Octavius 4D characterization for flattened and flattening filter free rotational deliveries.

PURPOSE: In this study the Octavius detector 729 ionization chamber (IC) array with the Octavius 4D phantom was characterized for flattening filter (FF) and flattening filter free (FFF) static and rotational beams. The device was assessed for verification with FF and FFF RapidArc treatment plans. METHODS: The response of the detectors to field size, dose linearity, and dose rate were assessed for 6 MV FF beams and also 6 and 10 MV FFF beams. Dosimetric and mechanical accuracy of the detector array within the Octavius 4D rotational phantom was evaluated against measurements made using semiflex and pinpoint ionization chambers, and radiochromic film. Verification FF and FFF RapidArc plans were assessed using a gamma function with 3%∕3 mm tolerances and 2%∕2 mm tolerances and further analysis of these plans was undertaken using film and a second detector array with higher spatial resolution. RESULTS: A warm-up dose of >6 Gy was required for detector stability. Dose-rate measurements were stable across a range from 0.26 to 15 Gy∕min and dose response was linear, although the device overestimated small doses compared with pinpoint ionization chamber measurements. Output factors agreed with ionization chamber measurements to within 0.6% for square fields of side between 3 and 25 cm and within 1.2% for 2 × 2 cm(2) fields. The Octavius 4D phantom was found to be consistent with measurements made with radiochromic film, where the gantry angle was found to be within 0.4° of that expected during rotational deliveries. RapidArc FF and FFF beams were found to have an accuracy of >97.9% and >90% of pixels passing 3%∕3 mm and 2%∕2 mm, respectively. Detector spatial resolution was observed to be a factor in determining the accurate delivery of each plan, particularly at steep dose gradients. This was confirmed using data from a second detector array with higher spatial resolution and with radiochromic film. CONCLUSIONS: The Octavius 4D phantom with associated Octavius detector 729 ionization chamber array is a dosimetrically and mechanically stable device for pretreatment verification of FF and FFF RapidArc treatments. Further improvements may be possible through use of a detector array with higher spatial resolution (detector size and∕or detector spacing).

Mechanical characterization of the varian Exact-arm and R-arm support systems for eight aS500 electronic portal imaging devices.

PURPOSE: The aim of this study is to compare the positioning accuracy at different gantry angles of two electronic portal imaging devices (EPIDs) support arm systems by using EPID difference images as a measure for displacement. This work presents a comparison of the mechanical performance of eight Varian aS500 (Varian Medical Systems, Palo Alto, CA) EPIDs, mounted using either the Varian Exact-arm or R-arm. METHODS: The mechanical performance of the two arm systems was compared by investigating the variation in sensitivity with gantry angle, both before and after the EPID position was adjusted after gantry rotation. Positional errors were investigated by subtracting images from a reference image taken at gantry 0 degrees, and the amplitude of the peaks and troughs at the field edges for longitudinal (radial) and lateral (transverse) profiles across the resulting image was related to the distance of displacement. Calibration curves based on a pixel-by-pixel shift were generated for each EPID and the Varian hand pendant accuracy was compared to the calibration data. RESULTS: The response of the EPIDs was found to change with gantry rotation, with the largest difference at 180 degrees. The Exact-arm was found to correct well for any displacement, while the R-arm tended to overcorrect following repositioning using the hand pendant. The calibration curves were consistent within each set of matched linacs, and the hand pendant accuracy was similar for both arm systems, although generally in different directions. With respect to gantry rotation effects, the mechanical performance of the Exact-arm systems was found to be much better than that of the R-arm systems. At gantry positions 90 degrees, 270 degrees, and 180 degrees the average misalignment in the longitudinal direction was +4.2 +/- 0.2, +1.8 +/- 1.6, and +7.4 +/- 0.5 mm for the R-arms, and +2.9 +/- 0.2, +2.1 +/- 0.8, and +4.9 +/- 0.7 mm for the Exact-arms. In the lateral direction the average positional errors were +2.1 +/- 0.4, -4.7 +/- 0.4, and -2.5 +/- 0.5 mm for the R-arms, and -0.3 +/- 0.3, -0.5 +/- 0.3, and -0.4 +/- 0.2 mm for the Exact-arms. The hand pendant correction had minimal impact in the lateral direction for both arm systems. However in the longitudinal direction the mean errors for the R-arms were +3.4 +/- 0.7, +1.5 +/- 0.6, and +4.6 +/- 0.7 mm at gantry angles 90 degrees, 270 degrees, and 180 degrees, and the equivalent Exact-arm errors were +0.9 +/- 0.3, +1.2 +/- 0.3, and +1.9 +/- 0.9 mm, respectively. CONCLUSIONS: The performance of the EPIDs demonstrate that the Exact-arm system provides a more reproducible position and better agreement with the EPID position as indicated on the EPID pendant at all gantry angles than the R-arm.