ESTRO 36 Abstract Book

S92 ESTRO 36 2017 _______________________________________________________________________________________________

decrease the risk of second IBTR with acceptable cosmetic results and low rate of late side effects.

Poster Viewing : Session 4: Brachytherapy miscellaneous

PV-0183 Microbrachytherapy: even more localised dose profiles? R. Brown 1,2 , X. Franceries 1,2 , M. Bardiès 1,2 1 INSERM, UMR1037 CRCT- F-31000, Toulouse, France 2 Université Paul Sabatier, UMR1037 CRCT- F-31000, Toulouse, France Purpose or Objective Owing to its intrinsic ability to deliver increased dose rates to tumours whilst respecting organ at risk (OAR) constraints, brachytherapy (BT) is being increasingly used for the treatment of radioresistant tumours. A new form of BT, microbrachytherapy (MBT), is proposed for small tumours. For this treatment, the grains used in BT are replaced by a solution containing β-emitters. More injections can be used with MBT than grains with BT, allowing for greater precision when targeting the tumour. As with all forms of radiotherapy, treatment planning is required. In this work, a method of generating optimal MBT treatment plans is proposed. Material and Methods The non-dominated sorting genetic algorithm II (NSGA2) [1] is used to generate treatment plans. This is a multi- objective algorithm, permitting the objective functions to be optimised independently. Two objective functions were used: the first to minimise the fraction of the tumour receiving less than the target absorbed dose (60 Gy) and the second to minimise the number of injections. The algorithm was validated on a spherical tumour of 20 mm radius. 20 mm was chosen because it represents the typical size of tumour that could be targeted with this new technique. Results The evolution of the Pareto front during the optimisation of the spherical tumour is shown in Figure 1. The optimisation finished after 200 iterations (generations), and so the final Pareto front represents the final results of the optimiser. Each point along the Pareto front represents a different treatment plan. The front can be seen as a set of compromises; it is impossible to decrease one objective function without increasing another. This enables the user to a posteriori decide relative objective importance and, hence, choose the ideal treatment plan for each patient.

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

As an example, the treatment plan using 30 injections was chosen. Its absorbed dose distribution through the central slice is shown in Figure 2. To highlight the steep absorbed dose gradient obtained with this treatment concentric spherical shells, surrounding the tumour were also included. Very satisfying treatment plans have been defined using this new method of MBC.