ESTRO 2021 Abstract Book

S554

ESTRO 2021

Abstract Text Radiotherapy for head and neck cancer is beneficial, as very complex and high dose gradients can be achieved thanks to the development of innovative techniques in computing, imaging and delivery systems. Consequently, tumour local control and organs at risk and sane tissues sparing could lead to dose escalation and have a tremendous impact on patient quality of life. Effectively, sharp falloff on dose distributions outside high doses in target volumes can be carried out, and thus complex techniques and geometric uncertainties must be minimised to prevent under dosage in target volumes and over dosage of surrounding critical structures. As a result, in order to account for geometrical uncertainties, i.e. setup errors and inter-intra fraction movements, robust immobilization and important image guided radiotherapy (IGRT) are essential for patients with head and neck cancer. Modern treatment units are equipped with different technologies of IGRT, for example embarked or not embarked imaging systems, using two or three dimensional modalities. Moreover, in addition to be used as a positioning and re-positioning system, IGRT could be executed with the aim of reducing CTV-to-PTV margins, with the consequences of lowering toxicities for such patients where very close proximity between sane tissues or critical organs and big target volumes exist. Of course, these IGRT systems are using X-Ray (with kV and/or MV energies) and could be used on a daily basis. Thus, accumulated dose due to imaging could be important and taken into account in clinical treatment plans. A further advantage in considering 3D imaging systems could be the development of adaptive radiotherapy, especially for patients presenting head and neck cancer. Effectively, the aim of adaptive radiotherapy is to correct treatment plan during patient treatment due to changes in organs at risk and sane tissues and target volumes. Anatomy of the head and neck region is complex, with close vicinity between tumour and very critical organs important to consider. This presentation aims to synthetize the development of IGRT techniques for head and neck cancer, providing recommendations in the clinic.

SP-0716 MRI guided Brachytherapy C. Kirisits Austria

Abstract not received

Symposium: Novel technology for treatment delivery and modulation: Flash, microbeams and spatio- temporal fractionation

SP-0717 Spatially fractionated radiotherapy Y. Prezado 1 1 Institut Curie, Signalisation, radiobiologie et cancer, Orsay, France

Abstract Text The therapeutic use of ionizing radiation has been largely guided by the goal of directly eliminating all cancer cells while minimizing the toxicity to adjacent tissues. Nowadays, technological advances in radiation delivery, including image guidance and particle therapy (i.e. proton therapy), have notably improved tumor dose conformation, thus reducing the dose to the organs-at-risk. Despite remarkable advancements, the dose tolerances of normal tissues continue to be the main limitation in RT and still compromise the treatment of some radioresistant tumors, large late stages cancers , tumors close to a sensitive structure (e.g. central nervous system (CNS)) and pediatric cancer. One possible way to overcome this limitation is to employ new modes of radiation dose deposition that activate biological processes different from those in standard radiotherapy. Along this line, spatially fractionated radiation therapy (SFRT) describes a radiotherapeutical approach that uses a strong spatial modulation of the dose to create alternating regions of high and low dose in order to increase the tolerance of normal tissue [1]. Despite the first treatments dating back to the early 20th century, SFRT remains rarely employed compared to conventional RT which is based on laterally homogeneous irradiation fields. However, decades of clinical and preclinical data have shown that SFRT has promising potentials as an extremely high therapeutic index treatment. Nowadays, there is a renewed interest in SFRT worldwide, with several clinical trials being anticipated in the near future. In this presentation the fundamental concepts of spatial fractionation will be described and the different forms of SFRT (GRID, Lattice, micro and minibeam RT) will be presented. The distinct radiobiological and dosimetric aspects will be also discussed. Finally, the recent exploration of the synergies between the advantages of SFRT and the benefits of charged particles for therapy will be described and main results presented [2-7]. Its exploration offers a whole new horizon of both scientific research and potential future clinical practice. [1] W. Yan et al. CTRO 20, 30-38 (2020)

[2] Y. Prezado et al, Sci. Reports 7, article number 17295 (2017). [3] Y. Prezado and G. Fois, Med. Phys. 40, 031712, 1–8 (2013). [4] Y. Prezado et al., Scie. Reports 7, article number 14403 (2017). [5] Y. Prezado et al. Scientific Reports 8, article number 16479 (2018). [6] C. Lamirault et al. Scientific Report 2020 [7] W. Gonzalez and Y. Prezado, Medical Physics 45, 2620-2627 (2018).

SP-0718 Targeting the Physics, but missing the Biology N. Suchowerska 1 1 Chris O'Brien Lifehouse Cancer Hospital, VectorLAB, Department of Radiation Oncology, Sydney, Australia

Abstract Text Radiation therapy, underpinned by physics and mathematics, have enabled us to understand the absorption of