ESTRO 2020 Abstract book

S463 ESTRO 2020

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. C. Salembier 1 1 Europe Hospitals - Site St Elisabeth, Department of Radiation Oncology, Brussels, Belgium and brachytherapy (BT) are standard treatment options for men with localized prostate cancer and confer long-term prostate cancer control outcomes. For decades, EBRT has been delivered every working day for a total of 35-45 treatments. However, technological improvements in treatment planning and delivery allow to safely administering high radiation doses in small volumes over a much shorter period of time with high conformality. Certainly for prostate cancer, as demonstrated by their estimated low alpha/beta ratio, the malignant cells might be more sensitive to larger doses per fraction. In consequence, multiple clinical randomized trials have evaluated the safety and efficacy of moderate (and extreme) hypo-fractionation for prostate cancer. These shorter treatment regimens are certainly more convenient for the patient and might reduce the costs but efficacy and safety should be demonstrated first. The actual clinical evidence will be discussed in regard to the examined prognostic risk groups. In selected patient groups, prostate seed implants have demonstrated excellent long-term cancer control associated with low toxicity profiles. HDR monotherapy in two (or more) fractions is emerging as a viable alternative to LDR brachytherapy and has less toxicity. In more locally advanced stages or higher risk groups, combination therapy of EBRT combined with BT can also be delivered using various combinations (intensity modulated radiotherapy +/- Androgen Deprivation Therapy (ADT) +/- either low-dose or high-dose rate brachytherapy . At the end, the available literature in regard to proton beam therapy will be shown. The goal of this presentation is to review the current clinical evidence for the existing radiation-based therapies in localized prostate cancer and Abstract text External-beam radiation therapy (EBRT)

to discuss appropriate regimens for routine clinical use according to patient risk groups, as well as future areas of research and development. 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 Abstract text Radiotherapy is a safe and effective treatment for prostate cancer, with huge changes in our technological capabilities over the last decade. SBRT has emerged from the convergence of radiobiological research, suggesting a low alpha/beta ratio, and technological innovation, which has made conformity and dose gradients previously thought to be impossible into a reality. Birthed out of these innovations, stereotactic body radiotherapy (SBRT) for prostate cancer, treating to high daily doses in 5 or less treatments, is gathering an evidence base. For many years this data was solely from Phase II or cohort studies, but now Phase III data is emerging. This talk will review data on SBRT now dating back over a decade, including very large datasets which have been recently published. What can we learn about the safety and efficacy of SBRT from this data and do the current guidelines around the globe reflect this data? Which patients may benefit more than others from SBRT and which patient groups have not yet been adequately studied? In the last few years, Phase III evidence in support of SBRT has been presented showing equivalence of ultrahypofractionated regimens in terms of 5 year biochemical outcomes (HYPO-RT-PC) and equivalence of short term toxicity for 5 fraction SBRT (PACE B). This talk will discuss whether SBRT should be standard of care for any patient in 2020. Finally, data still continues to accrue regarding the benefits and risks of SBRT. Several large trials will publish in the next few years, which may change the landscape for many men with prostate cancer. But where do we go next? Trials are ongoing with simultaneous integrated boost techniques, as well as treating higher risk men and treating the prostate and pelvis with SBRT. Predicting where the standard of care will be in 2030 is going to be challenging. SP-0757 Treatment planning and dose constraints for Prostate Sbrt P. Mancosu 1 1 Humanitas Research Hospital, Medical Physics Unit- Radiotherapy And Oncology Dept., Rozzano Milan, Italy Abstract text Stereotactic Body Radiation Therapy (SBRT) on prostate could be delivered using different machines (dedicated or not dedicated to SBRT). There is a need to harmonize the planning procedure in order to compare different findings. This presentation will focus the pro/cons of homogeneous and not homogeneous approaches in optimizing prostate SBRT. Constraints for prostate SBRT will be presented starting from the latest recent trials (in particular thePACE-B trial).