ESTRO 2020 Abstract book

S1110 ESTRO 2020

was also investigated by comparing with the patient’s shifts that did not require center couch. In both groups shift from second CBCT and final CBCT were analyzed separately. Two-tailed T-test was performed to evaluate statistical significance. In SRS group patient´s second CBCT and final CBCT shifts were compared to the maximum allowed shift. Results In SBRT group with center couch shifts both second and final CBCT mean shifts were smaller than 1 mm. Standard deviation of shifts is also £ 1 mm. We have found a statistically significant difference between second CBCT vertical and lateral shifts between these fractions where center couch was applied (69) and those where it is not (29). In group where additional immobilization were used all mean shifts were under 1 mm, also standard deviation was £ 1 mm. For second CBCT statistically significant difference was observed for lateral shift between treatment fractions for the patients with customized immobilization (82) and those without (16). In SRS group 176 fraction shifts were analyzed. 25 cases (14,2%) during second CBCT and 49 fractions (27,8%) during final CBCT had shifts larger than tolerance. However, only 3 fraction (1,7%) for second CBCT and 15 (8,5%) for final CBCT had shifts larger than 1 mm or 1⁰. Conclusion Additional immobilization for lung SBRT patients did not improved significantly neither patient position reproducibility, nor position stability during treatment. Although, there was statistical difference between patient positioning with center couch and without, the actual differences were smaller than tolerance. Strict criteria used for SRS treatment resulted in a number of patients being outside of tolerance. Further investigation and analyses is needed. PO-1893 Average body diameter as a predictive factor for the usability of inline CBCT for spine SBRT J. Stam 1 , G. Lim 1 , T. Wiersma 1 , F. Koetsveld 1 1 Netherlands Cancer Institute, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Stereotactic body radiation therapy (SBRT) for spine metastasis needs accurate treatment delivery. Our imaging protocol applies CBCT acquired just before (online), and during treatment delivery (inline) for patient positioning and monitoring of intrafraction motion. If the quality of the inline CBCT appears to be insufficient, an extra CBCT needs to be made between arcs and post- treatment. The aim of this study was to investigate if the average body diameter (AD) of the patient can be used to predict the usability of inline CBCT during SBRT for lumbal and sacral spine metastasis. Material and Methods Retrospectively 20 patients with lumbal or sacral spine metastasis were selected. All patients were treated on an Elekta linear accelerator with 10 MV Flattening Filter Free (FFF) dual arc Volumetric Modulated Arc Therapy (VMAT) technique, which had a minimum rotation of 356 degrees. The patients were positioned in a vacuum fixation (VacFix®) and a thoracic support. For all patients at least one inline CBCT was performed. For each patient the quality of the first inline CBCT is categorized as “sufficient” or “insufficient”. Sufficiency is jugded by the Radiation Therapist Technologists, such that the automatic registration algorithm correctly performs the registration. We defined image quality by the parameter contrast-noise ratio (CNR) of a scan as | µ B - µ S |/ σ S , where µ B is the mean voxel value of the body part of the central vertebra, and µ S and σ S are the mean and the standard deviation of the voxel values in the soft tissue surrounding the central vertebra. We determined up to

which CNR the CBCT was sufficient. The AD was measured on the CT slice in the isocenter. An exponential regression analysis was performed to assess the relation between the image quality and the absorption, i.e. CNR and the AD. We expect an exponential relationship based on the Lambert-Beer law which describes the absorption of radiation through a medium. Based on this analysis we defined a threshold for the usability of inline CBCT, up to a certain value of the AD. Results The AD of the patients varied from 22.7 cm to 34.5 cm (mean 27.8 cm). The CNR varied from 0.6 to 4.9 (mean 2.4). In 5 patients the CNR of the inline CBCT was ≤ 1.3 and categorized as insufficient. The AD for these patients was ≥ 30.6 cm. The result of the exponential regression analysis is shown in Figure 1.

Conclusion The AD of the patient can be used as a predictive factor for inline CBCT quality during SBRT for lumbal and sacral spine metastasis. Based on the exponential AD/CNR relation, we defined a threshold diameter of 32 cm above which the CNR of an inline CBCT scan is too low to correctly perform an automatic registration. PO-1894 AD-HOC adaptive radiotherapy: how often do anatomical changes lead to treatment adaptation? Y. Van Herten 1 , N. Van Wieringen 1 , J. Wiersma 1 , R. De Jong 1 , A. Bel 1 1 Amsterdam UMC - location AMC, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective In our hospital we apply online IGRT protocols based on cone beam CT (CBCT, Elekta) imaging for all indications. Each CBCT is not only used for position verification but also for monitoring anatomical changes which may lead to differences in dose delivery or target coverage. We introduced action levels for anatomical changes for RTTs evaluating the CBCTs. When an action level is exceeded a medical physicist estimates the impact on the dose distribution. The purpose of this study was to determine 1) how often an action level was exceeded and 2) how often this results in a treatment adaption. Material and Methods Data was collected of all anatomical changes exceeding our actions levels between August 2018 and May 2019. Action levels are: body contour changes more than 2cm in general; breast / head and neck more than 1cm (0.5cm for breast boost area) or any change yielding a target position