ESTRO 2020 Abstract Book

S901 ESTRO 2020

PTVs) except cord (0.79), esophagus (0.79), brachial plexes (0.62), and cochleas (0.60). The average difference between the same day wCT and the CBCT padded with wCT was -0.11 cGy (0% of planned fraction dose, SD 0.99 cGY); CBCT padded with pCT was -0.08 cGy (0%, SD 0.73 cGy); CBCT without padding was 3.75 cGy (0.25%, SD 2.03 cGy). Although padding with the wCT had a smaller dosimetric difference, time trends were evaluated using the Mann- Kendall test at a significance level of 0.05. None of the tests produced significant p-values, which points to the absence of trends in the dose differences over time between padding with the pCT or wCT. Conclusion DIR can be accurately performed between CT and CBCT. CBCT padding with planning or weekly CT images enables accurate dose calculations over the full FOV. Combined together, these tools enable full FOV dose accumulation over the course of treatment using CBCT to support adaptive RT in HN. PO-1643 Factors associated with image guided radiation therapy image rejection in a multi-site institution B. Traube 1 , M. Khan 2 , R. Kumar 2 , G. WALKER 2 1 University of Arizona, Medicine, Phoenix, USA ; 2 Banner MD Anderson Cancer Center, Radiation Oncology, Gilbert, USA Purpose or Objective The study evaluated the factors that influenced the approval vs rejection of image-guided radiation therapy images (IGRT) in a large practice. Material and Methods The approval and rejection incidents of IGRT recorded within an electronic imaging system was obtained from July 1, 2016-June 30, 2018. Variables included: the attending physician of the patient, the physician reviewing the image, total images reviewed the physician that day, time of day, day of week, treatment site, imaging modality, whether review occurred prior to a holiday, and degree of shift made. Logistic multivariate analysis was performed to determine factors associated with IGRT rejection rate controlling for treatment site and physician variability in approval rates. Results There were 51,797 image records obtained, of which 881 (1.70%) were rejected and 50,916 (98.30%) were approved. Multivariate analysis showed that the following factors were associated with an increased rate of IGRT rejection: images reviewed after regular clinic hours (OR 1.32, p=0.025), by physicians with high rejection rates (OR 3.41, p<0.001), by non-treating physician (OR 1.25, p=0.002), by physician reviewing less number of IGRT images (OR 0.99, p=0.009), by specific anatomical sites and cone beam computed tomography (CBCT) images (OR 1.62, p<0.001) (Table 1).

Conclusion CBCT scans, IGRT reviewed after regular clinic hours, by non-treating physicians, by physicians with less IGRT images to review were associated with increased rates of IGRT rejection. This data can help radiation oncologists know factors that improve the quality of IGRT review. In other words, it appears that more precise volumetric imaging, focused time with minimal distractions, “new eyes” and fewer cases to review were all important in image rejection, along with the disease site and proclivity of the physician. PO-1644 Inter-session variability of 4DMRI image quality after outlier rejection J.K. Veldman 1 , D. Den Boer 2 , A. Bel 2 , Z. Van Kesteren 2 ; 2 Amsterdam UMC - Location AMC, Department of Radiotherapy, Amsterdam, The Netherlands Purpose or Objective For the radiotherapy treatment of tumours in the abdomen, 4DMRI can be used to determine the influence of respiration. Irregularities in respiration may cause artefacts in 4D reconstructions, introducing inaccuracies in treatment. To further improve 4DMRI reconstruction and to be able to handle irregularities in respiration, outlier rejection has been proposed. For radiotherapy purposes, robustness of image quality is an important aspect as, ideally, a single planning image represents the following treatment fractions adequately. Our study aimed to analyze the inter-session variability (ISV) of 4DMRI image quality after outlier rejection (OR), by means of reconstruction quality parameters. Material and Methods Ten healthy volunteers were included, all scanned for two sessions with at least one week in between, each session consisting of three consecutive 4D acquisitions (total of 60 acquisitions). During each acquisition, 11 2D coronal slices were acquired repetitively (60 times) during free- breathing, using a T2W-TSE sequence. Prior to each slice a navigator signal was acquired (position of the diaphragm) and used to bin the 2D slices in ten bins. Subsequently, a 4DMRI was reconstructed using four OR strategies: discarding 5% of the extreme navigator positions whilst minimizing the amplitude range (Min95), phase binning without OR (Phase), amplitude binning without OR (MaxIE) and amplitude binning with thresholds at mean inhale and exhale levels (MeanIE), see Fig. 1A. We compared ISV based on three quality parameters (QP): 1. Intra Bin Variation (IBV): interquartile range of the diaphragm position within the bin-slice