ESTRO 38 Abstract book

S355 ESTRO 38

parts of the radiotherapy workflow, including delineation of organs at risk (OAR), treatment planning and reference images for patient positioning at treatment. For prostate cancer patients, MRI is optimal to differentiate between the prostate and surrounding soft tissues and OAR. To use the MRI for target definition requires a registration between the MRI and CT, often performed based on patient anatomy or fiducial markers in the prostate. This registration between two different image modalities are however associated with registration uncertainty. To avoid these registration uncertainties, which may occur when combining modalities, a radiotherapy workflow with MRI as the single modality has been proposed – called MRI- only. The reduction of uncertainty is especially important when radiotherapy is moving towards more precise techniques for treatment delivery and shrinking margins. An MRI-only radiotherapy workflow has great value for the patient, reducing the number of imaging sessions. To the clinic an efficient and possibly cost-reduced workflow are offered with simplified logistics. To use MR as the single imaging modality in the radiotherapy workflow introduces technical obstacles. The systems used today in radiotherapy are traditionally based on CT-information and tasks such as treatment planning and patient positioning are not a straightforward procedure using MRI. Building an MRI-only workflow includes several changes of the current practice, where new tasks and new methods for quality assurance are needed. This includes quality assurance of the MRI for the purpose of treatment planning, generation of electron density information from the MRI referred to a synthetic CT or a pseudo CT to be used for treatment planning, identification of fiducial markers in the MRI and patient positioning strategies based on the MRI or synthetic CT information etc. Training and education plays also an important role of the implementation of new techniques and the introduction of new methods need to be performed in a safe way. The clinical implementation of MRI-only has been limited to a few centers, but recently the interest has increased and new clinics have started to adapt their own radiotherapy workflows to MRI-only. The clinical implementations covers both commercial solutions as well as in-house developed methods. SP-0677 Oligometastatic Prostate SBRT: The How, What, Where and When J. Martin 1 , P. Blanchard 2 , P. Greer 1 , P. Keall 3 , D. Pryor 4 , M. Sidhom 5 , S. Siva 6 , S. Supiot 7 , S. Turner 8 1 Calvary Mater Newcastle, Radiation Oncology, Newcastle- Nsw, Australia ; 2 Institute Gustave Roussy, Radiation Oncology, Villejuif, France ; 3 University Of Sydney, Acrf Image X Institute, Sydney, Australiao; 4 princess Alexandra Hospital, Radiation Oncology, Brisbane, Australia ; 5 Liverpool Hospital, Radiation Oncology, Sydney, Australia ; 6 peter Maccallum Cancer Centre, Radiation Oncology, Melbourne, Australia; 7 Institut De Cancérologie De L’ouest René-Gauducheau, Radiotherapie, Nantes, France; 8 Westmead Hospital, Radiation Oncology, Sydney, Australia Abstract text Oligometastatic prostate cancer is a variably defined target, and an area of growing interest. One key question is how best to define this state, as prognosis will be dependent on the imaging modality used (conventional v functional), the tissues involved (nodal v bone v visceral), tempo of disease and the number of lesions detected (1 v 3 v 5 v more). Timing is also critical, given that the oligometastatic state can be detected either at diagnosis or following local treatment. There are options to Joint Symposium: ESTRO-RANZCR: Radiotherapeutical management of oligometastatic disease

often combined with an off-line way to detect tumor shrinkage. For cervix and bladder, inter/extrapolated motion prediction models can be generated based on pre- treatment imaging, though generation of these extra structures is still not straightforward and often based on in-house developed software. For prostate, different ways of motion measuring are possible (i.e. implanted markers, RFID or ultrasound) because the prostate volume and shape remains primarily constant, just its global position varies, depending on surrounding organ filling/muscle motion. Tracking and gating can be used for the faster (intra-fraction) prostate motion. For the library-based methods, plan selection is found to be robust: most studies show good correlation between online plan selection and offline clinician/expert panel plan selection; even with reduced image quality it is still possible to determine whether a target is within the pre- defined contours of the library. Reporting trustworthy dose-volume parameters to correlate with outcome is more of a challenge, as for the large and uncorrelated motion patterns in the pelvic region, DIR and dose warping algorithms are not well validated and re-delineation is too laborious (and not robust) on CBCT images to be used in clinical routine. Recent developments on CBCT based dose calculation are promising to improve this topic. An important aspect is patient safety: clinical (non- research) versions of commercial R&V software are not optimal for handling multiple plans of which only one is selected and the others disregarded, meaning that check and double check remains necessary to avoid mistakes. Robust and back-up procedures are necessary in case no decision can be made or if imaging fails. Abstract text Contemporary radiotherapy techniques of pelvic tumors may reduce normal tissue dose by conforming closely to target volumes. This allows reducing the toxicity of the treatment. To achieve this goal, it is necessary to reproduce the patient's position, the position of internal organs and their volume in accordance with the treatment plan performed in TPS (Treatment Planning System). The image guided radiation therapy techniques enable checking the position of internal organs as well as their volume. The protocol of preparation for radiotherapy includes obtaining the reproducibility of the bladder or rectum filling through the application of guidelines for patients. Many publications have shown the importance of bladder filling on CT images performed for treatment planning and bladder volume variations compared to the treatment plan during the whole course of radiotherapy. The main aim of this lecture is to summarize the available data in this topic, present our own experiences and give recommendations. This presentation will also focus on the discussion about various bladder preparation protocols, image guided radiotherapy techniques and on the impact of bladder volume on acute genitourinary toxicity. SP-0676 MR-based treatment planning for prostate cancer E. Persson 1 1 Skåne University Hospital, Hematologi- Oncology and Radiation physics, Lund, Sweden Abstract text MRI has today a given place in the radiotherapy workflow for several diagnoses. The superior soft tissue contrast of MRI makes it optimal for definition of the target area for radiotherapy. CT are traditionally used in the remaining SP-0675 Bladder filling - does it matter? D. Bodusz 1 1 Maria Sklodowska-Curie Memorial Cancer Center and Institute of Oncology, Radiotherapy Department, Gliwice, Poland