Development of cylindrical stepwedge phantom for routine quality controls of a helical tomotherapy machine

The aim of this study was to design a cylindrical stepwedge phantom and an appropriate treatment procedure, based on which parameters of tomotherapy machine and generated beam of radiation will be defined. The accuracy of parameter determination, which can be defined with the aid of the measurement system, was also evaluated. The cylindrical phantom that we developed and manufactured (stepwedge phantom) consists of four cylinders with different diameters made of polycaprolactam-PA-6, i.e. material with high mechanical strength, low water absorption (making measurements repeatable) and a density comparable to that of human soft tissues. The appropriate treatment procedure is carried out in a dynamic mode, which is focused on specific properties of the tomotherapy machine. It means that a phantom situated on the couch moves to the inside of the rotating linear accelerator. A total of 18 procedures were implemented in order to calculate the following parameters: couch velocity, dose rate value at a depth, Dose Ratio coefficients, dose variation (so-called Dose Flatness) coefficients, and the time of gantry rotation. Reference intervals for these parameters were determined to be as follows: for the couch velocity: ±1.2%, the average dose rate measured at depth: ±1.8%, the calculated values of the coefficients Dose Ratio: ±0.5% and Dose Flatness: (0.53-0.65)%, the time of gantry rotation: ±3%. The final results showed that during a single irradiation procedure, which lasts 5 min, the cylindrical stepwedge phantom allows to precisely determine the values of the above-mentioned parameters. Its use in the daily dosimetric measurements can ensure better control of the work of the tomotherapy machine.