ESTRO 36 Abstract Book

S154 ESTRO 36 _______________________________________________________________________________________________

expected from a new anti-cancer treatment. The ESMO- MCBS is an important first step to the critical public policy issue of value in cancer care, helping to frame the appropriate use of limited public and personal resources to deliver cost-effective and affordable cancer care. The ESMO-MCBS is a dynamic tool and its criteria will be revised on a regular basis. The next version will include also an approach to grade the clinical benefit data derived from the registration trials of medications approved on the basis of these single arm studies. Currently the grading of newly registered drugs is included in ESMO-guidelines. A similar approach to develop a scale can potentially be used for other treatment or diagnostic areas in oncology including radiotherapy. For a scale grading radiotherapy, there will likely be a number of similarities and differences versus a scale for drug treatment. Factors taken into account for the radiotherapy scale might well include the adjuvant and curative outcomes: overall survival, disease free survival, local recurrence free survival, pathological complete response and non- curative/palliative outcomes such as: single symptom relief (complete response, partial response, relief duration of response), control of hemorrhage, relief of obstruction, effects on skeletal events (pain, fracture) and neurological function. We anticipate methodological challenges in the relative weighting and scoring of palliative outcomes form localized radiotherapy as distinct from systemic therapies. SP-0298 For the motion B. Raaymakers 1 1 UMC Utrecht, Department of Radiation Oncology, Utrecht, The Netherlands The common ground for proton and photon guidance, that is MRI and CBCT guidance, is the desire to localize the target and the surrounding structures in order to improve the spatial accuracy of dose delivery. This is especially important to better target and to minimize the high dose volumes which are leading to the most acute toxicity and are often dose limiting. With modern accelerators, both proton- and photon therapy can generate a conformal high dose volume, while image guidance is the most important parameter on delivering this high dose volume to the correct position and with that minimize this high dose volume. Doing so, also hypo-fractionated treatments for more and more tumor sites can become feasible. MRI guidance is superior because: 1) Soft-tissue guidance of MRI will out-perform CBCT based set-up 2) MRI provides dynamic imaging to track breathing and peristalsis without the need for retrospective binning 3) MRI enables daily full re-planning 4) MRI provides intra-fraction (volumetric) imaging for dose reconstruction and plan adaptation 5) Integrated MRI provides functional response assessment during the course of radiotherapy CBCT has greatly improved radiotherapy by offering 3D imaging just prior to radiation delivery, these images can be used for improved patient set up and assessment of the breathing pattern. These data, even though they have limited soft-tissue contrast, are acquired just prior to treatment. Using these instead of relying on pre- treatment images of days (if not weeks) old, provides much more representative information on the target and surrounding structures and will improve patient set-up. With MRI integrated in the radiotherapy system, all the Debate: This house believes that proton guided photons (online MR guided therapy) will be superior to photon guided protons (CBCT proton therapy)

aims from CBCT guidance can be brought to the next level. MRI offers soft-tissue contrast, so one can much better distinguish tumor from surrounding tissues. Also dynamic MRI can provide 4D anatomical data with high temporal resolution (e.g. 3Hz) to detect breathing and peristaltic irregularities. The limitation of CBCT for needing bony landmarks, surrogates, the need for large tissue density differences or the retrospective binning to assess motion data will be solved when using MRI. So MRI is at the very least a much better CBCT in the sense that it provides direct visualization of target and surrounding structures. CBCT guided proton therapy is lagging behind on the much needed image guidance offered by MRI and hybrid MRI radiotherapy systems will improve position verification. On-line MRI will also enable on-line re-planning strategies that are not, or only for some sites, feasible with CBCT as an input. This on-line re-planning fits seamlessly into the large research interest of the radiotherapy community to adapt the dose more to the actual anatomy and deliver more conformal dose distributions, currently being implemented via library of plans or off-line re-planning strategies. Moreover, integrated MRI allows imaging during radiation delivery. This way, assumptions on anatomical stability or motion as determined on pre-treatment data can be verified. Also, the intra-fraction volumetric imaging provides the input for dose reconstruction, so even if the pre-treatment assumptions are failing and the anatomy is moving/deforming unexpectedly, one can reconstruct exactly what the dose delivered is. This can be used for off-line re-optimization for remaining fractions. Additionally, as this dose reconstruction can be done in near real-time, one can also built adaptation triggers on it such as gating and ultimately intra-fraction re-planning strategies. The latter would be truly interventional radiosurgery where the dose distribution is continuously adapted to the mobile anatomy. Another advantage of integrated MRI radiotherapy systems is the capability to assess functional parameters such as perfusion or water diffusion, from the patient in treatment position. This can provide great insight in treatment response and temporal behavior during the course of radiotherapy. In summary , there is a clear desire from the image guided radiotherapy community to use more and better imaging prior and during radiation delivery. MRI guided photon therapy can fulfill this desire and will contribute to more precise radiation delivery and to a more hypo-fractionated approach. With that hybrid MRI radiotherapy systems will become the first choice for radiotherapy and CBCT guided proton therapy is mainly indicated in case the integral dose is treatment limiting, e.g. for pediatrics. SP-0299 Against the motion A. Lomax 1 Paul Scherrer Institute PSI, CPT, Villigen PSI, Switzerland

Abstract not received

Proffered Papers: Intra-fraction motion management

OC-0300 Proof of tumor position during SBRT delivery using (limited-arc) CBCT imaging C. Hazelaar 1 , M. Dahele 1 , B. Slotman 1 , W. Verbakel 1 1 VU University Medical Center, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective SBRT requires accurate patient positioning and robust positional verification during irradiation itself is desirable. We investigated if CBCT scans reconstructed from (collimated) fluoroscopic kV images acquired during irradiation, including over a limited arc length, can