ESTRO 36 Abstract Book

S281 ESTRO 36 _______________________________________________________________________________________________

3 Azienda USL Toscana Centro- Sede di Firenze, Radiotherapy Unit, Florence, Italy

profiles of the individual beams into account was fitted to the data. The lateral profiles of the WFF beams were assumed to be uniform while those of the FFF beams were approximated using 4th or 6th order polynomials. The scatter functions of the FFF beams were recalculated using a uniform lateral profile (the same as the physical profile of the WFF beams) and are henceforth denoted as virtual uniform FFF beams (VUFFF). The field sizes of the VUFFF beams having the same scatter contribution as the corresponding FFF beams at a given field size were defined as the EQUSFs. Results In total, the data of four different LINACs and 18 different beam energies were analysed. The average values of EQUSFs over all investigated LINACs of the conventional 10 x 10 cm² reference fields of 6 MV and 10 MV FFF beams for C-arm LINACs and machine specific reference fields for CyberKnife and TomoTherapy were 9.5 cm, 9 cm, 5.3 cm and 6.5 cm, respectively. The standard deviation of these averaged EQUSFs was below 0.1 cm. Figure 1 illustrates the process of finding the EQUSFs of a FFF field. The scatter functions of a 10 MV WFF, a 10 MV FFF and a 10 MV VUFFF beam are plotted as a function of field size. In this example the 10 x 10 cm² reference field of the 10 MV FFF beam has the same scatter contribution as the 9 x 9 cm² field of the 10 MV VUFFF beam which was defined as EQUSFs. The scatter function of the 10 MV WFF was plotted for comparison purposes.

Purpose or Objective 2D detector arrays have become the standard device for verification of VMAT dose distributions. The detector pixel size is a key parameter to reproduce complex dose distributions. Aim of this work is to compare the gamma analysis of three ionization chamber systems and to test the ability of each system to detect deliberate errors. Material and Methods Measurements performed by PTW Octavius 4D 729 (5x5x5mm 3 ionization chamber, 10 mm spacing), PTW Octavius 4D 1500 (4.4x4.4x3mm 3 ionization chambers, 7.1 mm spacing) and PTW Octavius 4D 1000 SRS (2.3x 2.3x 0.5 mm 3 liquid filled ionization chamber, 2.5 mm spacing) in the PTW Octavius 4D phantom were used to validate the dosimetric accuracy of the VMAT delivery. Firstly, 50 VMAT SBRT treatment plans from a variety of clinical sites were considered. Secondly, systematic variations in collimator (2° and 5° rotation) and gantry angle (shift of 2° and 5°) and lack of monitor units were applied to four clinical treatments (2 lung tumors, 1 spine and 1 abdominal lymph node) in order to establish the detection sensitivity of the three devices. Measurements were compared with TPS Elekta Monaco computed doses via local gamma analysis ( 2%L/ 2 mm, 2%L/ 1 mm and 1% L/1 mm). For the 729 and 1500 detectors, the resolution was improved by merging two measurements performed with 5 mm couch shift. The threshold for a success in error detectability was established, by using the concept of confidence limit (CL), as suggested by AAPM Task Group 119 [1]: CL = (100 - D) + σ where D and σ are respectively the mean dose and the standard deviation of the distribution of the gamma passing rate (35 plans for 6 MV and 15 plans for 10 MV) measured by each dosimeter; the detectability threshold (DT) has been calculated as: DT = 100 - CL Results The average pass rate with 2%L/2 mm criterion for the 6MV cases was 86.6 ± 5.2 (no shift) and 90.3 ± 4.3 (merged) for 729, 91.5 ± 3.7 (no shift) and 94.7 ± 2.9 (merged) for 1500 and 98.9 ± 1.1 for 1000 SRS. Box plot relative to 6 MV VMAT SBRT 2%L/2 mm pass rate is presented in figure 1. Similar results were achieved for 10 MV plans. The results for the plans with errors, normalized to the DT for a success in error detectability, show that only the SRS system can distinguish the delivery errors, as shown in figure 2 for 6MV. Only a 2%L/1 mm gamma criterion proved to be sensitive enough to detect errors.

Conclusion It has been shown that with the introduction of a VUFFF beam, EQUSFs can be consistently defined for a variety of energies and collimations. These EQUSFs serve as basis for a unified reference dosimetry protocol for all different types of FFF machines. References [1] Fogliata A, Fleckenstein J, Schneider F, et al. Flattening filter free beams from TrueBeam and Versa HD units: Evaluation of the parameters for quality assurance. Med. Phys. 2016;43. [2] Chang Z, Wu Q, Adamson J, et al. Commissioning and dosimetric characteristics of TrueBeam system: Composite data of three TrueBeam machines. Med Phys. 2012;39:6981–7018. OC-0531 The influence of detector resolution on pre- treatment quality assurance in SBRT A. Bruschi 1 , S. Russo 2 , M. Esposito 2 , S. Pini 2 , A. Ghirelli 2 , G. Zatelli 2 , P. Bastiani 3 1 University od Florence, University of Florence, FIrenze, Italy 2 Azienda USL Toscana Centro- Sede di Firenze, Medical Physics Unit, Florence, Italy

Figure 1 : Box plot for 2%L/2 mm gamma passing rate of 6 MV VMAT SBRT plans for the investigated 2D arrays.