ESTRO 37 Abstract book

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ESTRO 37

for patients with locally advanced cervical cancer (LACC) were radiobiologically evaluated using models fitted to in vitro data which included the effect of hypoxia. The purpose was to elucidate differences between the models in estimated tumor effect. Material and Methods NHIK 3025 cervical cancer cells were grown in vitro and irradiated with doses from 0-27 Gy. Irradiation was done both under aerobic and hypoxic conditions. Cell surviving fraction (SF) was assessed by the colony formation assay. The LQ and USC models were fitted to the experimental SFs by regression. 3D brachytherapy dose distributions for twenty retrospective patients with LACC were imported and converted into isoeffective SF maps using the two models with regression parameters from the NHIK 3025 cell line. The isoeffective contour encompassing 90 % of the high-risk clinical target volume (HR-CTV) was estimated for selected hypoxic fractions with the two Both the LQ and USC model gave satisfactory fits to the in vitro data, but the USC model tended to slightly overestimate the survival at high doses. For the brachytherapy plans, median and range of the isoeffective SF contour for fully oxygenated tumors was 0.9 (0.2, 2.5) % and 1.1 (0.4, 2.6) % for LQ and USC, respectively. Increasing the hypoxic fraction increased the isoeffective levels, but the relative difference between LQ and USC was near constant and close to unity. Conclusion For the current cervical cancer cell line, the LQ model was superior to the USC. Still, based on the radiobiological evaluation of the brachytherapy plans, it is not expected that the small differences in model performance will result in clinically meaningful differences with respect to outcome prediction. EP-2232 Feasibility of dose escalation with individualised bladder filling for intrauterine brachytherapy J. Turner 1 , M. Dillon 1 , A. Taylor 1 1 The Royal Marsden Hospital, Clinical Oncology, London, United Kingdom Purpose or Objective With image-guided intrauterine brachytherapy (IGBT) the tumour dose is limited by the pelvic organs at risk (OAR) tolerances. Our current standard of care is to treat all patients with an empty bladder. Bladder filling is used in external beam radiotherapy to displace small bowel, however some studies have demonstrated higher bladder doses when this technique is employed with IGBT. This study aims to evaluate how bladder filling can be assessed on an individual basis to allow for variations in anatomy, thus allowing a customised optimal strategy for each patient. Material and Methods Patients receiving IGBT had planning CT and MRI scans, with a full (clamped after instilling 100mls water) and empty catheterised bladder. OARs were outlined and doses calculated with a full bladder (bladder-f) and empty bladder (bladder-e) for bladder, rectum, sigmoid and bowel 2cc and 10cc dose. Brachytherapy plans were optimised using GEC-ESTRO guidelines, selecting either bladder-e or bladder-f to deliver maximal dose to HR-CTV within OAR 2cc cumulative dose constraints. Results Twenty-eight patients (26-84 years) were treated between November 2015 and September 2017. The majority had FIGO stage 2B tumours (57%); 71% cervical squamous cell carcinoma, 7% cervical adenocarcinoma, 22% other pathologies. Median HR-CTV volume was 20cm 3 , range 7.1-82.4 cm 3 with median D90 83.6 Gy. The median difference in bladder volume was 113mls, range models. Results

58-178mls. In 24 cases (85.7%) bowel doses were improved with bladder filling. The median bowel D2cc reduced from 70.2% to 49.9% with median difference 15% (range 9.8% increase to 52.7% reduction). Median bladder D2cc dose increased with bladder filling from 101.3% to 112.1% with median difference 11.8% (34% increase to 16.4% reduction). Cumulative D2cc EQD2 for bowel were bladder-e: 64.5 Gy (48.0 – 90.3), reduced to bladder-f: 56.8 Gy (47.5 – 84.7) with difference -5.5 Gy (-20.0 to +6.1). Cumulative bladder D2cc EQD2 were bladder-e: 78.3 Gy (58.4 – 103.4) and bladder-f 84.3 Gy (63.4 – 107.4) with difference 7.1 Gy (-8.2 – 17.1 Gy). In 19 cases at least one OAR dose constraint on either bladder-e or bladder-f were exceeded; bladder 8, sigmoid 5, bowel 4, rectum 2. With individualised assessment of bladder filling 9 patients had potential for dose escalation: 7 with bladder-f and 2 with bladder-e.

Conclusion This novel strategy for assessing bladder filling can significantly change bladder and bowel doses. Wide variation in individual anatomy can affect the positioning of OARs. We therefore recommend assessing both bladder empty and full scenarios to ascertain the optimal approach for the specific patient. Individualised bladder filling provides the opportunity for dose escalation in IGBT.

Electronic Poster: Brachytherapy: Head and neck

EP-2233 Treatment of Squamous Cell Carcinoma by Alpha-radiation based Brachytherapy (Alpha DaRT) A. Popovitzer 1 , R. Ran Ben Hur 1 , R. Elli 2 , D. Dimirti Bragilovski 1 , D. David Silvern 1 , I. Itzhak Kelson2 3 , Y. Yona Keisari3 4 1 Rabin Med. Ctr. Beilinson Campus, Department of Radiation Oncology, Petah-Tikva, Israel 2 Rabin Med. Ctr. Beilinson Campus, Department of oral Maxiofacial Surgery, Petah-Tikva, Israel 3 Tel Aviv university, PHYSICS, Tel Aviv, Israel 4 Tel Aviv university, Department of Clinical Microbiology and Immunology, Tel Aviv, Israel Purpose or Objective Diffusing Alpha emitters Radiation Therapy (DaRT) is a unique intra-tumoral alpha radiation based tumor ablation treatment which was developed in our laboratories. Alpha DaRT seeds loaded with Radium-224 (3.77 days half-life) are inserted into solid tumors and release by recoil short-lived alpha-emitting atoms with cumulative half-life of about 12 hrs (Rn-220, Po-216. Pb- 212, Bi-212, Po-212, Tl-208). These atoms disperse in the tumor, and spray it with highly destructive alpha radiation. DaRT provides, for the first time, an efficient method for treatment of the entire volume of solid tumors by alpha radiation. Preclinical studies confirmed the destruction of tumors of various histotypes by DaRT. We performed a first in human clinical trial to examine the feasibility, safety and efficacy of Alpha DaRT in