ESTRO 36 Abstract Book

S411 ESTRO 36 2017 _______________________________________________________________________________________________

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) detectors. A rotational symmetric Gaussian horizontal beam profile and exponential decaying depth dose profile in the vicinity of the pellet was fitted to the measured profiles. Both 6 MV and 6 MV FFF beams were considered. Results The fit of the beam profile in three dimensions was based on two parameters: the variance for the Gaussian profile and the gradient of the depth profile. The parameters in turn were both changing as function of FS. Using the fitted beam profiles, an analytical model was developed for the calculation of volume correction factors k V for given FS (see Table 1). Table 1: Calculated volume correction factors k V , temperature and volume corrected output factors (OF) with SD being one standard deviation (SD) are displayed for the 6 MV and 6 MV FFF beams as function of the field size FS.

Conclusion Volume averaging was found to influence the alanine measurements by up to 6 % for the smallest field size. For a cylindrical detector irradiated along the symmetry axis of the detector, simple analytical expressions of the volume correction factors were obtained. The analytical expression gives valuable insight in the volume correction factor k V as function of field size and the radius of the sensitive volume of the detector. The method presented here would be applicable for other detectors. With a defined geometry of the sensitive volume of the detector relative to the central axis of the beam the volume correction factor can either be calculated analytically or numerically as function of FS. Poster: Physics track: Dose measurement and dose calculation PO-0785 A pencil beam algorithm for protons including magnetic fields effects F. Padilla 1 , H. Fuchs 1 , D. Georg 1 1 Medizinische Universität Wien Medical University of Vienna, Department of Radiation Oncology, Vienna, Austria

Purpose or Objective