ESTRO 2023 - Abstract Book

S1506

Digital Posters

ESTRO 2023

Results The registration process and generation of the virtual phantom took most of the overall time, between 15 and 25 minutes, for the patients considered. The dice coefficient was calculated to evaluate the registration, obtaining values between 0.5 and 0.6 for the patients considered (these values mean an acceptable agreement for a whole-body rigid registration). The calculation of out-of-field doses with P3D took less than 5 minutes. Even though cHL targets can present a wide variation depending on the patient, the calculated doses were not much different, being within the uncertainties of the model in the out-of-field area. For example, the mean dose to oesophagus and liver were found in the intervals 20-25 mGy/Gy and 1.5-1.7 mGy/Gy, respectively. Conclusion A successful implementation of the general framework for cHL patients undergoing VMAT was carried out. The system represents a valuable tool for out-of-field dosimetry, providing organ doses within about 30 minutes per patient. DVHs provided by the computation platform can be subsequently used for secondary cancer risk estimations.

PO-1784 A study of the impact of the dose per pulse on two different detectors for patient-specific QA

M.A. Benito Bejarano 1 , F. Gomez Enriquez 2 , F. Saez Beltran 1

1 Complejo Asistencial de Zamora, Medical Physics, Zamora, Spain; 2 Hospital Universitario Marqués de Valdecilla, Medical Physics, Santander, Spain Purpose or Objective The Truebeam linear accelerator (Varian Medical Systems) has two internal modes of operation to generate the modulation of the dose rate during VMAT treatment delivery. The influence of these modes over the dose response on a diode array and an ion chamber has been studied. Materials and Methods In the Truebeam the maximum dose rate of a given energy is achieved by generating a dose pulse for each radiofrequency pulse received from the modulator. In the first mode, the dose rate is modulated by dropping dose pulses for some radiofrequency pulses. In the second mode, in addition, the pulse amplitude is modified. The Delta4 Phantom+ (DPh+, Scandidos AB) is a biplanar diode array with 1069 p-type diodes, distributed in two orthogonal planes, embedded in a PMMA cylinder. The other measurement assembly consists of a PTW Semiflex TM31010 ion chamber (PTW Freiburg GmbH) inserted in a cylindrical solid water phantom. Two arc therapy plans with multiple beams have been created for the 6 MV, 6FFF and 10FFF energies, aiming to force dose rate modulation in each of the two modes. The DPh+ has been used to measure the delivered dose and the pulse repetition frequency (PRF) for each dose rate setting, in both operating modes. Based on these data, the dose per pulse (DPP) has been obtained and employing the formalism described in DIN 6800-2 the saturation factor for the Semiflex chamber has been determined. The dose measurements with both systems have been compared. Results The Truebeam accelerator automatically selected the mode of operation regardless of the nominal rate setting specified in the plan. Thus, the variation of the saturation factor for the Semiflex was less than 0.1% over the entire dose rate range. The effect of DPP on the measured doses was less than 0.6% for the Semiflex chamber and 0.2% for DPh+ in all cases. The Truebeam accelerator modulates the rate in a way that limits DPP fluctuations as much as possible, so on detectors with saturation effects that only depend on the DPP such as the Semiflex, the variation in saturation is negligible. The Delta4 Phantom+ also has no significant dose rate or DPP dependence and thus is very convenient for VMAT patient-specific QA verifications.