Page 55 - Fisica In Medicina n° 1/2017

^Äëíê~Åí=cáëáÅá= =The influence of detector resolution on pre- treatment quality assurance in SBRTA. BRUSCHI1, S. RUSSO2, M. ESPOSITO2, S. PINI2, A. GHIRELLI2, G. ZATELLI21 University of Florence2 Medical Physics Unit, Azienda Sanitaria di Firenze, FlorenceIntroduction2D 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.Materials and MethodsMeasurements performed by PTW Octavius 4D 729 (5x5x5mm3 ionization chamber, 10 mm spacing), 1500 (4.4x4.4x3mm3 ionization chambers, 7.1 mm spacing) and 1000 SRS (2.3x 2.3x 0.5 mm3 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)+σ46