Test purpose

The signal linearity test verifies that the signal detected by IQM is directly proportional to the dose output. If all other variables remain constant, the signal should double if the dose output is doubled.

Test method

The signal linearity test uses the reference field size of 10cm x 10cm
(defined by MLC and Jaws) for a 6MV photon beam. The dose output is increased through 8 measurements in the following steps:

2 MU / 5 MU / 10 MU / 20 MU / 50 MU / 100 MU / 200 MU / 500 MU

The coefficient of determination R2 indicates the variation of observed data from the line of best fit.
R2 is defined as:
1 − 𝑆𝑢𝑚 𝑜𝑓 𝑠𝑞𝑢𝑎𝑟𝑒𝑑 𝑟𝑒𝑠𝑖𝑑𝑢𝑎𝑙 / 𝑇𝑜𝑡𝑎𝑙 𝑠𝑢𝑚 𝑜𝑓 𝑠𝑞𝑢𝑎𝑟𝑒𝑠

The signal linearity test passes if the R2 value is 0.999 or better for a linear fit equation (IQM signal vs. delivered MU).

Test measurements were performed on 18 linear accelerators.

Test results

The measured IQM Signal for all doses and the calculated coefficient of determination is listed in Table 1 and diagrammed in the Figure below with the pass criteria (orange line). Rounded to three decimal places, the R2 coefficient of determination is 1.000 for all linear accelerators, at all sites, passing the criteria of  R2 at or better than 0.999.