Abstract Book

S355

ESTRO 37

deprivation therapy: ADT) or local salvage approaches with curative intent as salvage prostatectomy, re- irradiation, cryotherapy, high intensity focused ultrasound (HIFU). The majority of the series are small and include heterogeneous patients and treatments without a long follow-up. No randomized trials comparing salvage local and systemic strategies are available. The most effective local therapy has not been established and remains controversial in the absence of consensus. The patients with biochemical failure receive mainly long lasting ADT with negative impact in the quality of life. In selected patients with locally recurrent PCa, PSA < 10 ng/ml and a good life expectancy (>10 years) a local treatment with radical intent can be considered. However, numerous case series and large databases show that local salvage therapy is dramatically underused. This underuse of local salvage therapies can be explained by several issues including advanced patient age and presence of numerous comborbidities, lack of prospective studies in this area and fear of side effects of re- treatment employing both surgical and non surgical approach. Indeed, the primary RT dose prescribed for prostate cancer is already high (dose escalation), thus re- irradiation might lead to severe injury of surrounding organs like rectum and urinary bladder. Until recently, the vast majority of re-irradiation series included patients treated with brachytherapy (BRT). In the last years, along with the improvement in RT planning and delivery technology, first reports on salvage EBRT (re- EBRT) have been published (9 published series: Vavassori et al 2010, Zerini et al 2015, Fuller et al 2015, Zilli et al 2016, Rutenberg et al 2016, Janoray et al 2016, Detti et al. 2016, Leroy et al. 2017). Use of modern approaches like image guided stereotactic (SBRT) re-irradiation, offers a high precision in delivery of radiation dose to small targets minimizing the dose to the organs at risk (OAR). All these series show that Re-EBRT delivered with modern techniques like IMRT and SBRT is a safe, feasible and noninvasive treatment for the well selected locally recurrent PCa, offering a satisfactory tumor control (about 50-60% of the patients are free of progression at 2 years) and excellent toxicity profile. Further investigation is warranted to define the optimal patient selection and establish the optimal dose and volume parameters for this particular clinical scenario. SP-0673 Multi-parametric imaging for dose targeting in salvage radiotherapy of prostate cancer U. Van der Heide 1 , C. Dinis Fernandes 1 1 Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands Abstract text Multi-parametric MRI, combining T2-weighted MRI, Diffusion-weighted MRI and dynamic contrast-enhanced MRI, is widely used for localizing prostate cancer. In focal boost treatments such as investigated in the FLAME trial, these images are used to delineate the GTV inside the prostate as target for dose escalation. For patients with a post-radiotherapy recurrence focal treatment strategies including radiotherapy are considered. Here, multi-parametric MRI is also attractive for delineation of the intraprostatic GTV. However, for patients with a post-radiotherapy recurrence, the changes that the radiotherapy has caused in the tissue result in changes in mp-MRI. It is necessary to distinguish the effects of radiation on the images from those of the recurrent intraprostatic cancer. In this presentation, data from patients treated with a salvage prostatectomy will be presented. These patients were treated for primary prostate cancer with either external-beam radiotherapy or brachytherapy. The multi- parametric MRI data will be compared to histology. To differentiate between radiation effects and recurrent prostate cancer, we conducted a matched case control

study, where MRI from patients with biopsy-proven recurrent PCa and matched controls without evidence of recurrence are compared. The localization of recurrent prostate cancer on mp-MRI is confounded by radiation effects. T2 values in the peripheral zone tend to be lower than those observed in untreated patients. Tumor T2 values are lower but not significantly different than PZ, and comparable to CG. Mean K trans and k ep seem to be the most informative to distinguish tumour, even in the presence of elevated values around the urethra that appear in both cases and controls. SP-0674 Rectum tolerance dose and repair after previous radiotherapy A.H. Ree 1 1 Akershus University Hospital, Department of Oncology, Lørenskog, Norway Abstract text In radiation oncology, the success in eradicating tumor depends on the ability to deliver the cytotoxic radiation effects in a focused tumor volume. There is a steep dose- response relationship, and small dose escalations may translate into clinically relevant improvements. In prostate cancer, the corollary to more efficacious radiotherapy is that the rectum is at higher risk of suffering morbidity. This is a particular concern when prescribing prostate reirradiation. The combined insights into vulnerable anatomical structures of the pelvic cavity and classic principles of radiobiology, together with the utilization of modern radiotherapy technology, may improve patient outcome related to rectum tolerance following radiotherapy of prostate cancer. In this context, the presentation will address a number of central topics: The therapeutic ratio principle: - Tumor control probability (TCP) versus normal tissue complication probability (NTCP) - An anatomy tour of the (male) pelvic cavity - Radiotherapy technology/technique and the pelvic cavity Rectum tolerance dose: - Assessment of radiation volume - Toxicity assessment tools -Dose-limiting toxicities relevant for salvage prostateradiotherapy Repair: Late toxicities relevant for salvage prostate radiotherapy - Normal genetic variation Predictive modeling and evaluation of radiation tolerance: - The REQUITE project - Example of a Morbidity and Mortality Review (MMR) procedure SP-0675 Salvage high-dose-rate brachytherapy for prostate cancer recurrence P. Wojcieszek 1 1 Maria Sklodowska-Curie Memorial Cancer Center and Institute of Oncology, Brachytherapy Department, Gliwice, Poland Abstract text Radiotherapy has become one of the most important methods in the prostate cancer treatment. Despite its advantages and efficacy there are patients suffering from failure. Patients with confirmed localized (i.e. organ- confined±regional lymph nodes) recurrence need radical approach, especially those with long life expectancy or good performance. There are questions to be raised, before we consider what to do or which modality should be chosen in our management. Radiotherapy failure inside target volumes means that there are cancer cells resistant to irradiation. However there are data showing