Evaluation Output Variability in TrueBeam Linac using Process Capability Indices

Purpose or Objective: Process capability indices (PCI) are intended to provide single number assessment of ability to meet specification limits on quality characteristics of interest. It can be deduced from the multiple literature citations that PCI are both an effective and excellent means of measuring quality and performance, and have been widely used in the manufacturing industry. This work applied PCI to establish the control limits of the output constancy check for Varian TrueBeam quality assurance (QA), and to evaluate the efficiency of the QA process by using the process capability performance measurement index (Cpm).

Material and Methods: 400 output constancy checks were measured and evaluated with SNC Daily QA 3, and with the automated and integrated machine performance check (MPC) tool and were verified against independent detectors to evaluate its beam output detection abilities.

Results: A statistical analysis of the output constancy check was performed. The Cpm calculated to evaluate QA the capabilities and suitability of the SNC Daily QA 3 and MPC. The Cpm values of SNC Daily QA 3 was 1.82 and MPC was 1.89 the average 0.32% for SNC Daily QA 3 and 0.28 % for MPC.

Conclusion: Statistical process control is useful to quantifiably demonstrate the QA process confidence to specifications. Systematic errors were greater with the SNC Daily QA 3 process than with the MPC QA process. We speculate that this is due to the physical effort/manual intervention required to setup the SNC Daily QA 3 phantom for the measurements. Both the MPC and SNC Daily QA 3 have proven to be acceptable because the values are higher than 1.0