Therapeutic radiation beam output and energy variation across clinics, technologies, and time.

Over the past several decades, a medical physics service group covering 35 clinical sites has provided routine monthly output and energy quality assurance for over 75 linear accelerators. Based on the geographical spread of these clinics and the large number of physicists involved in data acquisition, a systematic calibration procedure was established to ensure uniformity. A consistent measurement geometry and data collection technique is used across all machines for every calendar month, using a standardized set of acrylic slabs. Charge readings in acrylic phantoms are linked to AAPM's TG-51 formalism via a parameter denoted kacrylic , used to convert raw charge readings to machine output values. Statistical analyses of energy ratios and kacrylic values are presented. Employing the kacrylic concept with a uniform measurement geometry of similar acrylic blocks was found to be a reproducible and simple way of referencing a calibration completed in water under reference conditions and comparing to other machines, with the ability to alert physicists of outliers.

VMAT testing for an Elekta accelerator.

Volumetric-modulated arc therapy (VMAT) has been shown to be able to deliver plans equivalent to intensity-modulated radiation therapy (IMRT) in a fraction of the treatment time. This improvement is important for patient immobilization/localization compliance due to comfort and treatment duration, as well as patient throughput. Previous authors have suggested commissioning methods for this modality. Here, we extend the methods reported for the Varian RapidArc system (which tested individual system components) to the Elekta linear accelerator, using custom files built using the Elekta iComCAT software. We also extend the method reported for VMAT commissioning of the Elekta accelerator by verifying maximum values of parameters (gantry speed, multileaf collimator (MLC) speed, and backup jaw speed), investigating: 1) beam profiles as a function of dose rate during an arc, 2) over/under dosing due to MLC reversals, and 3) over/under dosing at changing dose rate junctions. Equations for construction of the iComCAT files are given. Results indicate that the beam profile for lower dose rates varies less than 3% from that of the maximum dose rate, with no difference during an arc. The gantry, MLC, and backup jaw maximum speed are internally consistent. The monitor unit chamber is stable over the MUs and gantry movement conditions expected. MLC movement and position during VMAT delivery are within IMRT tolerances. Dose rate, gantry speed, and MLC speed are accurately controlled. Over/under dosing at junctions of MLC reversals or dose rate changes are within clinical acceptability.