ESTRO 36 Abstract Book

S71 ESTRO 36 2017 _______________________________________________________________________________________________

Material and Methods 8 LAICs (sensitive area with nominal diameter of 81.6mm) were investigated in this study, 4 of type PTW-34070 (LAIC Thick ) and 4 of type PTW-34080 (LAIC Thin ) with water- equivalent entrance window thicknesses of 4mm and 0.7mm, respectively. Measurements were performed in an X-ray unit (YXLON) using peak voltages of 100-200kVp and a collimated beam of 3.1mm FWHM. The LAICs were mounted on the moving mechanism of an MP3-P (PTW) and moved with a step size of 5mm to measure the chamber’s response at lateral positions. To account for beam positions where only a fraction of the beam overlapped with the sensitive area of the LAIC, a corrected response was calculated as the basis for determining relative response as a function of radial distance from the centre. The impact of a heterogeneous LAIC response, based on the obtained response maps was henceforth investigated for small field photon beams (as small as 6x6mm²) and proton pencil beams (FWHM=8mm). Results A pronounced heterogeneity of the spatial responses was observed in both the thick and thin window LAICs. These heterogeneities could be calculated as a function of the radial coordinate as there was no pronounced angular dependency. All 4 LAIC Thick followed a monotonously increasing response towards the chamber centre, while the absolute response values varied up to 1.5%, excluding the 2mm borders of the LAICs. In contrast the LAIC Thin trends were not uniform and responses varied by up to 10% (Fig 1). Investigating absolute dosimetry for a proton pencil beam the signal varies with a systematic offset between 2.4% and 4.1% for LAIC Thick and between -9.5% and 9.4% for LAIC Thin . For relative dosimetry (e.g. depth-dose profiles) the increase of beam size with increasing depth was investigated as the influencing factor. Systematic response variation by 0.4% and 1% at the most were found for the investigated LAICs. The systematic offset for absolute dose measurements for decreasing photon field size showed that for 6x6mm² field sizes the response was systematically 2.5-4.5% higher for LAIC Thick . For LAIC Thin the response varies even over a range of 20%. The entrance window thickness was evaluated to be constant within measurement uncertainty by performing measurement at multiple peak voltages.

is recommended 48- 60 Gy in three fractions for lesions with a diameter ≤3 cm, while for lesions with a diameter >3 cm a higher prescription dose, such as 60- 75 Gy is necessary to obtain similar local control [5]. 3. Spine The goal of spinal SBRT is local control and pain control. Several authors have reported that the 1-year local control rate ranges 80- 98% and provides pain relief. Therefore, several dose/fractionation schedules, such as 24 Gy in 1 fraction or 27 or 30 Gy in 3 fractions have been used and the optimal dose/fraction schedule is still unclear. 2) Comparison between surgery and SBRT for extracranial oligometastases According to several guidelines, surgery for extracranial oligometastases is still standard practice because of lack of evidence that SBRT has clinical advantages. A retrospective analysis comparing surgery with SBRT for 110 patients with pulmonary oligometastases demonstrated that 3-years overall survival rates were 62% for surgery and 60% for SBRT (p = 0.43) [6]. The authors concluded survival after surgery was not better than after SBRT although SBRT should be the second choice after surgery. However, no randomized trials have been conducted, and prospective randomized studies are required to define the effectiveness of each modality. 3) Cost-effectiveness Extracranial oligometastases have been usually managed with systemic therapy with or without surgery. However, systemic therapy, including molecular targeted drugs, is expensive. A cost-effectiveness analysis using a Markov modelling approach demonstrated that video-assisted thoracic surgery wedge resection or SBRT could be cost- effective in selected patients with pulmonary oligometastases [7]. Increases in medical expenses are a social problem worldwide, but it can be said that SBRT is a promising modality in this aspect. (References) [ 1] Lewis SL, Porceddu S, Nakamura N, et al. Am J Clin Oncol 2015. [2] Shultz DB, Filippi AR, Thariat J, et al. J Thorac Oncol 2014; 9: 1426-1433. [3] Ashworth A, Rodrigues G, Boldt G, et al. Lung Cancer 2013; 82: 197-203. [4] Binkley MS, Trakul N, Jacobs LS, et al. IJROBP 2015; 92:1044-1052. [5] Scorsetti M, Clerici E and Comito T. J Gastrointestes Oncol 2014; 5: 190-197. [6] Widder J, Klinkenberg TJ, Ubbels JF, et al. Radiother Oncol 2013; 107: 409-413. [7] Lester-Coll NH, Rutter CE, Bledsoe TJ, et al. IJROBP 2016; 95: 663- 672. OC-0149 Lateral response heterogeneity of Bragg peak ion chambers for narrow-beam photon &proton dosimetry P. Kuess 1 , T. Böhlen 2 , W. Lechner 1 , A. Elia 2 , D. Georg 1 , H. Palmans 2 1 Medizinische Universität Wien Medical University of Vienna, Department of Radiation Oncology and Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria 2 EBG MedAustron GmbH, Medical Physics, Wiener Neustadt, Austria Purpose or Objective A large area ionization chamber (LAIC) can be used to measure output factors of narrow beams. In principle, dose area product measurements are an alternative to central-axis point dose measurements. Using an LAIC requires detailed information on the uniformity of the signal response across its sensitive area. Proffered Papers: Best of particles