Room: AAPM ePoster Library
Purpose: evaluate accuracy of the IQM for different field sizes and to study how the signal changes across the IQM plane. Tolerance criteria for lung SBRT and nasopharynx VMAT plans were also investigated by assessing IQM sensitivity in detecting deliberately introduced errors in their delivery.
Methods: dependence of the measured signal on symmetric and asymmetric square field sizes, from 1×1 cm2 to 30×30 cm2, was investigated. The IQM’s ability to detect MLC field size and shift errors introduced to these fields was also characterized. Additionally, the system’s capability to detect delivery errors introduced to plans for two clinical sites was assessed. This included measuring 103 nasopharynx VMAT plans and 78 lung SBRT VMAT plans with introduced variations in gantry angle, collimator angle and MLC field size. Lastly, the results of the IQM sensitivity were compared to the ArcCheck detector.
Results: calculated and measured IQM signal deviation remained within ±2% for all symmetric field sizes used, while the asymmetric fields had greater signal variation (up to 20%). Greater variation (5% - 60%) was also seen for small field sizes (4 x 4 cm2 down to 1x1cm2). The IQM showed higher sensitivity to introduced field size errors than to field shift errors for both symmetric and asymmetric fields and also for the two clinical sites. The IQM was not able to detect errors in collimator angle, or MLC shifts, or small gantry angle errors for the two sets of clinical plans considered. The ArcCheck sensitivity was comparable to the IQM for lung SBRT, but better for the nasopharynx VMAT plans.
Conclusion: IQM device is shown as feasible for online monitoring of the consistency of treatment delivery with careful tolerance setting. Care must be taken when using it for plan verification especially for small field sizes.