ESTRO 37 Abstract book

S946

ESTRO 37

Results Clinical plans covered a wide range of PTV size (average = 129.5 cm 3 , minimum = 9.8 cm 3 , maximum = 327.5 cm 3 ) and dose prescription (average = 11.8 Gy, minimum = 6 Gy, maximum =18 Gy) resulting in 1866 MU on average (minimum = 806 MU, maximum = 3072).Diode array and portal imager QA process gave both gamma values higher than 97% for 2%-2 mm criteria (10% threshold). For plans with errors during delivery, both detectors gave similar results to Eclipse calculation but results varied depending on the lesion size. The large lesion was more sensitive to collimator rotation and dose difference. The small lesion is more sensitive to MLC errors. Conclusion EPI QA process for FFF beams is achievable. It is much faster than diode array process as there is no need for a phantom setup and it provides similar results. Nevertheless regular QA on collimator and gantry position should be performed on the machine as an error on these parameters is not always if not at all detected with EPID. EP-1763 The output factor measurements of CyberKnife system using various detectors C. Koksal 1 , N. Donmez Kesen 1 , U. Akbas 1 , L. Suncak 1 , M. Okutan 1 , H. Bilge 1 1 Istanbul Oncology Institute, Medical Physics, Istanbul, Turkey Purpose or Objective In Stereotactic Radiosurgery, the extremely high radiation dose is delivered to the target in a single or few fractions using narrow radiation fields. The radiation fields are collimated with 12 fixed circular collimators ranging from 5 to 60 mm in diameter in CyberKnife which is a robotic radiosurgery system. The accuracy of the output factors directly affects the accuracy of treatment delivery to the patients. The output factor (OF) measurement of small fields is very problematic and the small field measurements are more sensitive to physical properties of detectors. The aim of the present study is to perform the relative OF measurements of 12 fixed collimators in CyberKnife system using different detectors. Material and Methods In this study, the OF measurements of 12 fixed collimators were made with CyberKnife system at maximum dose depth in water equivalent RW3 slab phantom using PTW 60019 microDiamond, PTW 31014 0.015 cc PinPoint, PTW 31010 0.125 cc Semiflex, EBT3 Gafchromic film and GR-200A TLD. All detectors were positioned at the center of the radiation field with the point laser from treatment head. The measured data were compared with Monte Carlo (MC) factors. Results The relative OF data of 12 fixed collimators for each detector were shown in Figure 1. The OF values measured at 30 mm and greater collimator sizes were found similar for all detectors. The OF values decrease rapidly as the collimator size becomes smaller. The measured data with TLD and microDiamond detector at the smallest collimator sizes such as 7.5, 10, 12.5 and 15 mm were found very similar with MC factors.

Conclusion 0.125 cc Semiflex ion chamber may lead to large uncertainties in the OF measurements for the smallest collimator sizes. TLD and microDiamond detector show similar results for all collimator sizes except 5 mm. TLD is an utilizable dosimetric tool in the small field OF measurements by means of its small active volume. EP-1764 Comparison of COMPASS beam modelling with Monaco using Elekta express QA beams S. Sharma 1 , S. Chander 1 , V. Subramani 1 , P. Kumar 2 , N. Gopishankar 3 , S. Bhaskar 1 , S. Pathy 1 , S. Thulkar 4 , M. Singh 1 , A. Binjola 1 , N. Dhayanethi 1 , P. Kumar 1 1 All India Institute of Medical Sciences, Radiation Oncology, New Delhi, India 2 All India Institute of Medical Sciences, Medical Physics, New Delhi, India 3 All India Institute of Medical Sciences, Neurosurgery, New Delhi, India 4 All India Institute of Medical Sciences, Radio diagnosis, New Delhi, India Purpose or Objective Aim is to compare the beam modelling of COMPASS with Monaco treatment planning system using Elekta express quality assurance (QA) beams. Material and Methods Express QA package contains eight QA fields as follows (i) 10x10 (ii) 20x20 (iii) 3ABUT (iv) DMLC1 (v) HIMRT (vi) HDMLC(vii) 7SegA (viii) Four L. To compare MLC modelling in COMPASS (IBA, Germany) with treatment planning system (Monaco), all the Express QA beams were calculated in water phantom (30cmx30cmx30cm) using Monaco 5.11(Elekta) TPS using 0.5% per control point statistical uncertainty. All the plans (beams) were created for 6MV Versa HD linear accelerator. Then calculated RT-plan RT-structure set and RT-dose were exported to COMPASS in DICOM format to compare TPS and COMPASS (computed and reconstructed) distribution. All express QA beam were measured with COMPASS with I’matriXX detector attached with a gantry holder calibrated for 100.0cm SSD. After calculation and measurement with COMPASS, all measured (reconstructed) and COMPASS computed distribution were compared with 2D(two dimensional) planar evaluation. For planar comparison doses were exported to MY QA (IBA, Germany) software and compared gamma index criteria of 3% dose difference and 3mm distance to agreement with 20% threshold. Results Results are summarized in figure-1(showing gamma comparison results) and Figure-2 (showing the profile and Gamma comparison results of Monaco and COMPASS computed). The Gamma pass rate for express QA beams for Monaco versus COMPASS computed, Monaco versus COMPASS reconstructed & COMPASS computed versus COMPASS reconstructed were as follows: (1) 10x10 - 100.0%, 100.0%, 100.0% (2) 20x20 - 99.3% , 99.8%, 100.0% (3) 3ABUT - 100%, 99.2%, 100.0% (4) DMLC1 -99.3%, 95.5%, 99.9% (5) HIMRT - 98.9%, 99.9%, 99.9% (6) HDMLC -98.5%, 99.8%, 99.3% (7) 7SegA - 100.0%, 98.3%, 98.9% (8) FourL - 93.4%, 98.4%, 94.2% respectively. All beams showed gamma pass rate agreement above 95% except for FourL field. FourL field Monaco versus COMPASS reconstructed gamma pass rate was 98.4% but Monaco versus COMPASS computed gamma pass rate was 93.4%, if we try to adjust the modelling parameters in COMPASS beam modelling window for tuning that further all other beams passing rate goes down therefore we accepted this beam model of COMPASS for secondary dose calculation (of Monaco treatment planning system) and patient specific pre- treatment quality assurance.