Abstract Book

S126

ESTRO 37

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 measuring the breathing curve directly on the patient’s surface, surface scanners serve as ideal surrogates for robust and time efficient 4DCT scanning. As the user is free to select the spot on the patient’s surface, training and prior knowledge of the effects of choosing a region, which is not well correlated with internal tumor motion are vital to this application. There are multiple limitations in the use of optical surface scanning: For SBRT lung treatments for example, relying solely on

surface information can lead to significant errors or uncertainties. So far, no vendor can provide updateable models for internal-external correlation of the patient’s surface and tumor motion, neither from surrogate or linac side. In general, surface guided radiation therapy has the capabilities of providing extra safety, improved accuracy, and giving the possibilities of implementing new radiation therapy techniques, while one must keep in mind, that an extra effort must be made always, such as additional training, QA procedures, and time expenses. SP-0246 Image Guided Adaptive RT: Challenges, pitfalls and opportunities using plan selection in daily clinical practice R. De Jong 1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands Abstract text Target volumes in the pelvic region (cervix, bladder and rectum) are prone to large deformations caused by daily variation in bladder and bowel filling. These deformations cannot be corrected by table shifts nor managed by drinking protocols and/or dietary instructions. Therefore, large population-based margins are necessary to compensate for these deformations. This makes sparing of the organs at risk (OAR) very challenging, even with the use of intensity-modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT). To overcome this problem, our department has implemented plan selection; a strategy using multiple pre-treatment plans tailored to a range of possible shape variations of the target volume that are patient specific. For cervix and bladder a full and empty bladder pre-treatment CT (or MRI) scan are used to generate the target volume structures covering the range of possible shape variations. For the rectum a full bladder CT is acquired and variable PTV margins are applied to create the set of treatment plans. Based on daily Cone Beam CT (CBCT) the best fitting plan is selected for the treatment. With this strategy PTV margins can be reduced significantly and thereby the dose to OARs, resulting in a risk reduction of toxicity without compromising target coverage. Despite these favourable aspects, plan selection has its challenges in daily clinical practice. Although plan selection uses a full and empty bladder CT scan to capture the range of motions of the target volume (cervix and bladder), those two CT scans remain a ‘snapshot’ of that particular moment and may not represent the real motion throughout the treatment. Moreover, selecting the best fitting plan at the treatment machine requires detailed knowledge of the target volume and relies on the image quality of the CBCT. This requires an intensive training of those responsible and may involve a shift in responsibilities for adequate use of plan selection. A clinical (protocolled) workflow of plan selection is thus required. Once a protocolled plan selection workflow is established in the department, it offers opportunities for target regions outside the pelvic region, for example, lung and oesophagus. In these regions deformations and displacement are rather unpredictable. Therefore, population based margins may be insufficient to account for the variation presented by certain patients and these patients may benefit from ad hoc plan selection. In this lecture we will discuss the challenges and pitfalls of protocolled plan selection strategies in daily clinical practice. Additionally, we will demonstrate some ad hoc plan selection strategies outside the pelvic region.