ESTRO 36 Abstract Book

S518 ESTRO 36 2017 _______________________________________________________________________________________________

Cardenal Carro 2 , J.T. Anchuelo Latorre 2 , M. Ferri Molina 2 , A. Kannemann 2 , D. Guirado 3 , P.J. Prada 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 Purpose or Objective In vivo dosimetry (IVD) applied to HDR BT treatments allows to monitor real dose delivered to clinically relevant areas. MOSFET detectors are the most suitable devices for this task because of their tiny dimensions, which enables their introduction into identical needles to those used in treatments. However, these type of detectors show responses depending on source-to-detector angle and distance. Mathematical models describing these dependences can be obtained from a correct detector characterization. Applying these models on the measurements should minimize the impact of those dependences, improving precision and accuracy. The purpose of this study was to evaluate clinical data of IVD applied to HDR prostate BT using MOSFET TN-502RDM from Best Medical Canada with the Ir-192 Vr2 source contained in Flexitron aferloader (Nucletron-Elekta) and mathematical models describing those dependences obtained in a previous characterization work. Material and Methods Clinical data were taken from five patients suffering from prostate cancer. One to three measuring points were taken for each patient, where the MOSFET were positioned. Anatomical areas measured were neurovascular bundles, bulbourethral area and periurethral area. Nine measuring points were taken and evaluated. Real time ultrasound image guided technique was used to implant the treatment needles. An additional needle was needed for each measuring point. Oncentra Prostate 4.2.2.4. was used to calculate the treatment planning following a standard procedure. Subsequently, coordinates of measuring points and dwell positions were taken as well as dose contribution of each dwell position to each measuring point. After irradiation, mathematical models were applied on measured dose. Table 1 shows the three models considered, their parameters and the goodness of fit. TPS dose, direct MOSFET measured dose and calculated dose after applying the mathematical models on direct MOSFET dose were evaluated.

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 Figure 1 shows the results normalized to TPS d ose. All measurements suffer an approach to TP S dose after applying the mathematical models. The ave rage value of percentage difference between TPS dose and direct measured dose was 15% while the average percentage difference after applying the mathematical models decreases to 9% without any point exceeding 20%. Estimated global uncertainty associated to these corrected measurements were into the range 3.7-4.3%.