ESTRO 36 Abstract Book

S259 ESTRO 36 2017 _______________________________________________________________________________________________

is indexed on the patient couch. The cube can be moved to a predefined translational and rotational offset position on the couch. Two planar images were acquired and registered to the CT.The calculated correction vector was applied by the patient positioning system. This workflow was repeated at three index positions (equal distributed along the treatment volume), payloads (0kg, 100kg, 150kg), cube offsets (long.: 10mm, lat.: 15mm, vert.: - 3mm with/without rot 3°) and couch positions. After the alignment into the isocenter the absolute position was measured with a lasertracker (u: 0.1mm)(see figure 1b). To determine the total accuracy including the treatment beam and patient alignment system, an EBT3 film was inserted into the cube. Two fiducials in a row are used to shift the Bragg peak into the layer of the film (see figure 1c). For the combined workflow with an EBT3 film, a rotation error of 1° results in a fading of the fiducials.

Optimisation for prostate radiotherapy involves understanding the natural history of prostate cancer, its prognosticators, the underlying radiobiology and strategies for radiotherapy treatment as well as how best to deliver the therapy regimes devised. Prostate cancer presents with a large spectrum of risk factors that drives its natural history as such the presenting prostate specific antigen (PSA) levels, the summed Gleason score and the local tumour nodal and metastasis (TNM) staging. Taken together this combination of prognostic factors still provides the most reliable disease stratification compared to new tests or potential biomarkers that still require validation. It is well recognised that more reliable and sensitive predictive and prognostic factors are needed to distinguish risk groups within each risk stratification due to the large heterogeneity that exists within them. Using this approximately set of factors, prostate cancer can be divided into low, intermediate and high to very high risk groups for localised to regional disease that may receive radiotherapy curatively. This further guides the treatment volumes needed to achieve the best curative rate as well as the treatment strategies ie external beam photon therapy alone or in combination with high dose rate (HDR) brachytherapy or HDR monotherapy or proton therapy In addition, over the past one to two decades, the understanding of prostate radiobiology has changed from accepting the standard alpha-beta ratio of 10 assigned for the majority of cancer diseases to a much lower alpha- beta ratio much more akin to that of late reacting tissues. This lower alpha-beta ratio of around 1.2 to 2.5 has driven the radiotherapy rationale for using larger dose per fraction or hypofractionation. Earlier trials have suggested that this may provide a therapeutic benefit. Recently in the past year, 2 very large randomised trials have provided outcome equivalence for the use of moderate hypofractionation compared to conventional fractionation. The first randomised trial of over 800 cases from USA in early stage disease compared 2.75Gy to 1.8Gy dose fractions in a non-inferiority study can reported equivalence for the schedules. The second non-inferiority UK/International trial recruited nearly 3300 cases and reported equivalence of 3Gy to 2Gy dose fractions for early to intermediate stage prostate cancer. These outcomes have now established a new standard for the radiotherapeutic management of early to intermediate stage localised prostate cancer. These results should not be confused with the Dutch study looking for superiority of 3.3Gy over 2Gy dose fractions where the primary endpoint was not achieved. It remains to be determined if larger dose fractions (≥ 5Gy) delivered over 1 week such as that used in stereotactic radiotherapy may provide further benefit. Trials are currently on going and their results are eagerly awaited. As important as the dose fractionation are the therapy regimes and the potentially combinations listed above. It is recognised that local failure often occurs at the site of dominant clonagenic numbers thus the utilisation of simultaneous integrated boosts with hypofractionation has potential. This is being assessed in several studies such as the FLAME study where toxicity was demonstrated to be similar to standard and conventional regimes. Important questions that need to be addressed include identification of appropriate treatment volumes and potential regions of dose escalation or even dose de-escalation. Just as important will be methods of ensure that these shorter treatment regimes are delivered both accurately and reliably. These aspects will be reviewed by the co-lecturers within this symposium. SP-0493 The role of spacers in the era of highly conformal, hypo-fractionated, image guided, adaptive radiotherapy of the prostate P. Scherer 1 , F. Wolf 1 , C. Gaisberger 1 , F. Sedlmayer 1 1 Gemeinn. Sbg. Landeskliniken Betriebs. GmbH, University Clinic for Radiotherapy and RadioOncology, Salzburg, Austria

Results The mean 3D accuracy for all workflows was 0,34mm (n=24). Most of the residual rotational errors were below 0.15°. The mean deviation in x and y changes significantly in relation to the couch rotation (xmean for 0°: -0.74mm; xmean for 180°: +0.27mm)(see table 1). In figure 1d) an EBT3 film after image guided alignment and radiation with a proton beam is shown. For the combined workflow with treatment beam the translation in y and z axes was below 1mm and the rotation for rot Y and rot Z is lower than 1°.

Conclusion The dependencies for couch rotations might arise from a lateral offset of the imaging ring. A flexmap calibration with exactly known phantom position may reduce the lateral offset. The largest deviation (mean 3D vector = 0.78mm) was found for couch rotation 0°.The inserted EBT3 film allows determining the overall accuracy of the whole treatment workflow.The IGRT Phantom is an appropriate equipment for daily QA and for more detailed workflow tests.

Symposium: Focus on prostate cancer: what is the best of radiotherapy we need to treat our patients with

SP-0492 What are the best ingredients to deliver the optimal radiotherapy for prostate cancer V. Khoo 1 1 Royal Marsden Hospital Trust & Institute of Cancer Research, Department of Clinical Oncology, London, United Kingdom