ESTRO 36 Abstract Book

S547 ESTRO 36 2017 _______________________________________________________________________________________________

different fixation systems the mean (M), systematic (Σ) and random (σ) errors were determined over the patient population for the intrafraction translations and rotations, and interfraction rotations.

PO-1001 Evaluation of target volume delineation of the regional lymph nodes in breast cancer patients M. Mast 1 , E. Gagesteijn 1 , T. Stam 1 , N. Knotter 2 , E. Kouwenhoven 1 , A. Petoukhova 1 , E. Coerkamp 3 , J. Van der Steeg 1 , J. Van Egmond 1 , H. Struikmans 1 1 Haaglanden Medical Centre Location Antoniushove, Radiation therapy, Den Haag, The Netherlands 2 Leiden University Medical Centre, Clinical oncology, Leiden, The Netherlands 3 Haaglanden Medical Centre Location Westeinde, Radiology, Den Haag, The Netherlands Purpose or Objective New ESTRO guidelines have been developed for the delineation of the Clinical Target Volumes (CTVs) of the regional lymph nodes of the breast. Until now we used the methods based on the article of ‘Dijkema et al.’. In response to these new insights, we decided to develop a tool to implement this new ESTRO guideline. The main question, which will be answered, is: “What are the differences between delineating the regional lymph nodes of breast cancer according to the method ‘Dijkema et al.’, ‘the ESTRO guidelines’ and ‘the Tool combined with ESTRO guidelines (‘Tool’)’?” Material and Methods In ten patients CTVs of the regional lymph nodes of the breast were delineated (in Pinnacle [1]) by three dedicated radiation oncologists, according to the two different guidelines and the ‘Tool’. The ‘Tool’ is a method where the subclavian and the axillary vessels are delineated by a radiation therapist and is expanded in all directions with 5 mm. This volume is than adjusted by the radiation oncologist on the basis of the prescribed anatomical boundaries of ‘the ESTRO guidelines’. After that, all CTVs were exported to MATLAB to calculate the Conformity Index generalized (CIgen ) [2]. In MATLAB the differences in the various directions on the axial coupes of the treatment planning-Computed Tomography scans were analysed. Also the volumes of the CTVs were calculated in Pinnacle. Finally, the required delineating times per patient, per guideline and per radiation oncologist were compared and analyses were carried out using SPSS [3]. [1]: Pinnacle Treatment Planning System, version 9.10 (Philips Healthcare) [2]: E. Kouwenhoven, 2009, Phys Med Biol. [3]: IBM SPSS Statistics for Windows, IBM Corp., Armonk, The MATLAB analyses showed that the ‘Tool’ had the highest CIgen (0.64 and σ = 0.05) relative to the other two methods ( p<0.04 ) (table 1) . Furthermore, the delineating time was shortest (13.6 min and σ = 2.4) by using the ‘Tool’. The use of the ESTRO guideline without the ‘Tool’ resulted in the smallest average CTV volume (150.6 cm 3 and σ = 41.0). Furthermore, we saw a clear decrease of the standard deviations in most delineating directions when using the ‘Tool’, except in the ventral direction. Table 1. The differences of the CIgen between the three methods. CIgenDijkema et al. CIgenESTRO CIgenTool Average 0,58 0,57 0,64 Standard deviation (σ) 0,05 0,06 0,05 CIgen = Conformity Index generalized Conclusion Using the ‘Tool’ we found a significantly higher CIgen and a smaller CTV volume (compared with the method 'Dijkema et al.'). The advice is to use the ‘Tool’ as NY, USA Results

Results Figure 2 shows the systematic and random errors of the intrafraction translations (a,b) and interfraction rotations (c,d) of the three different fixation systems. Intrafraction translations were small for all systems, with maximum deviations generally lower than 1 mm for all fractions, and a systematic and random error both in the order of 0.3 mm. No statistically significant differences were found between the vacuum bag and Thermofit system, while the 3D head support showed a slight improvement for the systematic errors compared to the individually moulded head supports. Intrafraction rotations were typically in the order of 0.2 ⁰ , and no differences were observed between the three groups. The systematic and random errors of the interfraction rotations were in the order of 1 ⁰ and 0.6 ⁰ for all systems, with no significant differences between the three fixation systems, and maximum rotations of up to 4 ⁰ were observed occasionally.

Conclusion Inter- and intrafraction variations were analysed fo r three different cranial fixation systems. In trafraction translations were small for all systems, while interfraction rotations could be significant. The addition of an individual head support does not seem to decrease the interfraction rotations, and for intrafraction variations the results seem to even indicate a slight improvement when using a standard head support with a shape that provides good fixation for the head. Individual supports might have added value for patients with a deviating anatomy.

Poster: RTT track: Imaging acquisition and registration, OAR and target definition