99.92%±1.28% relative to CT based IMRT plan. Similarly for RA, it was 98.53%±0.78% and 100.27%±1.03% respectively. For rectum, the percentage difference between CT and pCBCT based IMRT plan's mean dose, D2% and D30% were 2.17%, 0.64% and 1.02% respectively. For RA, it was 2.30%, 0.72% and 0.70% respectively. For bladder, the percentage difference between CT and pCBCT based IMRT plan's mean and D35% doses were 1.07% and 0.26%. For RA, it was 1.22%, and −0.04% respectively. For femoral head, the percentage difference between CT and pCBCT IMRT plan's D2% dose is −0.46% and for RA, it was −0.35%. Conclusion: Both IMRT and RA plans shows similar dose variation due to the anatomical changes over weeks. The dose variation in target was minimal. However the OAR doses show a substantial variability over weeks.
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