ESTRO 36 Abstract Book

S795 ESTRO 36 2017 _______________________________________________________________________________________________

implant in a few seconds to the same level of accuracy as a detailed MC calculation. Conclusion RayStretch is a robust method for heterogeneity corrections in prostate BT supported on TG-43 data. Its compatibility with commercial TPSs and its high calculation speed makes it feasible for use in clinical settings for improving treatment quality. It will allow in a second phase of this project, its use during intraoperative ultrasound planning. EP-1501 Field-by-field and composite plan pseudo-3D verification of IMRT techniques with radiochromic film T. Hanušová 1 , I. Horáková 2 , I. Koniarová 2 1 Faculty of Nuclear Sciences and Physical Engineering- Czech Technical University in Prague, Department of Dosimetry and Application of Ionizing Radiation, Prague, Czech Republic 2 National Radiation Protection Institute, Section of medical exposures, Prague, Czech Republic Purpose or Objective The purpose of this study was to compare field-by-field pre-treatment verification of IMRT dose distributions, which is often performed clinically, to a pseudo-3D method that verifies the global plan dose distribution in several transversal, coronal and sagital planes. Material and Methods Sheets of EBT3 film were placed into an IMRT cube phantom into 5 transversal, 5 coronal and 5 sagital planes close to the isocenter. The phantom was irradiated in this setup with six IMRT step-and-shoot treatment plans. These included one head-and-neck case and five pelvic cases. Two of these plans had not previously met the clinical tolerance criteria and were not used for treatment. Dose distributions obtained with film were compared to predicted dose distributions in OmniPro I’mRT software. Gamma pass rates were obtained for 3 %/3 mm criteria. The same IMRT plans were then measured field-by-field with one EBT3 film sheet placed in the isocentric coronal plane in an RW3 slab phantom, with gantry at 0° for all fields. Again, gamma pass rates were obtained. Finally, the results were compared to clinically performed verification. This was done with a PTW seven29 detector placed in the isocentric coronal plane in an RW3 slab phantom and each field of the plan was tested with gantry at 0°. Gamma pass rates for the PTW array measurements were obtained in VeriSoft with the same criteria 3 %/3 mm. Treatment planning was performed in XiO version 4.80 and plan delivery was carried out on a Siemens Artiste linear accelerator at the Thomayer Hospital in Prague. Results EBT3 film gave higher gamma pass rates than the PTW seven29 array for field-by-field measurements for all patients. If all fields of each plan were averaged out, the average gamma score for both film and PTW detector was above the clinical tolerance limit of 90 % for all plans. This was not true for the composite plan measurements with film. While a certain plan met the tolerance limit if measured field-by-field, it could fail to meet the tolerance limit when the global plan dose distribution was measured. Moreover, for a given plan, different gamma score values could be seen with film for the three directions tested. While in some directions the plan met the clinical tolerance limit of a 90 % gamma score, it could fail to meet the limit in others. These findings can be influenced by film directional dependence, but this is supposed to be negligible. Conclusion Field-by-field pre-treatment verification of IMRT dose distributions, both with radiochromic film and an array of ion chambers, gave higher gamma scores than if the global plan dose distribution was measured in a pseudo-3D manner. Field-by-field measurements might be

insufficient to detect potential plan errors. More complex investigations are recommended at least when new IMRT techniques are being established in the clinic. EP-1502 Dosimetric assessment of brass bolus using radiochromic film P. Lonski 1 , L. Walton 2 , N. Anderson 2 , J. Lydon 1 , T. Kron 1 , B. Chesson 2 , R. Prabhakar 1 1 Peter MacCallum Cancer Centre, Physical Sciences, Melbourne, Australia 2 Peter MacCallum Cancer Centre, Radiation Therapy Services, Melbourne, Australia Purpose or Objective Brass bolus is a new type of bolus designed to enhance surface dose in radiotherapy. Manufacturers claim the impact on other radiation beam characteristics is negligible. The main advantage is the mesh-like grid of brass links can conform to complex patient contours which compared to conventional bolus reduces air gaps between bolus and patient skin. This study aims assess the dosimetric impact of brass bolus on surface dose in megavoltage photon beams. Material and Methods Brass bolus (Radiation Products Design Inc) with a nominal thickness of 1.5mm was used on a tissue equivalent slab phantom (RMI solid water). It was used as single layer and folded over in 2 or 4 layers. Radiochromic film (EBT3) was used to assess surface dose and dose variation on the phantom for a 6 MV and 18MV photon beams (Varian 21iX). Surface dose was measured with and without the brass bolus which was placed on top of the film. A photo of the brass is shown in Figure 1 (a) alongside a 50 mm section of steel ruler. A film calibration curve was derived by exposing samples from the same sheet to various known doses under reference conditions. Film was scanned 12 hours post exposure and manually analysed using ImageJ and MS Excel software. Results Surface dose measured using film in the absence of bolus was 20 % of dose at d-max for a 6 MV beam in a 10 cm x 10 cm jaw-defined square field. Surface dose with a single layer of the brass bolus increased to an average of 57 % of dose at d-max (1.5cm). The mesh-like structure of the brass resulted in a dose enhancement pattern which was non-uniform across the film, as shown for a 1 cm x 1 cm square region in Figure 1 (b), which shows the peaks and troughs resulting from the mesh. The maximum dose (peaks) was 62 % and the minimum (troughs) was 53 % of dose at d-max under reference conditions. Increasing the number of layers of bolus increased the surface dose.