ESTRO 2023 - Abstract Book

S1554

Digital Posters

ESTRO 2023

densities up to titanium. From a CT image, we can obtain the HU of each material, relate it to its electron densities and set up the curves. The analysis is done directly comparing the dose calculated with that measured with IC at the same point. A Semiflex chamber is placed in a dedicated insert of the phantom behind the titanium one, and the dose is measured for a 3x3 cm ² field and 100 MU. Same configuration was used in Monaco TPS in order to calculate the average dose in the chamber volume. Three different cases have been studied for dose calculation. First, a curve without titanium is introduced in the TPS (Figure 1). In this case the TPS will use the highest density entered in the curve as the titanium´s. Second, a CT image with titanium is obtained and the corresponding HU is used. The HU obtained now are lower than expected due to saturation of HU values in the CT scan. Finally, the electron density of titanium is forced to its actual value in the structure of interest.

Figure 1: CT2ED curves.

Results The results obtained for the dose measured with the IC and calculated for different energies, are shown in Table 1.

Table 1: Results

In the first case, without introducing titanium, differences are remarkably high, from 7% to 11%. Higher doses than the real ones are obtained as Monaco is underestimating the attenuation in titanium. Secondly, better results are observed, with differences ranging from 1.4% for 6 MV to 6.6% for 10FFF, however Monaco still calculates wrongly because the density it takes is not the actual one. Finally, when the titanium density is forced, considerably good results are obtained (less than 2%), except for 10FFF energy that we get 5%. Conclusion Monaco Monte Carlo algorithm calculates correctly with titanium if the correct electron density is forced, except for 10FFF, where a larger difference is seen. Further invertigation must be done about that. On the other hand, special care must be taken when entering the CT2ED curve in the TPS, since the problem of HU saturation might be found, using an inaccurate density and then obtaining an inaccurate dose calculation.

PO-1824 Intraoperative kilovoltage dose reduction after placing a hemostatic patch: dosimetric analysis

G.R. Sarria 1 , M. Grimmer 1 , P. Eich 1 , J.A. Holz 1 , F. Roehner 1 , L.C. Schmeel 1 , S. Garbe 1

1 University Hospital Bonn, Radiation Oncology, Bonn, Germany

Purpose or Objective Intraoperative radiotherapy (IORT) of cranial cavities is performed immediately after resection of brain metastases or glioblastoma. Surgical hemostatic patches are usually placed for bleeding control. We measured the depth dose curve (PDD) of the 50-kV intraoperative radiotherapy device INTRABEAM (Carl Zeiss Meditec AG, Oberkochen, Germany) on various surgical patch combinations. Materials and Methods A water phantom and a soft jet ionization chamber in a water tank were used for this purpose. An electrometer gathered information on 10-second charges. Five depth-dose curves were recorded at a 40- µ A beam flow and 50 kV, using a 3.5 cm spherical applicator. The applicator-chamber distance was adjusted in 13 different 1-mm layers and posteriorly displaced in a 3-mm fashion, from a 0.18 mm to a 1.18 mm depth. Two patches from different manufacturers were used for assessing different thickness levels, TachoSil (Corza Medical, MA, US) and Tabotamp (Ethicon, J&J Surgical Techonologies, NJ, US). Five measurements were performed with combined layers: 1) PDD reference curve (applicator only), 2) PDD + 1 TachoSil, 3) PDD + 2 TachoSil, 4) PDD + 2 TachoSil + 1 Tabotamp, 5) PDD + 2 Tabotamp