Initial follow-up of mechanical QC tests for RapidArc Dynamic (RAD)

RapidArc Dynamic (RAD) is a new solution for delivery of radiotherapy introduced by Varian. RAD blends Volumetric Modulated Arc Therapy (VMAT) and Intensity-Modulated Radiation Therapy (IMRT) by pausing gantry rotation at predefined positions and adds a degree of freedom: a dynamic collimator.

Two new tests validate the dynamic collimator's impact during RAD. The UZ Leuven developed picket fence test verifies MLC movement in combination with a rotating collimator. The Pac-Man test (designed by Varian) verifies the synchronous working of various collimator speeds and dose rates.

The RAD picket fence test was performed using radiochromic film. For the analysis, the measured distances between the pickets and a reference jaw were compared to the actual distances. To validate the results, the test was also performed for RapidArc. For the Pac-Man test, acquired using an Electronic Portal Imaging Device (EPID), five regions of interest (ROIs) with varying dose rates and collimator speeds were analyzed. All ROIs were normalized with an open field image, and the percentage deviation against the average of all ROIs ( $\mathrm{Diff}\left(x\right)$ ), as well as the absolute average of all differences ( $\mathrm{Dif}{f}_{\mathrm{Abs}}$ ) was calculated. To validate, the Pac-Man test was also executed using film. Both tests were conducted over 12 weeks and results were compared to thresholds found in literature (AAPM TG-142, MPPG 8.b). To verify stability, control limits were calculated based on AAPM TG-218.

The picket fence test results showed a maximum difference of 0.45 mm (±0.05) and a mean difference of 0.14 mm (± 0.01). For the Pac-Man test, the $\mathrm{Diff}\left(x\right)$ values ranged between -1.31% and 1.02% and the $\mathrm{Dif}{f}_{\mathrm{Abs}}$ value between 0.42% and 0.55%. All results fell within the calculated control limits, indicating stability.

These consistent results, which fall under thresholds set in literature, indicate that RAD plan delivery is mechanically feasible. The two introduced tests will from now on be included in the routine mechanical QC in UZ Leuven.