ESTRO 38 Abstract book

S250 ESTRO 38

dose distribution in terms of dose to 95% of the volume (D 95% ), dose to 2% of the volume (D 2% ) and average dose to the target. Results We obtained for ID an average DD% equal to (1.8 ± 0.9)% and an average %GP equal to (98 ± 2)%. For IVD, the mean DD% was equal to (-1.6 ± 5.4)% for D 95% , (1.0 ± 1.7)% for D 2% , and (0.3 ± 1.8)% for average dose. 74% of fractions showed DD% inferior to 2%. For 16% and 5% of the analyzed dose points, DD% was greater than 3% and 5%, respectively. Dose failures occur mainly in air-tissue interfaces and in superficial regions, in fact the deviations greater than 5% were observed in lung patients (30%), partial breast irradiation plans (27%) and palliative treatments (20%). For the 57% of fractions the delivered dose is higher than calculated one. For the fractions that exceed 3%, the errors are due to setup (29%), patient anatomy (40%), setup + anatomy (27%) and bolus positioning (4%). Among set-up errors, 48% is due to SSD variations, 5% are rotational errors, 12% are errors due to difficult art and shoulder positioning, and 35% is due to weight variation. No machine-based errors were observed for these patients. Conclusion EPID-based IVD is a powerful method to catch and quantify delivery discrepancies during radiotherapy process. Abstract text Hypofractionation is the application of dose fractions larger than the conventional 2 Gy per fraction. It includes moderate doses per fraction of approximately 3.5 Gy routinely used with curative intent, as well as stereotactic treatments with a few large fractions that has been widely applied in palliative radiotherapy, but now suggested to have a curative application, especially for oligometastases. Radiobiologically, when compared to conventional fractionation, hypofractionation would be expected to lower the therapeutic ratio between tumors and late-responding normal tissues. However, technological advances in image guidance and treatment delivery can reduce the dose delivered to normal tissues, allowing tumors to receive higher doses per fraction. There are reports that in addition to direct cell killing seen with high doses per fraction, one also finds dose- dependent secondary cell death that occurs following the induction of vascular damage. However, this is controversial, because changes to the tumour microenvironment (i.e., increased hypoxia) seen following vascular damage are not always apparent after stereotactic irradiation. In fact, hypoxia in tumours, a known factor for resistance to conventional radiotherapy, may play a more significant role with stereotactic radiation. These vascular and microenvironmental effects may depend on the dose per fraction. Recent studies suggest that high-doses per fraction has the potential to induce an immune response that not only improves response of the primary tumor, it also has the potential to cause an abscopal effect. However, the radiation dose and number of fractions necessary to induce this effect is unclear. In this presentation, we will review some of these radiobiological issues associated with hypofractionation and discuss how they can play a role in future radiation therapy. Joint Symposium: ESTRO-IAEA: The role of hypofractionation in current radiotherapy and its impact in planning radiotherapy services SP-0485 Hypofractionation from a radiobiological perspective M.R. Horsman 1 1 Aarhus University Hospital, Dept. Experimental Clinical Oncology, Aarhus C, Denmark

Supported by funding from the Danish Cancer Society and the Danish Council for Independent Research: Medical Sciences. SP-0486 Clinical outcome and effectiveness of extreme hypofractionation together with the different scenarios in terms of resources and costs J.A. Polo Rubio Hospital Ramon y Cajal, Madrid Spain

Abstract not received

SP-0487 How we deliver extreme-hypofractionated radiotherapy with current technology - a physicist

perspective J. Cuijpers 1

1 Amsterdam University Medical Centers, Radiation Oncology Department, Amsterdam, The Netherlands Abstract text The successful clinical implementation of hypo- fractionated radiotherapy is strongly connected to current advances in treatment technology that enable the safe and precise delivery of high fraction doses to the target with optimal sparing of the adjacent healthy tissue. This starts with a reliable definition of the target volumes and organs at risk using multimodality pretreatment imaging incorporating breathing motion for tumors in the lung or abdomen. Advanced dose delivery techniques, like multiple field IMRT or VMAT, using complex shaped beam portals are applied to achieve steep dose gradients outside the target. This sets high demands on dose planning, especially for small targets in low density lung tissue, and machine performance. Due to the multi-directional or rotational beam setup high beam energies are becoming of less importance, but instead the use of flattening filter free modalities will be important for reduction of treatment times for the delivery of high fraction doses. Of utmost importance is a reliable positioning of the tumor before but also during treatment using on-line image guidance, setting high standards in accuracy and precision for IGRT equipment. This is even more important in the case of hypo-fractionation since the dose averaging effect of geometric random errors does not occur if only a few fractions are administered. Ultimately, if the superior soft tissue contrast of MRI is required, an integrated MR- treatment unit will allow to meet these requirements. Also, demands for quality assurance programs and the necessary equipment will be affected by the requirements for high-precision hypo-fractionated treatments. The consequences on how this affects the program of requirements for radiotherapy equipment will be discussed in this presentation. SP-0488 What’s the impact of extreme- hypofractionated radiotherapy in operating a radiotherapy department - an RTT perspective Y.M. Tsang 1 1 Mount Vernon Cancer Centre, Radiotherapy, Northwood Middlesex, United Kingdom Abstract text Radiotherapy plays a key role in cancer management. The conventional radiotherapy treatment course is divided into a number of small doses called fractions, which are usually given once a day, Monday to Friday, over a number of weeks. Currently, there is a wide range of radiotherapy fractionation schemes available clinically based on treatment intents and disease sites. Hypofractionation, the delivery of a course of radiotherapy using a smaller number of fractions with a higher dose of radiation within each fraction than what is given conventionally as 1.8 - 2Gy, is increasingly studied and adopted for clinical use. In the last decade, there have been rapid changes and progressive developments in the technology used for