ESTRO 2020 Abstract Book

S418 ESTRO 2020

plastic catheter separated by spacers in between. This results in an overall applicator diameter of 12mm. With the help of the endoscope, the exact position and the extent of the tumour are determined, marked with a clip and contrast medium (Jopamiro 300mg J / ml, Patheon S.p.A Italy) injected. The applicator is then fixed to the patient and the endoscope removed. The patient is under general anaesthesia during the entire procedure. A CT with a 2mm layer thickness and an inserted applicator serves as imaging for the planning. The selection of the target volume dose, the treatment length, the treatment depth and the sectors to be irradiated is based on the endoscopy and CT images. Dwell-points are only set in probes which are relevant for the treatment of superficial tumours. Results: With a treatment depth of 5mm in the oesophageal wall over a range of 108 ° and a dose of D100% = 5-7 Gy per fraction for 3 fractions, the surface dose at the target volume is 333% and 145% at 3mm depth, the opposite being healthy Tissue received a dose of 36% at 5mm depth, 44% at 3mm depth and 64% at the surface. In comparison to the central source design applicator, the values are 220% on the applicator surface, 130% in 3mm depth and 100% in 5mm depth over the entire oesophagus diameter. Conclusion: The newly developed applicator enables a more individual adaptation to the target area while protecting the healthy tissue compared to the previous standard treatment using the Bonvoisin-Gerard applicator. The design also enables optimal placement of the applicator using an endoscope. The applicator has proven to be suitable in practice and has been used successfully for ten patients. SP-0753 SGRT on 4D dynamic approach: facts and trends F. Moura 1 1 Hospital Cuf Descobertas, Department of Radiotherapy, Lisboa, Portugal Abstract text Optical surface detection systems have been widely used in the radiation oncology clinical practice, providing safe and reliable patient positioning and localization accuracy throughout the whole treatment. This modality, also called as Surface Guided Radiation Therapy (SGRT), is a powerful tool for treatment reproducibility, with real time patient monitoring on a milliseconds period measurements. Reduction of both systematic and random errors, can be reached with this fast non-ionizing imaging modality, by providing means for positioning, intra- fraction motion monitoring and gated treatments by tracking patient´s breathing. Besides the precision optimization related to the system itself, it can promote a change on patient´s workflow, skin markerless and a significant reduction on ionizing radiation image guided RT (IGRT), via CBCT or Portal Imaging. From the perspective of precision and accuracy throughout the treatment delivery process, any treatment site could benefit from this technological development, which reveals its potential not just on free breathing but also for gated RT approaches. From the thoracic to abdominal and pelvic deep site treatment sites, the body surface is, in most of the cases, a good surrogate for the clinical applications, which could be correlated on a daily basis both with in 3D or 4D modalities. If taken into consideration the organs at risk sparing, left sided breast cancer patients, would be the strong candidates to use SGRT modality, which will allow for heart volume and other sub-structures sparing, by using gated Deep Inspiration Breath Hold (DIBH) with a

4D dynamic approach, alone or in combination with other IGRT systems. Regarding the combination with frameless open face immobilization systems, SGRT may also provide safe and effective treatment localization and intrafraction monitoring both for head and neck and intracranial treatment sites, which also enables for improved SRS detection methods for higher precision treatment delivery techniques. Several approaches will be covered during this lecture, for a global understanding of this straightforward and trendy available technology.

Symposium: Ultra hypofractionation for localised prostate cancer

SP-0754 Is prostate cancer sensitive to large doses per fraction M. Joiner 1 1 Wayne State University, Oncology, Detroit, USA Abstract text Better Physics just within this century now enables us to deliver radiation to a target volume with accuracy better than 1 mm. Given this accuracy, why fractionate? If we can put dose only on the cancer, never on critical normal tissue, then surely just give a high single dose to that cancer, and job done. Local tumor control is 100% with minimal toxicity. If only. Two linked issues keep Biology (clinical radiobiology) in the game. First, imaging resolution does not yet correspond with this sub- millimeter accuracy of radiotherapy delivery. Second, even if that imaging resolution is reached it could still not detect occult disease. Consequently, unless the cancer is truly isolated, perhaps in organ-confined early-stage prostate cancer, it is always necessary to “degrade” the treatment plan by defining a CTV and PTV into which the radiation delivery is expanded. This inevitably imposes a risk of normal-tissue radiotoxicity, therefore we can use fractionation and/or brachytherapy to minimize that risk. In fractionation, the Linear-Quadratic (LQ) model describes the relationship between total dose and dose per fraction, for isoeffect. A hypothesis for the different α/β values for early- and late-reacting tissues in LQ, is that a naturally low α/β for a cell population is smoothed out to a higher value as the sum of the responses of different proliferative subpopulations. In some malignancies, notably human prostate, clinical data indeed indicate an α/β as low as 1.5, which might also reflect more uniformity in response perhaps more characteristic of lower proliferative or early-stage disease. Thus in prostate cancers, hypofractionation is becoming standard of care. However LQ must overestimate the effect of very high doses per fraction because the effective D0 would become unrealistically low. Using LQ at very high doses per fraction or single dose, then plays safe in predicting toxicity but should overestimate the effect on the prostate cancer irself, of which there may currently be some evidence.

SP-0755 Comparative effectiveness of different radiation techniques for localized prostate cancer.

Presentation cancelled

SP-0756 Stereotactic body radiotherapy for localised prostate cancer – clinical evidence in 2020 A. Tree 1 1 Royal Marsden Hospital Trust & Institute of Cancer Research, Uro-Oncology, London, United Kingdom