ESTRO 2023 - Abstract Book

S1679

Digital Posters

ESTRO 2023

H. Benmakhlouf 1,2 , M. Fager 1 , A. Samadi 3 , M. Gubanski 4,3,2 , M. Ghazal 1,2

1 Karolinska University Hospital, Medical Radiation Physics and Nuclear Medicine, Stockholm, Sweden; 2 Karolinska Institute, Oncology-Pathology, Stockholm, Sweden; 3 Karolinska University Hospital, Neurosurgery, Stockholm, Sweden; 4 Karolinska University Hospital, Radiotherapy, Stockholm, Sweden Purpose or Objective In Gamma Knife RS, the target coordinates are based on the location of the fixation system relative to the skull and is established by a pre-treatment CT. Any movement of the fixation system relative to the skull after the CT will translocate the target relative to the coordinate system which will result in a shift of the whole treatment dose distribution if not corrected for. The new Vantage system (mainly consisting of a stereotactic frame) was recently introduced in our center as one of the first centers globally. With this frame it is possible to track the motion of a marker placed on the nose. The purpose of this work is to evaluate the geometric stability of the new frame in the time period between the initial stereotactic CT-image and post-treatment on-board CBCT in patients undergoing GK treatments. Materials and Methods Stereotactic CT-images and pre-treatment on-board CBCT (CBCT1) were acquired for the first 255 patients treated with the new system. Post-treatment CBCT (CBCT2) images were in addition acquired for 144 patients. For 90 of these patients, intra-fraction tracking of the nose marker was registered. The shift of the target coordinate relative to that established by the stereotactic CT was determined by co-registering CT with CBCT1 and CBCT1 with CBCT2. An in-house matlab code was developed to determine the translation of the target using the co-registration transformation matrix data found in the LGP files. Results Figure 1 and 2 shows the target translation vector from CT to CBCT1 and CBCT1 to CBCT2, respectively. The former represents the pre-fraction movement of the target whereas the latter represents intra-fraction movement. The translation vector is up to 1.09mm with a mean of 0.31mm in figure 1 whereas it is up to 0.43mm with a mean of 0.09mm in figure 2. This confirm that the stability of the new system is approximately 3.5 times lower intra-fraction compared to pre-fraction (that this, the time between the CT and pre-fraction CBCT). This can be attributed to the difference in setup between CT and CBCT, larger uncertainties in inter-modality co-registration and to differences in the elapsed time between CT-CBC1 and CBCT1-CBCT2 where the former is usually much longer. The largest intra-fraction motion of the nose marker was found in the cranio-caudal direction with a maximum radial shift of 0.51mm and average 0.1mm. This data did not correlate well with nose movement found when comparing CBCT1 to CBCT2 and is thought to be due to facial expression and mimic.