ESTRO 38 Abstract book

S1079 ESTRO 38

and more than 7% of the time (up to 19% (patient6)) for the 3 others. When intrafraction CBCT was performed, a strong correlation was observed with TP-US. While punctual differences could be explained by the differences in latency between the 2 systems, large prostate rotations, the presence of gas and EM-T location in the prostate are likely to explain more important TP-US is a promising option since it is non-invasive and enables visualization of the target and organs at risk. However the accuracy needs to be evaluated in presence of rotations. EM-T is a reliable technique for monitoring prostate. However location in the prostate volume is of important to be more accurate. EP-1977 Intrafraction motion in CNS radiotherapy with an open mask system using an optical surface imaging D. Reitz 1 , S. Schönecker 1 , M. Pazos 1 , P. Freislederer 1 , M. Reiner 1 , M. Niyazi 1 , U. Ganswindt 2 , C. Belka 1 , S. Corradini 1 1 University Hospital LMU Munich, Department of Radiation Oncology, München, Germany ; 2 University of Innsbruck, Department of Radiation Therapy, Innsbruck, Austria Purpose or Objective Intrafraction motion control is of special interest in modern radiotherapy. Open masks allow for a continuous patient monitoring, reduce claustrophobia and anxiety and improve patient`s compliance compared to closed masks. In this prospective study we evaluated the magnitude of intrafraction motion in patients receiving radiotherapy for CNS tumors with an open mask system. Material and Methods Data of 114 fractions in 10 patients that underwent non- stereotactic radiotherapy for CNS tumors (primary or secondary) were analyzed. During each treatment session patients were monitored using the Catalyst TM optical surface scanner (C-RAD AB, Sweden). Three-dimensional deviations and relative position differences during the whole treatment fraction were calculated and analyzed statistically. Results Overall, the maximum magnitude of the mean deviation vector was 0.62 mm ± 0.62 mm (standard deviation) (95%- CI: [0.07 – 2.2] mm) and a median of 0.35 mm during dose application (beam-on time only). Along the lateral (-0.12 ± 0.61 mm; 95%-CI: [-1.73 – 0.99] mm) and longitudinal (-0.07 ± 0.56 mm; 95%-CI: [-1.38 – 1.09] mm) axis changes were quite similar, while for the vertical axis deviation was tendentially lower (-0.03 ± 0.25 mm; 95%-CI: [-0.46 – 0.53] mm). 99% of the whole beam- on time the magnitude of the deviation vector was < 2.55 mm. The median net beam-on time of radiation therapy was 92 seconds. According to Friedman’s test differences in the distributions between the three possible directions (lateral, longitudinal and vertical) were significant (p<0.01), in Post-Hoc-analysis a dissimilarity between lateral and vertical as well as longitudinal and vertical direction could be verified (p<0.01) whereas between lateral and longitudinal direction a dissimilarity could not be verified (p=1.0). Conclusion Real-time intrafraction motion was < 2.5 millimeters in all directions and open masks can therefore be considered a suitable and reliable treatment option in clinical practice. EP-1978 Surface guided coplanar and non-coplanar stereotactic radiotherapy with open masks – a phantom study E. Konradsson 1,2 , M. Kügele 1,2 , K. Petersson 2 , L. Berg 2 , M. Gebre-Medhin 2 , S. Ceberg 1 differences. Conclusion

and could result in sub-optimal treatment. For a patient with small differences between DIBH scans, no concerning differences in dose due to intra-fraction uncertainty could be identified. EP-1976 Clinical evaluation of two monitoring devices for prostate radiotherapy treatment M. Biston 1 , L. Delcoudert 1 , T. Zaragori 1 , A. Munoz 1 , D. Sarrut 2 , P. Pommier 1 1 Léon Bérard Cancer Center, Radiotherapy, Lyon, France ; 2 Lyon university, Creatis, Lyon, France Purpose or Objective The objective of this work was to co-evaluate 2 in-beam monitoring devices for prostate radiotherapy: intra- prostatic electromagnetic transmitters (EM-T) (RayPilot®, Micropos Medical) and ultrasound imaging using transperineal probe (TP-US) (Clarity®, Elekta). We report the monitoring results obtained with the 2 devices used concomitantly on 9 patients. Material and Methods The accuracy of the 2 systems was first investigated in a phantom study. Then intra-fraction motions measured with the 2 devices used simultaneously were analyzed for 9 intermediate risk prostate cancer patients (155 sessions). They were implanted with the EM-T and 2 fiducial markers 8 days before the simulation CT. Pre- treatment positioning was performed with the TP-US and validated by a Cone Beam CT (CBCT) imaging (+ fiducial markers)/CT registration. During CBCT imaging the 2 devices monitoring mode were started. Irradiation was stopped and patient positioning adjusted for shifts above a threshold of 3mm for at least 15s for both devices. Each time threshold was exceeded a CBCT was performed to confirm the obtained shifts. The percent time the differences between TP-US and EM-T were >1, >1.5, >2, >2.5, >3 and 5mm, was scored for each direction and each patient.

Results Experiments on phantom confirmed the lack of interference between the 2 systems, and showed deviations of less than 0.5mm when translations were applied progressively. In presence of rotations (5 to 15° around left-right (LR) and antero-posterior (AP) axis), each system compensated with systematic shifts up to 6.9 mm for the TP-US and up to 3.8 mm for the EM-T. Mean differences between displacements observed on patients with EM-T and TP-US were ≤0.55mm in all directions except for patient 6 (Table 1). A strong correlation was found in LR direction: differences >2mm were monitored only less than 0.22% of the time for all patients and differences >2.5 mm never exceeded 3s, except for patient 6 (>2.5 mm 9.3% of the time). More important differences were observed in supero-inferior (SI) and AP directions, where more intrafraction displacements occurred. In AP direction they were >2mm less than 3.8% of the time for all patients but patient 6 (26% of the time). They were >2mm less than 1% of the time for 6 patients