174. Large scale adoption of statistical process control for patient-specific VMAT quality assurance

Abstract Purpose Statistical Process Control (SPC) is a tool widely used in industrial engineering for monitoring, controlling and improving a process through statistical analysis. We applied this strategy for patient-specific VMAT pre-treatment verification. Methods From 2009, more than 1700 patients were treated with Elekta VMAT at our institution. Plans were re-grouped according to treatment technique and disease sites Group 1: 736 complex treatments using SIB for head-neck, pelvic, brain and other sites; (2) 441 low-risk prostate treatments and (3) 558 extracranial metastasis treated with SBRT. 4942 planar dose measurements were performed with the PTW Seven29 array/Octavius phantom. Three metrics were simultaneously evaluated: (a) γ %: points-percentage with γ -value less than one, (b) γ mean and (c) γ 1%: the near-maximum gamma. Clinical specifications were: γ % > 90%, γ mean  γ 1 % 2 . Shewhart charts were used to calculate the central (CL), upper control (UCL) and lower control limits (LCL). The capability of the processes was evaluated by means of Cpk indexes. Results Using 3%/3 mm γ -analysis, results significantly depend on plan complexity. For γ %, CL and LCL were 93.8%, 99.1%, 99.5% and 87.9%, 96.6%, 97.9%, for group 1,2 and 3 respectively. For γ mean, CL and UCL were 0.42, 0.36, 0.29 and 0.54, 0.49, 0.40, for groups 1, 2 and 3 respectively. For the groups 2 and 3, all processes are in control and within clinical specifications. The Cpl/Cpu indices for γ %, γ mean and γ 1% resulted equal to 2.85, 1.64 and 2.11 in group 2 and 4.20, 2.60 and 2.01 in group 3. For group 1 only the γ mean process is in control; the γ % process is out of control and out of clinical specifications (returning in control with 5%/3 mm specification, with Cpl and Cpu equal to 1.87 and 1.57, respectively). Conclusions SPC is a powerful tool to quantifiably evaluate the QA process performance in advanced radiotherapy.