ESTRO 36 Abstract Book

S94 ESTRO 36 _______________________________________________________________________________________________

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

Conclusion A new form of BT, MBT, has been proposed, as well as a promising method of generating optimal treatment plans. It can be seen that the treatment plans proposed by the optimiser (NSGA2) deliver satisfactory absorbed dose distributions to the tumour, whilst sparing surrounding tissue, which in turn spares more OARs. This method can be used in real time during clinical treatment of MBT. References [1] K. Deb, A. Pratap, S. Agarwal, and T. Meyarivan, “A fast and elitist multiobjective genetic algorithm: NSGA- II,” IEEE Trans. Evol. Comput. , vol. 6, no. 2, pp. 182–197, 2002. PV-0184 Quantitative study on position margin in Intraluminal Brachytherapy Planning for lung treatment C.W. Kong 1 , H. Geng 1 , Y.W. Ho 1 , W.W. Lam 1 , K.Y. Cheung 1 , S.K. Yu 1 1 Hong Kong Sanatorium & Hospital, Medical Physics and Research Department, Happy Valley, Hong Kong SAR China Purpose or Objective In Intraluminal Brachytherapy for lung treatment, a Lumincath applicator, normally 5F flexible nylon catheter, is inserted through the Trachea and Bronchus. High activity radioactive source is loaded through the catheter for treating the tumor site. Unlike external radiotherapy, there is no motion control technique for afterloading brachytherapy treatment. Breathing motion should affect the position accuracy of Intraluminal Brachytherapy as both Trachea and Bronchus move with the breathing motion of the patient. It is not practical for the patient to do breath-hold during treatment since the whole treatment can last for several cycles of breathing depending on the source activity. The additional margin for treatment length should be considered in Intraluminal Brachytherapy to compensate such effect. The objective of this study is to investigate the position margin of treatment planning on intraluminal brachytherapy for lung treatment. Material and Methods We applied two-dimensional (2D) projection reconstruction methods to measure the movement of catheter due to the breathing motion. In 2D projection reconstruction an orthogonal pair of isocentric radiographs were taken on the patient inserted with the Lumincath catheter. By localizing difference position markers on the catheter in two separate projections, the catheter can be reconstructed in three-dimensional (3D) space for the planning calculation. The average position difference of reconstructed points between two projections reflects the accuracy of 2D reconstruction method. By comparing the reconstruction accuracy between two scenarios: patient doing free breathing and breath-hold, the impact of breathing motion on the position of catheter can be derived. In the study an orthogonal pair of radiographs were done on patients with free breathing and breath-hold; The discrepancy in the average position difference between 2D projection reconstructions with free breathing and breath-hold was