ESTRO 36 Abstract Book

S523 ESTRO 36 _______________________________________________________________________________________________

distinguish two regions between flap and film. In one region, the film is at a distance of 5 mm from the applicator, and in the other region at a distance of 7 and 9 mm (5mm of PMMA plus 2 or 4mm air gap respectively).Two different treatment plans have been designed, in the first one the source stops in the center of the spheres and in the other one at the edge, to compare the difference between dwell positions. The dwell times are set to get the dose distribution as uniform as possible, prescribing 6 Gy at a depth of 5 mm.

Conclusion The application of mathematical models describing the significant dependences of MOSFET TN-502RDM on their measurements results in an accuracy increase besides an improvement in precision. However, IVD implementation in HDR prostate BT treatments as a possibility of real-time decision making related to an error detection needs a retrospective evaluation of a larger sample data to define correctly these error detection thresholds. PO-0944 Dosimetric influence produced by the presence of an air gap between the skin and the freiburg flap M. Fernandez Montes 1 , S. Ruíz Arrebola 1 , R. Fabregat orrás 1 , E. Rodríguez Serafín 1 , J.A. Vázquez Rodríguez 1 , M.T. Pacheco Baldor 1 , N. Ferreiros Vázquez 1 , M.A. Mendiguren Santiago 1 , J.I. Raba Díez 1 , M.M. Fernández Macho 1 , J.T. Anchuelo Latorre 2 , M. Ferri Molina 2 , A. García Blanco 2 , I. Díaz de Cerio 2 , M.A. Cobo Belmonte 2 , A. Kannemann 2 , J. Andreescu Yagüe 2 , M. Arangüena Peñacoba 2 , N. Sierrasesumaga Martín 2 , D. Guirado llorente 3 , I. Bernat Piña 4 , P.J. Prada Gómez 2 1 Hospital Universitario Marqués de Valdecilla, RADIOPHYSICS, Santander, Spain 2 Hospital Universitario Marqués de Valdecilla, Radiation ONCOLOGY, Santander, Spain 3 Hospital Universitario San Cecilio, Radiophysics, Granada, Spain 4 Hospital Universitario Marqués de Valdecilla, Medical Oncology, Santander, Spain Purpose or Objective Surface applicators were proposed as a way to treat superficial lesions with HDR brachytherapy. The Freiburg Flap (FF) is an applicator used in this type of treatment that has limited flexibility, so that in certain situations it is not perfectly adapted to the surface treatment. The purpose of this study is to quantify the discrepancy in the TPS dose calculation produced by unsuitable positioning of the applicator, as opposed to the ideal situation, when the applicator is perfectly adapted to the patient's skin leaving no air gap. Material and Methods Nucletron FF, is an applicator comprising of silicone spheres attached to each other, 1 cm in diameter, arranged in parallel rows, capable of adapting to the surface to be treated. The TPS Brachy Nucletron Oncentra (Elekta, v-4.5.2) was used for dose calculation using an 192 Ir radiation source and radiochromic film (Gafchromic EBT3) have been used for dose measures which were subsequently analyzed with ImageJ To quantify the discrepancy between the TPS dose calculation and the real administrated dose when adaptation to the surface is not suitable, the experimental setup designed shown in figure 1 was made, where we can

Results Results obtained are shown in table 1. Underdosage is observed, produced by air layers, ranging from 4.8% to 10.8% when dwell positions are at the center of the spheres, and from 6.2% to 11.8% when dwell positions are at the edge of the spheres, with 2 and 4 mm air gap respectively.

Conclusion In view of the results obtained, it can be concluded that several layers of air between the applicator flap and the skin can lead to considerable variation in dosimetry, which may involve the loss of effectiveness of treatments with this type of applicators. Thus, utmost care is required during the placement of the flap to minimize the error due to the air gap, therefore avoiding an underdosage in the volume to be treated. PO-0945 Pretreatment verification for brachytherapy G. Fonseca 1 , M. Podesta 1 , M. Bellezzo 1 , B. Reniers 2 , F. Verhaegen 1 1 Maastro Clinic, Physics, Maastricht, The Netherlands 2 University of Hasselt, NuTeC, Hasselt, Belgium Purpose or Objective Individual plan QA is not performed in brachytherapy mostly due to the large uncertainty associated with dose measurements. Traditional setups require precise and accurate positioning, and therefore usually laborious procedures to detect anything other than large discrepancies with an unclear distinction between source or detector mispositioning. This study evaluates the use of an Electronic Portal Imaging (EPID) to verify the treatment plan. Material and Methods The EPID panel response was characterized with an High Dose Rate (HDR) Ir-192 source. A robotic arm was employed for positioning within a water tank (Figure 1a) assuring 0.2 mm accuracy during the calibration, which covered a clinically relevant range for the distance between the source and the panel (from 6 up to 25 cm). Experiments were performed with an acquisition rate of 6.7 fps for a single catheter and for a gynecological