ESTRO 38 Abstract book

S238 ESTRO 38

SP-0460 Integration of MR and particle therapy – how far are we? A. Hoffmann 1 1 OncoRay - Center for Radiation Research in Oncology, Medical Radiation Physics, Dresden, Germany Abstract text Precise targeting in proton therapy (PT) is even more important than in conventional photon-based radiation therapy (XT), because the usage of fewer radiation fields make PT dose distributions more sensitive to anatomical variations ( e.g. , organ motion and deformation) and patient set-up inaccuracies than XT. This is due to the steep dose fall-off behind the Bragg peak and to the fact that the range of the proton beam strongly depends on the material composition in the beam path. Especially for moving tumours, these uncertainties translate into relatively large safety margins, thus compromising the dosimetric benefit of PT. This urges the need for real-time image guidance during PT dose delivery. Magnetic resonance imaging (MRI) offers real-time image guidance with unparalleled soft-tissue contrast and absence of radiation dose. PT is therefore expected to benefit even more from the integration with real-time MRI than XT. Hence, in recent years there has been a growing interest to investigate the potential of MR-integrated PT (MRiPT), but so far no hybrid system has been realized due to a number of hitherto open technological problems. To study the technological feasibility of MRiPT, mutual interactions between the PT and MRI system have to be taken into account, some of which have already been addressed or are subject of ongoing research. Lorenz-force induced dose distortions ( i.e. beam deflection and electron return effect for transverse magnetic fields and beam focussing effect for longitudinal magnetic fields) need to be quantified and taken into account during treatment planning and dose delivery. The magnetic interaction with dosimetry detectors may compromise the results of such measurements. Methods and procedures for proton beam dosimetry in the presence of magnetic fields have to be established. The mutual electromagnetic interaction between the MRI scanner and the PT system needs to be assessed and understood as this can compromise both the beam and image quality. For simultaneous operation of both systems, magnetic shielding may be required in addition to RF shielding. For on-line adaptive treatment planning, the proton dose distribution needs to be calculated directly from MR images. Fast and accurate methods that translate MR image information into electronic stopping power need to be developed. Recent research efforts have realized a proof-of-concept system where first MR images of tissue-mimicking phantoms have been successfully acquired with a clinical in-beam MR scanner during proton beam irradiation. This offers the prospect that the development of a clinical prototype MRiPT system within the next 5 years should not be considered beyond the realms of possibility. The contribution provides an overview of the current status of MRiPT research achievements and discusses technology issues that need further investigation. Implications for future developments and clinical treatment workflow are also addressed. SP-0461 MR-based functional imaging R. Beets-Tan 1 1 Maastricht Radiation Oncology (Maastro), Maastricht, The Netherlands

1 N.N. Alexandrov National Cancer Center, Department of Radiology, Minsk, Belarus Abstract text MRI is used at different stages of oncological patient management including tumor staging, treatment (surgery, radiation therapy) planning and tumor response assessment. Traditional radiation therapy planning workflow integrates high contrast resolution and anatomical details provided by MRI with electron density values obtained from CT. More recently MRI-only radiotherapy planning workflow has been implemented with synthetic CT images generated form MRI data. On- treatment MRI-guidance is a new paradigm of precision radiotherapy that enables a scan-plan-treat approach or adaptive radiotherapy. MRI linac machines integrating clinical MRI scanner with linear accelerator are current reality in some cancer centers around the globe. SP-0463 Towards less radiotherapy in breast cancer treatment I. Meattini 1 1 University of Florence - Azienda Ospedaliero Universitaria Careggi, Radiation Oncology Unit - Oncology Department, Florence, Italy Abstract text Overdiagnosis and overtreatment represent great challenges in modern treatment for breast cancer. During the last decades several trials aiming less radiotherapy have been performed: reduction of overall treatment course (hypofractionation), reduction of overall treatment courses and breast target volumes (partial breast irradiation), omission of radiotherapy (whole breast and/or tumour bed boost). Hypofractionated whole breast irradiation, based on precedent studies over the past two decades, offers an opportunity for improved patient convenience, lower healthcare costs, and greater access to care without sacrificing treatment outcomes. Four randomized trials fully supported the use of hypofractionated whole breast irradiation as standard of treatment: the RMH/GOC trial, the UK START trial A and B, and the Canadian trial [1-6]. Partial breast irradiation has been introduced as an alternative treatment method for selected patients with early stage breast cancer. Potential advantages include shorter treatment time (accelerated), improved cosmesis secondary to a reduction in volume of breast tissue treated, and cost reduction compared with standard fractionation. Initial trials investigating partial breast irradiation showed conflicting results. The recent new high-level evidence provided by the results of the IMPORT- LOW trial clearly showed that adequate patients' selection led to low LRR. Tumour biology seems to play a crucial role on treatment outcome, thus it should always be considered together with disease stage, comorbidities, and patient's quality of life. Breast cancer should be always considered as a heterogeneous disease, for which a one treatment fit all strategy rarely represented the best option e giving the patients the final word in the shared decision-making process [7-10]. Whether radiotherapy is beneficial in elderly patients undergoing conservative surgery for early breast cancer has long been controversial. In the PRIME II trial, patients ≥65 years with tumours <3 cm in size with primarily grade I/II disease, all oestrogen receptor-positive and pathologically node-negative, were randomly assigned to whole-breast radiation therapy plus hormone therapy vs hormone therapy alone. The findings parallel those of CALGB 9343, which included an even lower-risk cohort of Symposium: Improved outcome by smarter use of radiotherapy

Abstract not received

SP-0462 Adaptive workflow - current status and challenges S. Kharuzhyk 1