ESTRO 36 Abstract Book

S805 ESTRO 36 _______________________________________________________________________________________________

EP-1500 Application of RayStretch in clinical cases: Heterogeneity corrections in LDR prostate brachytherapy J. Vijande 1 , F. Ballester 1 , J. Perez-calatayud 2 , F. Hueso- González 3 , F. Siebert 4 1 Universitat de Valencia Dep. de Fisica Atomica- Molecular Y Nuclear, Atomic Molecular and Nuclear Physics, Burjassot, Spain 2 University and Polytechnic Hospital La Fe, Physics Section- Radiotherapy Department, Valencia, Spain 3 Target Systemelektronik GmbH, Wuppertal, Germany 4 UK S-H- Campus Kiel- Klinik für Strahlentherapie, Radioonkologie, kiel, Germany Purpose or Objective Tissue heterogeneities and calcifications have significant impact on the dosimetry of low energy brachytherapy (BT). RayStretch is an analytical algorithm developed in our institution to incorporate heterogeneity corrections in LDR prostate brachytherapy. The aim of this work is to study its application in clinical cases by comparing its predictions with the results obtained with TG-43 and Monte Carlo (MC) simulations. Material and Methods A clinical implant (71 I-125 seeds, 15 needles) from a real patient was considered. On this patient, different volumes with calcifications were considered. Its properties were evaluated in three ways by i) the Treatment planning system (TPS) (TG-43), ii) a MC study using the Penelope2009 code, and iii) RayStretch . To analyse the performance of RayStretch , calcifications located in the prostate lobules covering 11% of the total prostate volume and larger calcifications located in the lobules and underneath the urethra for a total occupied volume of 30% were considered. Three mass densities (1.05, 1.20, and 1.35 g/cm 3 ) were explored for the calcifications. Therefore, 6 different scenarios ranging from small low density calcifications to large high density ones have been discussed. Results DVH and D90 results given by RayStretch agree within 1% with the full MC simulations. Although no effort has been done to improve RayStretch numerical performance, its present implementation is able to evaluate a clinical 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.