ESTRO 36 Abstract Book

S93 ESTRO 36 2017 _______________________________________________________________________________________________

calculated. Such comparsion was done for 12 times on different patients. Results The average position difference between two radiographs in the breath-hold reconstruction was 1.3 ± 0.5 mm among different patients. Such difference was greatly increased to 6.5 ± 2.5 mm in free-breathing reconstruction. Assume the position difference in the reconstruction due to breathing motion was independent from other factors such as isocenter precision and reconstruction calculation accuracy, the derived average position error of catheter in the reconstructions due to breathing motion was 6.4 ± 2.5 mm. Conclusion Our study showed that in Intraluminal Brachytherapy for lung treatment, the breathing motion can significantly affect the catheter position by 6.4 ± 2.5 mm on average. Position margin of such value should be added in the treatment length during Intraluminal Brachytherapy planning to compensate such effect. PV-0185 Retina dose as risk factor for worse visual outcome in 106Ru plaque brachytherapy of uveal melanoma G. Heilemann 1 , L. Fetty 1 , M. Blaickner 2 , N. Nesvacil 3 , D. Georg 3 , R. Dunavoelgyi 4 1 Medical University of Vienna/AKH Vienna, Department of Radiotherapy, Vienna, Austria 2 Austrian Institute of Technology GmbH, Health and Environment Department Biomedical Systems, Vienna, Austria 3 Medical University of Vienna/AKH Vienna, Department of Radiotherapy/Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Vienna, Austria 4 Medical University of Vienna/ AKH Vienna, Department for Ophthalmology and Optometry, Vienna, Austria Purpose or Objective Visual acuity is a common side effect in 106 Ru plaque brachytherapy. The purpose of this study was to evaluate the retina dose as a risk factor associated with visual outcome. Material and Methods 45 Patients treated with 106 Ru plaque brachytherapy were included in this retrospective study. A minimum of 100 Gy was prescribed to the tumor apex using one of two available plaque (types CCB, CCA) manufactured by BEBIG (Eckert & Ziegler, Germany). Treatment planning and dose calculation was performed using an in-house developed 3D treatment planning system with Monte Carlo based dose calculation. Dose volume histograms (DVH) were generated for both physical absorbed dose and biological equivalent dose (BED), according to the definition introduced by Dale and Jones [1]. Visual acuity was reported using Snellen charts. To analyze potential predictors in anterior tumor locations, a subgroup of 20 patients was selected presenting with a minimum distance of 5 mm between tumor and macula. Statistical calculations were performed in SPSS (version 21, IBM). Risk factors associated with loss of visual acuity were evaluated using the Cox proportional hazards models. The loss of visual acuity was correlated to risk factors using Pearson correlation coefficients. Statistical significance was assumed to be p ≤ 0.05. Results Median follow-up time was 29.5 months (IQR, 15.0-29.8). A median apex dose of 131 Gy (IQR, 113.0-150.4) was delivered to tumors with median apex heights of 4.6 mm (IQR, 3.5-6.0)), largest basal diameters of 10.8 mm (IQR, 8.3-12.6) and smallest diameter of 9.3 mm (IQR, 7.9- 11.4). The baseline visual acuity (Snellen 0.82 ± 0.23 SD) was significantly higher (p < 0.001) than the mean visual acuity at last individual follow-up (0.59 ± 028 SD). The Pearson Correlation analysis showed a significant

correlation of visual acuity loss with the mean (r = 0.49, p = 0.001) and maximum (r = 0.47, p = 0.001) retina dose and tumor basal diameter (r = 0.50, p < 0.001). The dose to the macula showed no correlation with visual outcome (r = 0.24, p = 0.12). In the subgroup of patients with anterior tumor locations the maximum retina dose remained the only predictive factor (r = 0.46, p = 0.043). Evaluating the Cox proportional hazards model yielded a significantly higher risk for visual acuity loss (of more than 0.3 Snellen) for patients receiving a maximum dose of 500 Gy or higher (p = 0.009). A Cox multivariate analysis including the macula dose (p = 0.11) and basal diameter (p = 0.78) showed that a high maximum retinal dose is the highest risk factor (p = 0.017). The evaluation of the BED metrics showed no better correlation with the investigated endpoints and in some cases BED was even inferior. Conclusion The study showed that retina dose (D 2 and D mean ) is a suitable predictor for visual acuity loss, especially in case of anterior tumors where other risk factors (i.e. basal diameter) fail. References [1] R.G. Dale and B. Jones. The clinical radiobiology of brachytherapy. Br. J. Radiol. 71 , 465-483 (1998) PV-0186 MaxiCalc: a tool to calculate dose distributions from measured source positions in HDR brachytherapy M. Hanlon 1 , R.L. Smith 2 , R.D. Franich 1 1 RMIT University, School of Science, Melbourne, Australia 2 The Alfred Hospital, Alfred Health Radiation Oncology, Melbourne, Australia Purpose or Objective Dosimetric treatment verification via source tracking in HDR brachytherapy requires evaluation of the delivered dose as source dwell positions are detected. Current TPSs are not configured to perform this function, hence a fast dose calculation engine (DCE) that can accept the input of arbitrary dwell positions from the source tracking system is required. Here we present a TG-43 based DCE that computes 3D dose grids for measured dwell positions and performs a comparison with the treatment plan. Material and Methods The DCE, dubbed MaxiCalc, takes the input of measured dwell positions and times and calculates a dose grid of nominated dimensions and grid spacing for direct comparison to the treatment plan. MaxiCalc was validated against Oncentra Brachy (OCB v4.3) at 27 single dose points, as per OCB commissioning, as well as a 3D dose grid of 13 dwells. Dwell positions and times delivered in a phantom were measured by our source tracking system, as previously published. 1 The measured dwell positions were then used as input to MaxiCalc and the resultant dose grid compared to that from OCB. Observed dose differences due to source position measurement uncertainties were investigated. Results For the 27 dose points, MaxiCalc differed from OCB by a mean of 0.08% (σ=0.07%, max 0.41%) demonstrating differences that are similar to those between published values 2 and OCB. In a multi-source plan for doses between 50-200% of the prescription dose, MaxiCalc yields a maximum difference of <1%, which arises due to minor calculation differences in the steep dose gradients near the source. There was a gamma pass rate of >99% at 1mm/1%. A dose grid was calculated for a plan of 25 dwell positions acquired using our source tracking system, there was maximum difference of 12.2% (mean = 0.7%). The maximum difference arises from a small shift in the apparent dwell positions causing large differences due to the high dose gradients near the source, which is only significant within 10 mm of the source. For this volume of