Tolerance Analysis

TrueBeam Radiotherapy System

Summary

  • Developed a tolerance stack-up model on the TrueBeam Radiotherapy System using CETOL 6 Sigma software

  • Performed RSS and worst-case analysis and identified 4 out of 3,000+ parts as critical misalignment sources, projected to yield annual savings of approximately 750k

Mission

During my internship at Varian Medical Systems, I took on the challenge of developing a tolerance stack-up model for the TrueBeam Radiotherapy System to resolve the product’s component misalignment issues.

Background

Alignment for a radiotherapy machine is critical as it needs to deliver exact X-ray beam output to treat a patient’s tumor.

Several critical sub-assemblies determine the beam alignment: gantry, sled, ion chamber, target, carousel, and collimator (as seen in the image above).

It is of top priority that the beams are aligned with the isocenter.

My Role

  • learn CETOL Six Sigma software

  • select an appropriate analysis scope

  • import part dimensions and tolerances into the software for Root Sum Square analysis

  • identify parts that are critical to misalignment

To construct a simplified model of TrueBeam, I narrowed down my analysis scope from the original 3,000+ parts to 34 parts after a detailed study of the system.

Through Root Sum Square and worst-case analysis, I identified 4 critical parts responsible for significant misalignment.

Results

The insights from my analysis directly informed redesigns, projected to yield annual savings of approximately 750k. The model I developed also serves as a template for future tolerance analyses at Varian.

Huge thanks to my colleagues at Varian who helped me throughout this internship.