Statistical process control analysis for patient-specific IMRT and VMAT QA

摘要

A relatively large cohort of 278 nasopharyngeal carcinoma plans in almost 2 years (2007–2009) of patient-specific IMRT QA has been considered using a MapCHECK 2D diode array. This device is composed of 445 n-type solid state detectors with an area of 22.0 × 22.0 cm2. The inner 221 detectors cover the central part of 10 × 10 cm2 with 0.707 cm diagonal spacing, while the outer 224 detectors have 1.414 cm diagonal spacing. The active area of each detector is 0.8 × 0.8 mm [2, 11]. Almost all of the nasopharyngeal carcinoma IMRT plans were performed with nine equi-angular beam arrangements of coplanar technique with dynamic MLC generated by the Eclipse treatment planning system (version 7.3.10; Varian Medical Systems, Inc., Palo Alto, CA, USA), using 6 MV on a Varian Clinac 21 (80 leaves) or 23EX linear accelerator (120 leaves) (Varian Medical Systems, Inc.). The isodose distribution is shown in Fig. 1. After the patient plan was approved by a radiation oncologist, medical physicists created the pretreatment verification plan based on the composite plan at a 0° gantry angle with the beam directed perpendicular to the verification plan. MapCHECK has 1.35 cm inherent buildup equivalent to 2 g/cm2 water, the source-to-skin distance (SSD) on the MapCHECK surface was set at 98.65 cm and then it was covered by 3 cm of solid water phantom as shown in Fig. 2a. Therefore, the water equivalent depth of total buildup was 5 cm, which was approximately equivalent to the average depth of the head and neck region. However, before the IMRT plans were verified, the absolute dose of 200 cGy was calibrated at every QA session to reduce the effect of output variation. Because with version 6.1 of the SNC Patient software (Sun Nuclear Corp., Melbourne, FL, USA) it is possible to compare the dosage in composite beams, this study evaluated the plan in composited beam verification with an absolute dose comparison using the gamma index. The criteria were 3% dose difference and 3 mm distance-to-agreement (γ_3%/3mm) between the measurement and calculation with 10% threshold [12]. The gamma index was applied from the ellipse formula by using the dose and distance difference between measurement and calculation. A point that had a gamma value higher than 1.00 would not pass the criteria. The percentage of points that pass the criteria can be called the % gamma pass or gamma pass rate. One year after the VMAT machine (Varian Clinac iX linear accelerator; RapidArc; Varian Medical Systems) had been installed in our institute (year 2011), 159 VMAT plans of nasopharyngeal carcinoma were optimized and calculated using version 8.9.17of the Eclipse treatment planning system. From our experience, 6 MV with a 2.5 arc (185° to 175° CW and CCW rotation and 185° to 0° CW) was an optimal technique to treat the head and neck region. The maximum repetition rate was set to 600 MU/min. The collimator rotation was set at 340° for the CW arc and 20° for the CCW arc to reduce the tongue-and-groove effect. The field size was manually set by the planner before optimization. A medium thickness of the Exact IGRT couch top structure was inserted into the planning to improve the accuracy of dose calculation. The medical physicist planned both IMRT and VMAT techniques as shown in Fig. 1, the IMRT treatments were planned first and the dose constraints from IMRT were used to optimize the VMAT plan. The oncologist selected the appropriate VMAT or IMRT plan to treat the patient by considering dose-volume histogram and isodose evaluation. If the VMAT plan was selected, the ArcCHECK cylindrical diode array was employed for patient-specific QA. This device is composed of 1386 n-type solid state detectors with an active size of 0.8 × 0.8 mm2 in each detector. The detectors are arranged in a spiral array with 1 cm spacing along the cylinder and 1 cm along the circumference for 21.0 cm in detector array length (spiral height). The detectors are embedded in a 2.85-cm depth of acrylic buildup that is equivalent to a 3.28-cm water equivalent depth [13]. All detectors of ArcCHECK were perpendicular to the beam for all gantry angles. ArcCHECK was divided into two sections. The outer section has 6 cm thickness and the inner section is 15 cm in diameter of acrylic insertion capable to insert a thimble ionization chamber for central axis dose measurement. If the inner section is removed, the VMAT plan can be used to check the inhomogeneity correction of air in treatment planning. After the VMAT plan was approved by a radiation oncologist, the VMAT plan was recalculated in the ArcCHECK phantom based on a composite plan for the same patient plan of monitor unit to get the same MLC movement, the same dose rate variation and the same gantry speed modulation. The isocenter was set at the center of ArcCHECK by using lasers and cross-hairs, the SSD of 86.60 cm would read on the ArcCHECK surface as shown in Fig. 2 b. The ArcCHECK