ESTRO 37 Abstract book

S205

ESTRO 37

Results The EBT3 calibration curves for the lowest energy beams (HVL=0.12 and 0.816 mm Al) differed by up to 20% from one another, however there was good agreement (to 4%) between the higher energy beams (0.816 to 2.18 mm Al). This indicates that the energy dependence of EBT3 becomes significant for photon beam qualities below 0.816 mm Al (Eeff=21 keV).

derived from flawed BBD. From the 62 BBD, only one was without systematic DDC or OF errors. Most BBD contained DDC or OF errors of up to 5% in a subset of measurements. Five BBD were so flawed that no model could be built and the entire BBD had to be re-measured. Two main sources of errors could be detected. The first kind were detector specific errors like the spectral response of e.g. diodes in depth, the calibration of the detector and other effects. The second kind were setup errors like wrong relative position of measurement, the placement of electrometer in the linac room or the alignment of the water phantom (WP). In several BDD, an over estimation of DDC for large fields/depths could be detected which came from missing water in the WP, resulting in backscatter from the WP carriage. About ¾ of BBD had severe errors that would have compromised patient dose computation. Conclusion Results show that QA of BBD is feasible by MC beam modelling with a high sensitivity. Random and systematic errors can be detected and the overall quality and self- consistency of BBD can be validated with a precision of typically +-0.5% and +-1.0% in worst cases. This leads to a huge improvement of BBD accuracy and by implication to a higher accuracy in patient dose calculation. This method could be a valuable addition to dosimetry audits, because it eliminates very specifically a frequent source of dose computation errors. OC-0406 Validation of the INTRABEAM system dosimetry with ionization chamber and EBT3 film measurements P. Watson 1 , H. Bekerat 2 , P. Papaconstadopoulos 2 , J. Seuntjens 1 1 McGIll University Health Center, Medical Physics Unit, Montreal, Canada 2 Jewish General Hospital, Medical Physics Unit, Montreal, Canada Purpose or Objective The INTRABEAM system (Carl Zeiss Meditech AG) is one of the most frequently used miniature x-ray sources for IORT. In the absence of an absorbed dose to water primary standard for this device, the INTRABEAM is calibrated by the manufacturer. The purpose of this work is to independently validate the dosimetry of the INTRABEAM system using both ionization chamber and EBT3 Gafchromic film measurements of absorbed dose. Material and Methods Ionization chamber and EBT3 film irradiations were performed in a Zeiss INTRABEAM water phantom, positioned from 5 to 30 mm from a submerged INTRABEAM source. The absorbed dose to water measured by a PTW 34013 soft x-ray chamber was calculated by three methods: the method recommended by the water phantom manual ("Zeiss" dose), the manufacturer calibration dose method ("TARGIT" dose), and our dose formalism relying on an MC-calculated chamber conversion factor ("CQ" dose).To position the EBT3 films during irradiation, they were placed in a custom PMMA holder. The dose perturbation due to the presence of the films and film holder was accounted for with an MC- calculated correction factor. Measurements were repeated with three sets of films, including a control film which was submerged but not irradiated. The netOD was calculated for each film in the region corresponding to the position of the ionization chamber. To account for the energy dependence of EBT3 film, a netOD-to-dose calibration curve was created for various beam qualities relevant to the INTRABEAM photon spectrum in water (HVL = 0.12 to 2.18 mm Al) using a Gulmay orthovoltage x-ray unit. The appropriate film calibration curve was found by interpolating to the expected INTRABEAM HVL at each depth in water.

In general, the EBT3 dose measurements agreed with the ionization chamber dose calculated with the Zeiss and CQ methods within uncertainties. However, the TARGIT dose was found to be significantly (30% to 60%) less than the EBT3 dose, and up to 80% less than the CQ dose. This result suggests that the TARGIT dose severely under- estimates the physical dose to water.

Conclusion We have demonstrated with ionization chamber and EBT3 Gafchromic film measurements that the manufacturer reported TARGIT dose of the INTRABEAM system significantly underestimates the absorbed dose to water, with dose differences of 30% to 60% compared to EBT3, and up to 80% compared to CQ-calculated ionization chamber measurements. OC-0407 Real-time dose verification of dynamic MLC tracking using a monolithic 2D silicon diode array M. Duncan 1 , M.K. Newall 1 , V. Caillet 2 , J.T. Booth 2 , M.L.F. Lerch 1 , V. Perevertaylo 3 , A.B. Rosenfeld 1 , M. Petasecca 1 1 University of Wollongong, Centre for Medical Radiation Physics, WOLLONGONG, Australia 2 Northern Sydney Cancer Centre, Radiotherapy, Sydney, Australia 3 Spa Bit, Spa Bit, Ukraine, Ukraine Purpose or Objective Patient motion management is important to avoid misalignment of the tumour and toxicities to healthy tissue during radiotherapy. One method of motion management is real time MLC tracking of the tumour. This technique has been recently applied clinically on a