ESTRO 2022 - Abstract Book

S1371

Abstract book

ESTRO 2022

directly through the surrogate, RB and GDL were found to be equivalent up to 4.5mm and 2.2mm water, respectively. By comparing dose distributions in the TPS, DSC was found to be 0.989 and 0.997 for GDL and 0.897 and 0.954 for RB for a static field and a VMAT plan, respectively. The RB and the GDL could be tracked in ranges [330˚, 60˚] and [330˚, 15˚], [- 18˚, 18˚] and [-18°, +3˚], [-18˚, 18˚] and [-18˚, 11˚] in yaw, pitch and roll, respectively.

Figure 2: Film measurements. (A) Profiles for GDL, RB and MRB at the entrance dose of the beam compared to no surrogate (NS). (B) The position of the chosen profiles below the three surrogates. Conclusion Gating Device L was found to have a smaller dosimetric footprint than the Reflector Block. GDL had a lesser impact on dose distributions compared to RB. While GDL was trackable in a smaller volume of space, tracking was identical for GDL and RB in a clinically relevant range. Gating Device L is therefore a promising alternative to Varian’s Reflector Block.

PO-1590 The effect of high density material in breast expanders on the dose distribution

L. Paelinck 1 , C. Monten 1 , C. De Wagter 1 , Y. Lievens 1

1 University Hospital Ghent, Radiotherapy, Ghent, Belgium

Purpose or Objective A breast expander is a temporal prosthesis implanted in the breast during mastectomy and consists of a magnet, injection port and an expansion envelope. The high densities of the magnet and injection port have an influence on the image quality and the accuracy of dose calculations. The purpose of this study was to investigate which density overrides are appropriate to use in our TPS Raystation 6 (RaySearch, Stockholm, Sweden) by comparing calculations and radiochromic film (Gafchromic, Ashland Specialty Ingredients, USA) measurements. Materials and Methods A schematic representation of the measurement setup through a (non-isocentric) transversal plane is shown in figure 1. The breast expander with a filled expansion envelope was immersed in a polystyrene box filled with water and placed on the top of a slabbed polystyrene phantom. Additional polystyrene plates were placed next to the water filled box. This allows the placement of radiochromic films in a coronal and sagittal plane just under and next to the box. Two beam setups, G = 0° and G = 90°, were used. The beam size and film size fitted the dimensions of the box in the coronal and sagittal measurement planes. Each time 200MU was delivered. A calibration curve was measured and the film analysis was performed by in-house made software. A CT scan (Aquilion, Toshiba Medical Systems, Tokyo, Japan) of the phantom setup was made and imported in the TPS. The contouring of the magnet and port were respectively based on a L/W of 6000/10000 and 2000/1000. Four density override configurations were investigated: 1) magnet and port on titanium, rest of the phantom on water, 2) magnet on titanium, port and rest of the phantom on water, 3) no override on magnet and port, rest of the phantom on water and 4) magnet on titanium, port on aluminum, rest of the phantom on water. In all cases, the predefined materials in RayStation ‘water’, ‘titanium’ and ‘aluminum 2’ were used. All calculations were performed with a collapsed cone algorithm. Results In figure 1 an image of the measured dose in both orientations is shown. The attenuation caused by the magnet and port is clearly visible on both films. In table 1 the results of the calculations and measurements in the homogenous region a few cm away from the magnet and port and just downstream the magnet and port are displayed in absolute dose values (cGy). The maximum measured attenuation caused by the magnet in the sagittal and coronal measurement plane is respectively 20% and 15%. The case in which magnet and port are overridden with density titanium and the rest of the phantom with water is in best agreement with the calculations for both measurement orientations.

Conclusion