ESTRO 36 Abstract Book

S519 ESTRO 36 2017 _______________________________________________________________________________________________

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.

cylinder applicator (Figure 1b) with 5 catheters. Inter- dwell distances of 2 and 5 mm were employed and the experiments performed for source activities between 5 and 10 Ci. The EPID response is proportional to the source activity so it is possible to obtain the activity by sending the source to pre-defined dwell position.

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.

Results 3D Cartesian coordinates can be obtained with 0.2 mm accuracy using a single EPID panel. The panel can clearly identify dwell positions 2 mm apart even with the catheter at 24 cm distance (Figure 1c) from the panel. Absolute coordinates can be obtained by adding reference points (representing the corners of the water phantom) in the treatment plan that can be related with the position of the water phantom over the panel during the experiments. An in-house developed software compares all dwell positions/times against the treatment plan. The software can also monitor the sequence of the treatment identifying the afterloader channel connected to each catheter. Therefore, it is possible to detect catheter misplacements, swapped transfer tube connections, wrong dwell times and/or positions and also verify the source activity. Conclusion This work describes an experimental system that can be implemented in the clinic providing experimental pre- treatment verification that is not currently available. This method provides several advantages when compared against other dosimeters such as films or MOSFETs as it combines a 2D dosimeter, which has an online response. Our system can detect several problems that would be unnoticed during the treatment if only traditional QA is performed. PO-0946 Entropic model for real-time dose calculation: I-125 prostate brachytherapy application. G. Birindelli 1 , J.L. Feugeas 1 , B. Dubroca 1 , J. Caron 1,2 , J. Page 1 , T. Pichard 1 , V. Tikhonchuk 1 , P. Nicolaï 1 1 Centre Lasers Intenses et Applications, Interaction- Fusion par Confinement Inertiel- Astrophysique, Talence, France 2 Institut Bergonié Comprehensive Cancer Center, Department of radiotherapy, Bordeaux, France Purpose or Objective This work proposes a completely new Grid Based Boltzmann Solver (GBBS) conceived for the description of the transport and energy deposition by energetic particles for brachytherapy purposes. Its entropic closure and mathematical formulation allow our code (M 1 ) to calculate the delivered dose with an accuracy comparable to the Monte Carlo (MC) codes with a computational time that is reduced to the order of few seconds without any special processing power requirement.

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