Abstract Book

S1289

ESTRO 37

for both groups; on average 766 seconds (12.7 min) for the lung SBRT patients and 738 seconds (12.3 min) for the spinal SBRT patients. A two-tailed t -test with 5% significance level yields p =0.73, indicating that the average treatment delivery times do not differ significantly.

Electronic Poster: RTT track: Patient preparation, positioning and immobilisation

EP-2336 Validation of VacFix for SBRT treatments of spine oligo metastasis L. Wiersema 1 , A. Licup 1 , M. Buijs 1 , A. Van Mourik 1 , G. Borst 1 , P. Remeijer 1 1 Netherlands Cancer Institute, radiotherapy, Amsterdam, The Netherlands Purpose or Objective In stereotactic body radiotherapy treatment (SBRT) of spinal metastasis small PTV margins are preferred allowing high doses to tumor and sparing of the spinal cord. In our institute we use the vacuum fixation (VacFix) to immobilize the patient, reducing the intrafraction motion (IFM). The aim of this study was to validate the positioning accuracy by comparing the IFM of the spinal VacFix to the IFM of patients treated with SBRT for lung lesions (i.e. without VacFix). Material and Methods N=20 patients with VacFix based SBRT for spinal metastasis (n= 64 fractions) and 18 patients treated with SBRT for lung tumors (n= 81 fractions) were selected. The patients were positioned with a thoracic support and VacFix or mattress for spine and lung irradiation, respectively. IGRT was used for these treatments to correct and validate the setup error according to the area specific thresholds. Based on the analysis of the validation (V1)/inline (C1) CBCTs and validation (V2)/ post (PV) CBCTs (see figure 1) the systematic and random error of the IFM were calculated based on the spine position. In case the threshold exceeded the tolerance a correction intervention was performed in between arcs. To calculate the IFM actual interventions were taken into account in the calculations. The IFM excluding the correction intervention was calculated to assess the need of these corrections. The statistical significance for the systematic and random errors were tested using Levene’s test and Mann-Whitney U-test, respectively. To take treatment procedure times into account these were also analyzed (from first CBCT to inline CBCT 2 nd ARC).

Conclusion Although the intrafraction motion is small in the SBRT lung as well as the SBRT spine metastasis patients, there is a small but significant reduction in random errors found in intrafraction motion by using a VacFix. Performing correction intervention in between arcs will reduce the random error even further. EP-2337 Comparison between two frameless immobilization techniques for Brain stereotactic radiosurgery I. Lvovich 1 , O. Kaidar-Person 1 , R. Bar-Deroma 1 , R. Carmi 1 1 Rambam Health Care Campus - Faculty of Medicine, Oncology, Haifa, Israel I. Purpose or Objective Advances in imaging, localization, immobilization and 6 degrees of freedom couches facilitate the delivery of highly conformal radiosurgery for brain lesions. Early reports suggest that clinical outcomes are similar with frameless vs frame-based radiosurgery techniques, therefore patients with benign CNS disease, who were previously treated by surgery or in specialized units using radiosurgery framed-techniques are commonly treated with frameless-techniques allowing for fractionated stereotactic radiation (SRT) . Today, there are a number of frameless immobilization devices for brain SRT. In the current study we evaluated two fixation systems used in our institution: a full thermoplastic mask (FT) and a short thermoplastic mask with a mouthpiece (STM) (Fraxion system by Elekta). Material and Methods All patients treated between January to June 2017 for benign CNS lesions were included in the study. Patients were treated in the Elekta Axesse linear accelerator with the HexaPod table. Parameters evaluated included simulation time, treatment time, and positional accuracy. Treatment and simulation time was defined as the time taken to enter and leave the RT suite. Accuracy in positioning was determined by cone beam CT (CBCT) scans. Three scans were performed: CT1 to determine the correct location of the treatment site and CT2 to verify location following correction using the HexaPod table. Post-treatment scan(CT3) was performed to verify actual site of treatment. Positional accuracy was defined as the difference between the relative positional deviation from the planning CT of CT2 and CT3. To minimize operator influence, the automatic mode that uses bone matching (translational and rotational) of the CBCT system was used in a target area set to brain + A total of 25 patients were included, 12 in the FT group and 13 in the STM group. All were irradiated to a single intracranial lesion. Patient distribution according to number of fractions is shown in Table 1. Mean simulation time was 20 min and 45 min for FT and STM respectively. 0.5cm. Results

Figure 1: Schematic overview for calculations

Results The IFM of the spine in both groups are found to be ≤1 mm for the group mean, systematic and random errors (see table 1). Although Levene’s test for the homogeneity of group variances indicates that the average intrafraction motion between both groups were not significantly different, the difference in the random errors (correction interventions included or excluded) are statistically significant as indicated by the Mann-Whitney U-test. The duration of the treatment procedure was comparable