ESTRO 36 Abstract Book

S415 ESTRO 36 _______________________________________________________________________________________________

Results The detector presents dependency on energy that is reflected in the response variation with depth and field size (2.2% under-response for 6 MV, 20x20 cm 2 at 20 cm depth). The anisotropy study shows important deviations: 28% for lateral incidences and 7% for posterior incidence. The detector sensitivity for leaf positioning measurement is 1.8 % per tenth of millimeter in the penumbra. The output factor corresponding to 6 MV and 1x1 cm 2 shows +2% deviation compared with the measurements obtained using a SFD diode and a CC13 gas ionization chamber. The results are normalized to a 5x5 cm 2 . For a 10x10 cm 2 this deviation is -1%. If the energy increases the deviations decrease (+1% for 1x1 cm 2 and -0.5% for 10x10 cm 2 in 10 MV and 15 MV). In the measurement of small field profiles the gamma comparison between measurements with the liquid ionization array and radiographic film shows 100% passing rates with tolerances 1% - 1mm. Several patient treatments have been verified. In table 1 the comparison between the treatment planning system and the array measurement for a particular case is shown. We show differences in gamma passing rates when anisotropy corrections are applied or not. Figure 1 shows one of such comparisons. Conclusion A new detector array is presented for the verification of patient treatments of high complexity. The detector presents a small dependence on energy, which causes a small over-response for the output factors of small fields and an under-response for output factors of large fields. The anisotropy of the device is significant (28% and 7% for lateral and posterior incidences), but can be compensated during treatment verification by using angle-dependent correction factors. The usefulness for the patient treatment verification has been demonstrated by measuring different patient treatments. The results obtained confirm the validity of this array for dose distribution measurements of complex treatments with small fields and high gradients. PO-0783 Planverification in Robotic Stereotactic Radiotherapy with the Delta4-Dosimetry-System W. Baus 1 , G. Altenstein 1 1 Universität zu Köln, Department of Medical Physics, Köln, Germany Purpose or Objective Stereotactic robotic radiotherapy with the CyberKnife (Accuray, Sunnyvale) might not be fluency modulated radiotherapy (IMRT) in the strict sense. However, the technique is comparable in complexity because of a large number of small (5 to 60 mm), highly non-coplanar fields. Therefore, the manufacturer recommends individual plan verification (DQA, Delivery Quality Assurance), though only on a point dose measurement basis. The report of the AAPM task group on robotic radiotherapy (TG 135) [1] advises the use of film. However, because film dosimetry is rather cumbersome in most cases, it foregoes to demand it for every plan. Film dosimetry provides 2D- measurement, but also laborious calibration, fading and non-linear sensitivity. Arrays of dosemeters have the advantage of comparably easy and direct evaluation, though at a distinctively lower resolution. The aim of this work is to investigate the usefulness of an existing dosimetry system based on diode arrays – the Delta4+ Phantom (ScandiDos, Uppsala) – for DQA of the CyberKnife robotic treatment system. Material and Methods Several patient plans with PTVs ranging from 5.6 to 112 cm³ were investigated. The irradiation was performed with a CyberKnife (G4, rel. 9.5, 6 MV photons, no flattening filter), treatment planning system was

Multiplan (rel. 4.5). The Delta4+ dosimetry system consists of a PMMA-cylinder of 22 cm diameter in which two orthogonal silicon diode arrays are housed, adjecent to electrometers. There are 1069 detectors, an inner region with detector spacing of 5 mm (6x6 cm²) and 10 mm spacing in the outer region. The measurements were carried out with software Vers. 2015/10. The measurements were compared to film (Gafchromic EBT3 Film, ISP, Wayne) and a high-resolution ion chamber array (Octavius 1000 SRS, PTW, Freiburg). A speciality of the CyberKnife treatment is the fact that correct image guided positioning – using X-ray opaque markers, e.g. – is mandatory. Therefore, special considerations have to be taken for marker placement. Results Positioning and localisation of the Delta4 was possible and the plan verification could be carried out. The evaluation with the scandidos software produced results with good agreement between plan and measurement (see fig. 1). The evaluation was somewhat compromised by system breakdowns (maybe caused by treatment times of typ.an hour) and the non-complanarity of the plans which prevented the correction for gantry angle usually exploited by the software. The scandidos software allows for a 3-dimensional evaluation of the dose distribution. The lower spacial resolution compared to film or the 1000 SRS seems to be less important, on the other hand. Conclusion In principle, the Delta4 dosimetry system seems to be highly suited for DQA of CyberKnife treatment. However, the manufacturer should improve the system in terms of radiaton resistance and a proper implementation of fiducial markers to make it wholly suitable for Cyberknife DQA. [1] S. Dieterich et al., Report of AAPM TG 135, Med. Phys. 2011 PO-0784 Volume correction factors for alanine dosimetry in small MV photon fields H.L. Riis 1 , S.J. Zimmermann 1 , J. Helt-Hansen 2 , C.E. Andersen 2 1 Odense University Hospital, Department of Oncology, Odense, Denmark 2 Technical University of Denmark, Center for Nuclear Technologies, Roskilde, Denmark Purpose or Objective Alanine is a passive solid-state dosimeter material with potential applications for remote auditing and dosimetry in complex fields or non-reference conditions. Alanine has a highly linear dose response which is essentially independent of dose rate and energy for clinical MV photon beams. Alanine is available as pellets with a 5 mm diameter, and irradiations in flattening filter-free (FFF) beams or other non-uniform beams are therefore subject to volume averaging. In this work, we report on a simple model that can provide volume correction factor for improved output factor measurements in small MV photon beams. Material and Methods The x-ray beam was delivered by an Elekta Versa HD linac with an Agility MLC160 radiation head. Square field sizes (FS) 0.8, 1.0, 1.4, 2.0, 3.0, 4.0, 5.0, 7.0, 10.0, 20.0, 30.0, 40.0 cm were investigated. The data were acquired at SSD=90 cm, depth 10 cm. The alanine pellets were the standard Harwell/NPL type (Ø4.83×2.80 mm). The pellets were placed in water with a latex sleeve to protect against water. A Bruker EMX-micro EPR spectrometer equipped with an EMX X-band high sensitivity resonator was used to read out the dose deposited in the alanine pellets. The horizontal beam profiles were measured using the IBA Dosimetry photon field detector (PFD) for all FSs while depth dose profiles were measured using the PTW microLion (FS < 8 cm) and PTW semiflex (FS > 8 cm)