Influence of the grid size on the dosimetric characteristics of IMRT beams and on overall treatment plans G. Pittomvils1, L. Olteanu1, B. Vanderstraeten1, C. De Wagter1, Y. Lievens1. 1Ghent University Hospital, Radiotherapy OP7, Gent, Belgium. OBJECTIVES METHODS NCS report 22 states that “an appropriate calculation grid size for dose calculation is advised and special attention should be given to the shape of the depth dose curves for each clinically used grid size”. The same report recommends to verify output factors and depth dose curves of small fields. 5 % agreement between measured and planned data is recommended.  During commissioning of a new model in Pinnacle of the Elekta linacs of the department, special attention was given to the limits of the small IMRT beams modeling in order to apply the most appropriate calculation grid size. Tangential (60°), in-axis and off-axis fields are often used in IMRT treatments and those fields were investigated with a width varying from 8 mm to 30 mm. The length of the fields was 60 mm. Percent depth dose curves were computed and compared using 4/2/1 mm grid sizes and output factors were measured using a small cylindrical and a liquid filled ionization chamber for off-axis field settings. Chamber orientation was selected for maximal spatial resulotion; axial for the pinpoint chamber (PTW 31006) and lengthwise parallel for the liquid ion chamber (PTW 31018). Both chambers are dedicated for small beams but both are underestimate also the output of the smallest field due to the volume effect. The influence of the grid size on treatment plans was evaluated by comparing TPS calculated data with data recorded by the cylindrical Delta-4 phantom (Scandidos AB, Upsala Sweden) RESULTS Figure 1 : OF calculation versus Figure 2 : Orthogonal DDC measurement for 1-2-4 mm³ grid size Figure 3 : Tangential DDC Table 1 : Gamma evaluation results, version 9.0 vs version 9.8 for 2 and 4 mm³ grid sizes Figure 4 : Clinical IMRT beam measured on Delta-4 phantom (green curve) versus planned dose curve using 4 mm³ grid size (red curve) Output factors are compared with measurements for 3 cm off axis fields at SSD=90 cm depth 10 cm. A good mutual agreement is observed between 1mm³ and 2mm³ grid sizes. Dose prediction is also within tolerances (5%) with the measurements for field width ≥ 10 mm. For the 4 mm³ grid size large deviations are observed for widths <18 mm (figure 1). Orthogonal in axis beams show the typical pattern (figure 2), a build up zone and an exponential decay. A significant decraese in calculated dose is observed for 4 mm³ grid size compared to 2/1 mm³. A similar output difference is observed for off axis and tangential beams but the typical profile is lost for field widths <14mm (figure 3). In figure 4 an individual IMRT beam parallel to one of the detectors plates of the delta-4 phantom of a breast treatment with lymph note involvment is depicted. A similar pattern as for small tangential fields is observed. Such breast treatments are critical while oblique off axis beams are included in the treatment protocol. In table 1 the differences in the 3%/3mm Gamma evaluation on delta-4 is depicted. The new Pinnacle model (9.8), a model with more accurate leaf tip modelling, is evaluated for 1/2 and 4 mm³ grid sizes and compared to the old Pinnalce model (9.0). The influence of the grid size on the Gamma index is obvious and more significant than the model version number. References CONCLUSIONS The influence of the grid size on de the dose prediction in the treatment planning system is confirmed. Commisionning tests revealed inaccuracy for the 4 mm grid sizes for small, in axis, off axis and oblique segments. This inaccuracy increases with decreasing field size. This inaccuracy was confirmed on Delta-4 pre-treatment evaluations. The selection of an appropriate grid size was of more importance than the selection of a new better leaf tip model. NCS-22 : Code of practice for the quality assurance and control for intensity modulated radiotherapy, 2013, E. van der Wal, J. Wiersma, A.H. Ausma, J.P. Cuijpers, M. Tomsej, L.J. Bos, G. Pittomvils, L. Murrer, J.B. van de Kamer