Test purpose

Evaluation of the IQM sensitivity to errors in segments using EGSnrc/BEAMnrc Monte Carlo (MC) codes.

Test method

Sensitivity analysis (SA) techniques were applied to study the significance of small alterations of the field (segment) sizes on the IQM signal response. One hundred and eighty multi leaf segments were analyzed with methods that included scatter plots (SP), brute force, variance-based (VAR), and standard regression coefficient SA.
The segments were altered randomly within ±1, ±2, and ±3 mm leaf steps for 10 MV photon beams.

Test results

In total, 6 original segments were each altered 30 times randomly with ±1, ±2, and ±3 mm positional errors. For each segment, a Monte Carlo simulation was done to determine the IQM signal. Sensitivity analysis results indicated that the IQM is sensitive to detect these alterations in SA. The sensitivity is more pronounced in small SA.

Conclusion

The sensitivity of the IQM in this study shows its potential to detect small alterations in SA. The SP and VAR relation to SA is approximately constant at large SA but displays power function relationships at smaller SA values. All sensitivity analysis methods employed in this study indicated that the IQM signal (𝑆) will indicate small segments alterations even for the larger segments.