ESTRO 36 Abstract Book

S877 ESTRO 36 _______________________________________________________________________________________________

challenged by patient motion during treatment delivery. Movement during treatment can be induced by pain. To prevent movement, individualized vacuum cushions are commonly used immobilization devices in SBRT. This study evaluates the impact of the use of a vacuum cushion on intra-fraction movement in patients during SBRT based on cone beam CT (CBCT) data and the impact of pain on motion. Material and Methods Intra-fraction motion was measured in two groups of patients treated with SBRT: 25 patients with spinal metastases using a vacuum cushion (BlueBAG™, Elekta, Stockholm, Sweden, n= 56 fractions) and 19 patients with lung lesions treated without a vacuum cushion (n= 68 fractions). For the purpose of this study, the comparison group was considered to have a fictive thoracic spine target volume. Intra-fraction motion was assessed by registering the post-treatment CBCT scan to the planning CT, based on the volume of interest around the (fictive) spine metastasis. Translations and rotations were determined based on a CBCT bone density match using the Elekta Medical Systems XVI software. Absolute values of displacements in translations and rotations after each fraction were calculated. Pain at baseline was registered. Treatment time was similar in both groups. Statistical significant differences between the two groups regarding displacements in all directions were tested with Mann- Whitney. This test was also performed for movement in patients with and without pain. Mixed models were used to analyze the differences in movement between two groups because of multiple measurements (i.e. fractions) within patients. Results Significant differences in intra-fraction movement were found in 5 out of 6 directions (Table 1) in favor of the vacuum cushion. Mixed models confirmed a significant difference in movement in the longitudinal and vertical direction and pitch axis between patients treated with and without a vacuum cushion. Intra-fraction movement of ≥ 2mm (employed PTV margin) was less frequent in patients treated with a vacuum cushion (Figure 1). In patients treated with a vacuum cushion no significant differences in movement were observed in painful patients versus patients without pain (Table 1).

dose delivery. In these patients, pain did not influence movement during radiotherapy. EP-1621 Intrafraction errors in cranial radiotherapy with standard VMAT mask: implications for SRS/SRT. F. Azoury 1 , D. Nasr 1 , C. El Khoury 1 , N. Khater 1 , J. Barouky 1 , R. Sayah 1 , N. Farah 1 , S. Achkar 1 , E. Nasr 1 1 Hotel Dieu de France Hospital - Saint Joseph University, Radiation Oncology, Beirut, Lebanon Purpose or Objective Frameless SRS/SRT for intracranial tumors enhances patient comfort. Reproducibility of setup is assured by systematic CBCT with 6D couch correction before treatment delivery; however concerns of intrafraction motion that could limit the use of frameless reinforced VMAT masks in SRS/SRT still remain. Here we study the magnitude of errors in rotations and translations when using standard reinforced VMAT thermoplastic masks. Material and Methods We analyzed 100 fractions of patientswho underwent cranial VMAT onTrueBeam with Perfect Pitch 6DoF couch (Varian Medical Systems). The patients were immobilized with reinforced IMRT Masks (CIVCO) and standard neck support. For patients requiring additional support to the neck personalized Accuform cushions (CIVCO) were used. Initial repositioning using CBCT before every fraction and 6DoF corrections was performed. Post-treatment CBCT repositioning was also performed to assess intra-fractional motion. Positional errors in all six directions were compiled in addition to 3D total vector errors (TVE). Results

Intrafraction translation errors (cm) for the vertical, cranio-caudal and lateral directions were: 0.01 ± 0.03, 0.02 ± 0.05 and -0.01 ± 0.04 respectively (Mean± SD). 3D TVE was 0.07 ± 0.04(Mean± SD). Intrafraction rotational errors for pitch, roll and rotation were 0.13 0 ± 0.33 0 , 0.04 0 ±0.18 0 and -0.03 0 ± 0.35 0 respectively(Mean± SD) (Table.1). Boxplots presented in Figure.1 show a small variability of the TVE with a range of errors when we eliminate outliers of 0.12cm; however 92% of the cases were within 0.1cm deviation. Conclusion The use of personalized single layer masks with custom made Accuform cushions produces stable positioning for use on TrueBeam with Perfect Pitch platforms. Intrafraction motion showed a mean TVE of 0.07 ± 0.04cm. These results suggest that a PTV margin of 0.1cm for SRS cases and 0.2cm for SRT cases is justified to mitigate

Conclusion The use of a vacuum cushion resulted in a clinically relevant decrease of intra-fraction movement in multiple directions during SBRT, which results in a more accurate