ESTRO 2020 Abstract Book

S61 ESTRO 2020

76 (56%) pts reported G0, G1 and G2 events, respectively. No significant MB modulation was found in G0 (at p level<0.01), while 7 genera were significantly modulated by RT in G1: Faecalibacterium, Flavonifractor, Gemmiger, Odoribacter, Phascolarctobacterium, Roseburia, Sutterella (p range: 0.003-0.008). 3 genera were significantly modulated in G2: Akkermansia, Bilophila, Faecalibacterium (p range: 0.001-0.008). When considering GU tox, 80 (59%) and 42 (31%) pts reported G1 and G2 events, respectively. No significant MB modulation (at p level<0.01) in G0 was identified.5 genera were modulated in G1: Faecalibacterium, Flavonifractor, Phascolarctobacterium, Ruminococcus, Sutterella (p<0.01). Lactobacillus was significantly modulated exclusively in pts with G2 GU tox (p=0.008). The detailed results about association between tox and MB species are reported in fig 2

Telomere shortening has been associated with a higher risk of cancer and its progression, in contrast, previous studies also reported an association between shorter telomere length and improved outcome for a variety of cancers. The purpose of this study was to evaluate the relationship between leucocyte relative telomere length (RTL) and mortality in prostate cancer patients. Material and Methods Blood samples from 533 patients were gathered before start of irradiation for prostate cancer. RTL of peripheral blood leucocytes was determined by a quantitative polymerase chain reaction method and was divided into quartiles based on the distribution in the entire cohort. The study endpoints were overall and cancer- specific mortality. Results During a median follow-up time of 149 months, 188 patients (35.3%) died, 58 patients (10.9%) died of prostate cancer. Univariate Cox regression analysis revealed a significant association of RTL quartiles and overall mortality (HR 1.20; 95% Cl 1.05 – 1.36; p = 0.006). In multivariate Cox regression, which included age at diagnosis, androgen deprivation therapy and risk group based on GS, T stage and PSA level, RTL quartiles were still a significant predictor of higher overall mortality (HR 1.22; 95% Cl 1.07 – 1.39; p = 0.003). Multivariate Cox regression analysis also showed a significant association of RTL quartiles with cancer specific mortality (HR 1.27; 95% CI 1.00 - 1.60; p = 0.047). Conclusion Longer leucocyte RTL correlates with higher mortality in prostate cancer patients treated with radiotherapy. PH-0121 Selecting the optimal fiducial marker to reduce the PTV margins for partial breast irradiation N. Hoekstra 1 , S. Habraken 1 , A. Swaak - Kragten 1 , M. Hoogeman 1 , J. Pignol 2 1 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands ; 2 Dalhousie University, Radiation Oncology, Halifax, Canada Purpose or Objective In adjuvant partial breast irradiation (PBI), the geometric accuracy of the treatment is critical to avoid geographical miss. This is even more important if the number of fractions is reduced. The tumor bed is often not clearly visible on pretreatment imaging, so fiducials can be used for daily image-guidance. There are various types of fiducials available, and it is currently unknown what the impact is of a given fiducial type on the PTV margin. The purpose of this study is to select the optimal fiducial for patient positioning in PBI regarding the required PTV margin. Material and Methods Fourteen patients from the CK-APBI trial (NL6802) were included and ten patients analyzed, excluding the first 4 participants assuming a learning curve. All patients had ≥3 tantalum surgical clips placed during lumpectomy, 3 interstitial gold markers placed outside the lumpectomy area, and a gold marker taped on the areola skin. Breath- hold planning CT and 5 daily CT scans were available for all patients. The daily CT scans were acquired with an in- room CT-on-rails integrated with a CyberKnife system. The surgical clips, interstitial markers, and skin markers were identified in all CT scans. In MIM (version 6.9.3), the tumor bed in the planning CT was registered to the tumor bed in each daily CT. Subsequently, the displacement of the center of mass (CoM) per set of fiducials was calculated. This gives the residual error for the use of fiducials for daily image guidance. We calculated the group mean Poster Highlights: Poster highlights 4 PH: Intrafractional motion

Conclusion This analysis demonstrated the possibility to quantitatively study RT-induced alteration of gut MB in a clinical setting including “real world” pts. Moreover, association between acute tox and different pattern of MB alteration was highlighted: a dysbiotic MB becomes a strongly suspected actor in appearance and evolution of RT-induced tox PH-0120 The influence of telomere length on mortality in prostate cancer patients treated with radiotherapy. T. Langsenlehner 1 , K. Lukasiak 2 , E. Thurner 2 , U. Langsenlehner 3 , H. Stranzl-Lawatsch 2 , W. Renner 4 1 Medical University of Graz, Department of Radiation Oncology, Graz, Austria ; 2 Medical University of Graz, Department of Therapeutic Radiology and Oncology, Graz, Austria ; 3 FÄZ Graz, Department of Internal Medicine, Graz, Austria ; 4 Medical University of Graz, Clinical Institute of Medical and Chemical Laboratory Diagnostics, Graz, Austria Purpose or Objective Telomeres, the end parts of linear chromosomes, play a crucial role in protecting DNA during each cell division.