Test purpose

Evaluation of the IQM beam monitoring performance for the accuracy and reproducibility of LINAC beam delivery, and those of beam monitoring performance by the IQM system to define a segment wise tolerance band.

Test method

32 Head & Neck VMAT test fields (with 110 segments each) were calculated by the IQM CALC application, then delivered by a TrueBeam LINAC  for 15 fractions over a course of 50 days.

Assessment of LINAC’s beam delivery reproducibility, and accuracy of the delivered MU, and start/ end segment gantry angle for each segment was done by direct comparison of Trajectory Log Files (TLF) with corresponding planned parameters.
Similarly, gantry angles determined by the IQM accelerometer were compared to those from the trajectory log files (TLF).

Test results

A total of 52800 segments were delivered and measured. TLF analysis revealed the accuracy in LINAC’s VMAT segment boundary detection and MU delivery were within 0.3 degrees and 0.2 MU respectively.

The deviations were found to be highly reproducible, reflecting on stability of the LINAC & limitations in LINAC’s control system. The IQM accelerometer rotational accuracy was found to be within 0.4 degrees.


The variations in segment-wise beam delivery by the LINAC, and in monitoring by IQM, should be considered when performing calculation versus measurement analysis.
The calculation model performance was found to be satisfactory for Head & Neck VMAT beam monitoring.