[P219] Analysis of the dose-rate dependence of the ArcCHECK device

Purpose The ArcCHECK device (Sun Nuclear) is a diode array designed to verify IMRT and VMAT treatments. During its commissioning, a dose-rate dependence higher than the specified in the manual (±1% over the range 50–1400 cGy/min) was observed. This may be relevant in VMAT verifications, as the dose-rate varies. Therefore, some tests were performed to study the dose-rate dependence of the ArcCHECK device and to estimate its relevance in VMAT verifications. Methods The ArcCHECK device was irradiated using beams of 6 MV provided by an Elekta Infinity LINAC. The measures were corrected with the measurements of a cc13 ionization chamber (Iba). To determine the dose-rate dependence, the device was irradiated with a sequence of beams of a field size of 5 × 25 cm varying its gantry angle by 4° until the whole device is covered, receiving all the detectors the same dose. Dose-rates over a range of 50–600 MU/min were employed. Then, the measurements of each diode were fitted with the dose-rates by an adjustment function. To study the influence of having different dose-rates in the same treatment, the device was divided in two sectors, one of 280° irradiated with a dose-rate of 74 MU/min and another of 80° with 597 MU/min. The response was compared with the value predicted by the adjustment function. Results It was found a higher response of the system as the dose-rate increased, having a difference of 2.4 ± 0.7% (k = 2) between the responses for 50 and 600 MU/min. The measurements was adjusted by a function in the form: D = a · dD dt b + dD dt c D is the response of the device, dD dt the dose-rate and a , b and c parameters. The best fit is achieved with these values: a = 97.36 ( 96 ) , b = 0.7 ( 2 ) , and c = 0.9996 ( 2 ) , with χ 2 / v = 0.25 . The measurement in the second test was 0.7 ± 0.4% (k = 2) lower than if the irradiation were entirely at 600MU/min, indicating that is necessary to correct the values in VMAT verifications. Conclusions The dose-rate dependence of the ArcCHECK device is higher than the specified and the measures must be corrected when performing VMAT verifications. Further tests using real VMAT treatments shall be done to do a better estimation of the influence of this dependence.