Pediatric cone beam CT on Varian Halcyon and TrueBeam radiotherapy systems: radiation dose and positioning accuracy evaluations.

Medical exposure to ionizing irradiation has become a recognised carcinogenic risk. Balancing the concomitant imaging dose and positioning accuracy is critical in image-guided-radiotherapy (IGRT) especially for children, whose higher biological susceptibility and longer expected life make them more vulnerable to develop secondary cancer. This work aims to evaluate and benchmark the imaging dose and positioning accuracy of a new MV cone-beam-CT (CBCT)-guided IGRT system, Varian Halcyon, against conventional kV CBCT. Weighted-CT-dose-index (CTDIw) were measured for Varian TrueBeam kV CBCT, and computed for Halcyon MV CBCT using Eclipse system as validated before. Simulating the IGRT workflow, the positioning accuracy of correcting a known shift was tested on various regions of 1-year, 5-year and adult anthropomorphic phantoms, respectively. Inter-scanner and inter-protocol comparisons of dose and accuracy were performed. kV CTDIw for 'Head', 'Thorax', 'Pelvis' and 'Image Gently' (in CTDIΦ16cm/CTDIΦ32cm phantoms, respectively) protocols were measured as 4.5 mGy, 5.4 mGy, 19.3 mGy, and 1.1 mGy/0.5 mGy, respectively. Using 'High Quality' mode, MV CTDIw in the CTDIΦ16cm and CTDI Φ32cm phantoms were computed as 84.5 mGy and 63.8 mGy (imaging length = 28 cm), 68.8 mGy and 55.5 mGy (imaging length = 16 cm), respectively, which were about twice of 'Low Dose' mode. The maximum positioning deviation observed on Halcyon was 0.51 mm ('Low Dose' adult thorax), which was lower than that of standard (0.58 mm, 'Pelvis' adult pelvis) and 'Image Gently' kV CBCT (1.57 mm, adult pelvis). Accuracy of 'Image Gently' kV CBCT head & neck and thoracic imaging were clinically acceptable for adults (maximum deviation = 0.54 mm, adult thorax). Complying with Image Gently campaign, dose-efficient protocols should be used for pediatric IGRT, achieving comparable positioning accuracy on the new Halcyon MV CBCT system relative to the conventional kV CBCT.

Dose calculation deviations induced by fractional image-guided-couch-shifts for Varian Halcyon MV cone beam CT.

PURPOSE: To assess the criticality of calculation deviations induced by fractional image-guided-couch-shifts for Halcyon MV cone beam CT (CBCT) dose, which is incorporated as part of total treatment dose. METHODS: Eclipse-calculated imaging dose was first validated in 'Cheese Phantom' by measurement. Then, the actual imaging dose (Dact) for 18 historical patients of various sites were recalculated based on 513 MV CBCT-guided-couch-shift data, and compared with reference computations based on treatment isocentre (Dref). Patient- and plan-specific dose from treatment fields was integrated with Dact and Dref respectively for comparison. RESULTS: The average absolute relative disagreements between the measured and calculated dose were less than 1.23%. The mean ±â€¯1SD of gamma passing rates of the accumulated imaging dose and total dose were 80.71 ±â€¯6.22% and 99.81 ±â€¯0.32% respectively based on 3 mm/3%/local/10% threshold criteria. The accumulated errors of minimum imaging dose to PTV were no larger than -14.38 cGy, which were reduced to -0.82 cGy after the heterogeneous treatment dose was overlaid. The mean relative discrepancies of PTV minimum dose were -0.61 cGy (-0.71%) and -0.00 (0.00%), before and after incorporating the treatment dose respectively. CONCLUSIONS: The Eclipse-calculated Halcyon MV CBCT dose was validated. Although the isocentre displacement-induced imaging dose calculation errors for Halcyon MV CBCT were partially cancelled out by couch shifts of various directions and distances, especially after the incorporation of heterogeneous treatment dose, it was still advisable to monitor the accumulated deviations and replan when unacceptable target under-dose or organ over-dose were observed.