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ESTRO 37

Conclusion With the proposed fully automated re-optimization technique, the percentage of daily plans with adequate target coverage increased from 1.1% to 97.7%, and all plans showed decreased OAR doses. With an average re- optimization time of 3 minutes, this method is an important step towards real-time adaptive proton therapy.

optimization towards a Pareto optimal solution for the new anatomy. The method was evaluated using the planning CT and 7-9 repeat CTs of 11 prostate cancer patients. Prostate, lymph nodes and seminal vesicles were delineated as target structures. Dose was prescribed according to a simultaneously integrated boost scheme assigning 74 Gy to the high-dose PTV, (prostate) and 55 Gy to the low- dose PTV (lymph nodes and seminal vesicles). Re- optimization was performed for each repeat CT using tight margins of 2/3.5 mm (high-dose/low-dose PTV), only meant to account for intra-fraction motion. The original plan was created with enlarged margins (4/7 mm) to provide a spot placement covering most target deformations in the repeat CTs. The original and re- optimized spot positions and weights were evaluated on the repeat CTs using the 2/3.5 mm-PTVs. Results Evaluated on the repeat CTs, the original plans achieved sufficient target coverage (PTV V 95% ≥ 98% and V 107% ≤ 2%) in only 1 out of 88 CTs (see Fig 1). With re-optimization this improved to 86/88 CTs. Fig 2 shows the differences between the original and re-optimized plans for the OARs. Median improvements up to 12%-point for the rectum and bladder V 45Gy were observed, with outliers up to 35%-point. For most CTs, all dosimetric parameter values were lower for the re-optimized plans. Four CTs had a lower bladder D mean for the original plan. All re- optimizations were completed within 3.5 minutes (average 3 min).

Proffered Papers: RTT 1: Innovative strategies for improving patient care

OC-0091 Prognostic value of calcium score in breast cancer patients treated with radiotherapy C. Roos 1 , V. Van den Bogaard 1 , M. Greuter 2 , R. Vliegenthart 2 , E. Schuit 3 , J. Langendijk 1 , A. Van der Schaaf 1 , A. Crijns 1 , J. Maduro 1 1 University of Groningen- University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands 2 University of Groningen- University Medical Center Groningen, Center for Medical Imaging, Groningen, The Netherlands 3 University Medical Center Utrecht, Julius Center for Health Sciences and Primary Care, Utrecht, The Netherlands Purpose or Objective Recent studies on dose-effect relationships showed that acute coronary events (ACE) already occur in breast cancer (BC) patients within the first 9 years after RT and that the absolute excess risk depends on pre-existent cardiovascular risk factors and age. Therefore, it becomes increasingly important for radiation oncologists to identify which baseline factors are important for BC patients. The coronary artery calcium (CAC) score is a well-established and reliable early predictor of ACE in the general population. The CAC score is normally measured using diagnostic electrocardiogram (ECG) triggered Computed Tomography (CT) scans. In this study we tested whether pre-treatment CAC based on non-triggered planning CT scans was associated with the cumulative incidence of ACE among BC patients treated with postoperative RT. Material and Methods The study population consisted of 939 consecutive female BC patients treated with RT. The CAC score was established on non-triggered pre-treatment RT planning CT scans. The CAC score was classified into widely used clinical CAC score categories: CAC zero (0), low CAC (>0 - <100), intermediate CAC (100 – 400) and high CAC (≥400). Due to limited number of events in the high CAC score category the high CAC score category was combined with intermediate category to maintain sufficient statistical power. The association between CAC and ACE was tested using Cox-proportional hazard models. Known cardiovascular risk factors for ACE and the individual mean heart dose (MHD), collected from the three- dimensional CT planning scans, were tested for confounding of the association between CAC and ACE. Results CAC scores varied from 0 to 2,859 (median 0). The 9-year cumulative incidence of ACE was 3.2% and was significantly associated with the pre-treatment CAC score (low CAC p=0.043, Intermediate + high CAC p=<0.001). After correction for confounders, age, history of ischemic heart disease, diabetes, Body Mass Index ≥30, MHD, hypercholesterolemia and hypertension, the hazard ratio for ACE for the low and the combined intermediate and high CAC score category were 1.42 (95% CI: 0.49-4.17; p=0.519) and 4.95 (95%CI:1.69-14.53; p=0.004) respectively, compared to the CAC zero category.