ESTRO 37 Abstract book

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ESTRO 37

potential tool to help the user finding the appropriate tradeoff. SP-0243 A bright future for the PTV?: gating, tracking, on-line re-planning T. Depuydt 1 1 University Hospital Gasthuisberg, Radiation Oncology, Leuven, Belgium Abstract text To move away from the PTV concept, the probabilistic approaches use a-priori available population or patient- specific information on target location uncertainties to provide a level of robustness ensuring tumor coverage and OAR sparing. Today the image information on inter- fraction and intra-fraction anatomical/geometrical difference acquired using devices integrated in treatment units has become of high quality, both in volumetric information (CT, CBCT, MRI) and time-resolved respiratory related motion. Consequently real-time online adaptive radiotherapy could be considered as another route to retire the PTV concept. An overview will be given on the feasibility of that idea and the challenges still to be conquered. Abstract text Image guidance has been an integral part of radiation oncology for many decades. Various kV imaging systems were already integrated on cobalt units in the 1950s and 60s. These systems, however, were not commercially developed for a large market. Alternatively if treatment fields had to be verified the patient was re-sent to the simulator. The mainstay of IGRT in the previous century was based on portal imaging, initially using analogue films and cassettes which had a high work load and were limited to visual comparison with simulator images. In the 80’s electronic portal imaging systems started to be developed. Initially a large number of MU were needed to provide an image of the bony anatomy and treatment field. Later the current amorphous silicon portal image devices were introduced resulting in a large reduction in MU. These 2D digital images now allowed (automatic) registration with digitally reconstructed radiographs (DRR’s) improving the accuracy and efficiency of patient setup. The MV beam only enables the visualization of high contrast objects. Verification was therefore limited to the bony anatomy leaving tumor motion relative to the bony anatomy uncorrected unless fiducial markers were implanted. The introduction of IMRT providing increased conformity, required further improvements in the accuracy of treatment delivery. In the late 90s, the linac integrated CBCT idea was introduced mounting a kV tube and detector orthogonal to the treatment beam. These 3D images provided soft tissue contrast while simultaneously reducing the imaging dose compared to portal imaging. This provided the radiation oncologists with the option of reducing treatment margins and reducing toxicity. 3D MV volume imaging became available around the same time but soft tissue remained poorer than kV imaging. The availability of daily soft- tissue contrast revealed the complexity of the day-to-day changes both in terms of organ motion and anatomical changes as well as treatment response. The next logical step was to develop 4D imaging for tumours and organs at Symposium: IGRT, IGART and SGRT SP-0244 IGRT from 2D to 4D, changing the verification paradigm M. Rossi 1 1 Netherlands Cancer Institute, Radiotherapy Department, Amsterdam, The Netherlands

risk moving with respiration such as the lung, liver and adrenals. 3D imaging of moving tumours results in a blurred image. 4D CBCT provides sharper images, improves registration accuracy of targets moving with large amplitudes and allows verification of both the position and amplitude of the target on a daily basis. Similarly, time resolved planar imaging enables kilo- voltage intrafraction monitoring (KIM) for verification of active motion management such as gating or tracking. The most recent development in IGRT is the integration of an MR scanner with a radiation treatment delivery device. MRI is a versatile imaging modality providing various soft tissue contrasts in 1D, 2D, 3D and 4D. This allows for daily treatment adaptation, which not only corrects for target motion but adapts the treatment plan to potentially account for day-to-day anatomical changes and facilitates advanced motion management strategies. The development in image guided techniques from 2D to 4D has resulted in a tremendous improvement in the accuracy and precision of treatment delivery, allowing reduction of treatment margins for all target areas, thereby reducing toxicity and\or providing for dose escalation. SP-0245 Surface Guided Radiation Therapy: A new reality, pros and cons P. Freislederer 1 1 University Hospital- LMU Munich, Department of Radiation Oncology, Munich, Germany Abstract text Surface Guided Radiation Therapy has made exceptional advances in the last years. There are numerous applications for this technique, including patient positioning & monitoring, Deep-Inspiration Breath-Hold (DIBH), whole brain radiotherapy (WBRT) with open masks and stereotactic radiosurgery (SRS). Also, it can be used for the acquisition of respiratory correlated computed tomography (referred to as 4DCT). For daily patient positioning, optical surface scanning has potential advantages when comparing it to patient positioning based on skin markers, such as an increased accuracy within body regions, where no skin markers are drawn on. But it does suffer from decreased positioning precision in pelvic regions, as registration algorithms cannot be very effective and reliable for tube-like shaped objects. Also, even close to unnoticeable time delays in the calculation of surface scanners during patient positioning could lead to a decreased acceptance of RTTs. When it comes to intrafractional patient monitoring, surface guidance has undisputable advantages: Throughout the whole fraction, the patient surface can be constantly monitored and intrafractional shifts can easily and quickly be observed. Automatic beam hold based on the patient surface also is an important additional safety feature. For DIBH techniques, optical surface scanning does enable fast and reliable treatment during daily clinical routine, while obsoleting the need of invasive procedures, such as spirometry. Currently, most surface scanners use only one single point on the patient surface to perform gated treatments. In a future perspective, a surveillance of the whole treatment region could improve reproducibility and safety. Another application of surface guidance is WBRT using open masks for patients who are not capable of being immobilized due to anxiety, claustrophobia, or various other anatomical or physiological reasons. With exact patient monitoring, the possibility of 6D intrafractional isocenter correction, and automated beam hold, this technique can be useful to this certain patient cohort. Additionally, this technique could be potentially used during cranial SRS treatments for monitoring the patient’s position for non-coplanar treatment fields. Optical surface scanners can also be helpful with the acquisition of 4DCTs. Being capable of