Application of Statistical Process Control (spc) in the Manufacturing of Medical Accelerators

Most radiation therapy machines are based on microwave linear accelerators. Manufacturing reliable accelerators and hence, reliable radiation therapy machines requires accurate, efficient, and well-controlled processes throughout the steps of electron accelerator manufacturing. Statistical Process Control (SPC) techniques are utilized at Siemens – Oncology Care Systems (OCS) to meet these requirements. SPC is routinely used to monitor different accelerator manufacturing processes. Analysis of the resulting data is used to initiate actions to preserve consistent and highquality performance. This results in processes that are stable and predictable at both higher effective capacity and lower cost. In addition, it is expected that high-energy physics projects such as the Next Linear Collider (NLC) should benefit from these techniques. Quality control techniques similar to those used by Siemens are essential to the fabrication of the large number of cavities and accelerator sections in an industrial environment. QUALITY ASSURANCE IN THE MANUFACTURING OF MEDICAL DEVICES Consistent high-quality and defect-free production is a clear requirement in medical devices. To achieve this requirement, medical device manufacturers usually employ Quality Management Systems (QMS) to maintain quality. Additionally, QMS help medical device manufacturers meet various regulatory quality standards such as the ISO 9001 (as a generic quality system standard) and the ISO 13485 (as a quality system standard for medical devices). In recent years, Statistical Quality Control (SQC) and its subset the Statistical Process Control (SPC), [1-3] have proven effective in improving and maintaining the quality of the medical device industry and supporting the FDA’s good manufacturing practices. STATISTICAL QUALITY CONTROL Statistical Quality Control (SQC) is a scientific method to analyze manufacturing data. Based on this analysis, measures are taken to maintain the quality of the manufactured product. One of the techniques that are used to monitor manufacturing processes and provide feedback is Statistical Process Control (SPC). The feedback is used to maintain and improve the capability of the process and to ensure product conformance. SPC is used to control the process by signaling when adjustments may be necessary. Some techniques associated with SPC include frequency histograms and control charts. A control chart is the tool used to monitor the variation in a process and ensure that the process is in a state of control. This allows the operator to monitor the trends occurring in the process. The control chart reflects the specification limits, namely, the Upper Specification Limit (USL) and the Lower Specification Limit (LSL). In addition, it has upper and lower control limits that lie within the specification limits. The Upper Control Limit (UCL) and the Lower Control Limit (LCL) are determined by evaluating the dispersion (variability) in process, see Fig. 1. In a well-controlled process, these limits can be chosen to be equal to μ ± 3σ respectively, where σ is the process standard deviation and μ is the process mean. These statistical limits are normally called the “3 sigma control limits”. In a normal (Gaussian) distribution, 99.73% of the values measured lie in interval of width 6 σ. Figure 1: Typical SPC chart. APPLICATIONS OF SPC IN THE MANUFACTURING OF SIEMENS MEDICAL ACCELERATORS At Siemens Medical Solutions USA, Inc., Oncology Care Systems Group, we have been implementing various Computer–Aided Processes (CAP) in several critical areas of the accelerator manufacturing. The CAP approach provides accurate and efficient techniques to characterize accelerator structures [4-6]. These techniques are implemented at different phases of development and of ____________________________________________ *E-mail: [email protected]. General Form of SPC Charts 0 10 20 30 40 50 60 70 80 90