ESTRO 38 Abstract book

S362 ESTRO 38

assumed that margins for microscopic disease (CTV) and geometrical uncertainties (PTV) should be kept separated. These issues will all be discussed. Finally, the main effect of the new imaging modalities on radiotherapy safety margins might be indirect. The improved accuracy and smaller margins allow for ever higher treatment and fraction doses. There is now evidence that a different radio-biology applies to single- dose radiotherapy (SDRT). Apart from the standard radiation mechanism that targets misrepair of double strand breaks, higher fraction doses (> 12Gy) are believed to also induce tumor cell kill by injuries to the tumor microvasculature. We will discuss the possible consequences of SDRT on safety margins; margin factors A and B might change and even negative margins are not unthinkable. SP-0701 Personalized phantoms through 3D printing S. Crowe 1 1 Royal Brisbane and Women's Hospital, Cancer Care Services, Herston, Australia Abstract text The application of 3D printing (or additive manufacturing) technologies and techniques in medical physics has exploded in the past five years. 3D printing has been widely used for treatment equipment, including bolus, compensators, shielding, immobilisation devices and brachytherapy applicators. These bespoke patient- specific solutions can be precisely fabricated, in a cost- effective way, with limited expertise. For the medical physicist, 3D printing allows the fabrication of a wide variety of tools for quality assurance, of varying complexity. The simplest applications include ancillary dosimetry equipment such as jigs or build-up caps; followed by simple phantoms for mechanical, dosimetric and imaging QA; and custom inserts for existing QA phantoms. Perhaps the most exciting application is the fabrication of anthropomorphic phantoms, suitable for evaluating new treatment and imaging technologies and techniques, end- to-end audits, trial accreditations, and answering research questions. 3D printed phantoms can include (and have included) variable density media, radionuclides, programmable motion, deformable components and embedded 3D gel dosimeters. This presentation summarizes the scientific literature surrounding 3D printing as applied to medical physics phantoms; provides advice for design, fabrication and QA; and describes the local experience of a radiotherapy department attached to a 3D printing and biofabrication research institute. SP-0702 Do we need to touch? Latest developments in physical and digital phantoms for 4D radiotherapy C. Mcgarry 1,2 1 Belfast Health and Social Care Trust, Northern Ireland Cancer Centre, Belfast, United Kingdom; 2 queen's University Belfast, Centre for Cancer Research AQd Cell Biology, Belfast, United Kingdom Abstract text Physical and digital anthropomorphic phantoms have been developed to represent the human body’s anatomy and attenuation characteristics for imaging and dosimetric studies in radiotherapy over many years. With the advancement of multimodality imaging used to capture intra-fractional motion at the pre-treatment and delivery stages of the radiotherapy process, dynamic phantoms have emerged with varying levels of complexity for Symposium: A new era for radiotherapy (anthropomorphic) phantoms

has been applied in the brain, where it might help contouring glioma invading the white matter. Although different MR imaging contrasts might give different GTVs, we know that in imaging we will never see all tumor tissue. Therefore, CTV margins are used in daily clinical practice. The better the imaging resembles the histology of a tumor, the smaller the CTV margin that has to be applied. The extent of the tumor outside the delineated volume can be determined using pathological validation of the tumor using histology. As MR imaging develops, the historical CTV margins might be too large as was illustrated for laryngeal cancer [20]. For brain tumors also blood oxygen level dependent MRI also referred to as functional MRI (BOLD fMRI) can be used to avoid functional regions in the brain. This technique shows the vascular response to neuronal activity. Besides contrast also high resolution can help to better define small structures, such as cranial nerves in case of perineural growth or small lymph nodes. Besides MR imaging, also MR spectroscopy has played a role, but never became a widely spread method, probably due to the demanding level of expertise. Particularly for brain and prostate, 1H MR spectroscopic imaging has shown its value. Due to the emerging MR guided radiotherapy, highly accelerated techniques for motion characterization, tracking and gating in combination with sufficient contrast to distinguish the tumor and OAR are being developed. As response monitoring during treatment is important for adaptation of radiotherapy, especially DWI is being further developed. In summary, the use of MRI in radiotherapy will increase in the near future as delineation will not only be part of the pre-treatment imaging but also of the treatment guidance. Therefore, the choice of MR imaging techniques which give a high contrast and allow fast acquisition are desired to make improve delineation accuracy and variation. SP-0700 The future of margins in the era of new (multi-modality) imaging technology J. Stroom 1 , S. Vieira 1 , C. Greco 1 1 Fundação Champalimaud, Radiation Oncology, Lisboa, Portugal Abstract text A large part of technical development in radiotherapy this century has concentrated on improving treatment accuracy through use of new imaging technology. During planning, MRI and PET imaging techniques have been applied to add precision to target delineation. During treatment, off-line correction strategies based on MV portal images have been gradually replaced by online corrections and plan adaptions using cone-beam CT. On- line MRI capabilities are now being introduced to refine treatment setup even further, and several systems for intra-fraction monitoring of moving patient anatomy are available. However, in this presentation we will try to make the point that even with all the new imaging techniques, it might be worthwhile to hold on to safety margins for standard fractionated treatments for a while. Whereas systematic and random uncertainties, often denoted by standard deviations Σ and σ, should diminish when applying the above-mentioned imaging techniques, they will not reduce to zero. So, for fractionated radiotherapy, margin recipes of e.g. the form AΣ + Bσ might still be applied, albeit with smaller values for Σ and σ as before. On the other hand, even though they are widely used, there still are several unresolved issues regarding margin recipes. To start, no randomized clinical studies have proven one margin recipe over another. Then, the added value of using ITV for breathing motion is highly debatable and delineation uncertainties are frequently ignored, just like uncertainties in microscopic disease. Moreover, it is