ESTRO 37 Abstract book

S936

ESTRO 37

EP-1745 Differences in central line dose profiles for RayStation, Eclipse and Pinnacle, around a lung insert E. Bogaert 1 , G. Pittomvils 1 , C. De Wagter 1 1 Ghent University Hospital, Radiotherapy, Gent, Belgium Purpose or Objective Precise treatment planning in thoracic region implies correct prediction of dose in heterogeneous media. The accuracy of dose calculation depends on the performance of the calculation algorithms in these situations of electronic disequilibrium. Here, differences between algorithms become most visible for narrow high energy photon beams. In view of RayStation (RaySearch Laboratories, Sweden) being recently introduced in our department, a comparison is made with the existing Treatment Planning Systems (TPS) Eclipse and Pinnacle, regarding performance in and behind a lung insert. Material and Methods Two accelerators, Varian Clinac iX with Millennium MLC and Elekta Synergy with MLCi were modelled in Varian Eclipse v 8.9 (AAA algorithm AAA ec ) and Philips Pinnacle v 9.2 (Collapsed Cone CC Pin algorithm) respectively, before modelling in RayStation v5 (Collapsed Cone v 3.2 CC RS ). CT images of a polystyrene 12x12x12 cm 3 cubic slab phantom with a Gammex RMI lung equivalent insert (ρ = 0,3 g/cm 3 , diameter 6 cm) behind buildup of 2 cm polystyrene was used with an isotropic calculation grid of 2.5mm. Four 6MV beams (gantry 0°) with field sizes 16x16, 6x6, 3x3 and 1x10 cm 3 , 200 MU each, were evaluated separately. Both machines are absolutely calibrated according to NCS18. No density override and no dose scaling took place. Line dose profiles along the central axis were extracted for pairwise comparison. Results Figure 1 shows the resulting depth profiles. Extend and location of the lung insert are depicted. Overall agreement for the two large fields between CC RS and the other algorithms is apparent. However, AAA ec shows re- buildup behind the lung insert which is not visible with CC RS nor CC Pin . For the two small fields, the lower output for the Elekta Synergy machine is visible. This is due to accelerator head design with beam defining devices closer to the target, implying less head scatter and the fact that no scaling was used. In the lung cavity CC RS predicts a slightly higher dose for both AAA ec and CC Pin , even though the CC Pin algorithm is of the same type as CC RS . This effect is more pronounced for 1x10 cm 3 field. Also for the 1x10 cm 3 field, the line doses of CCRS and AAA ec are crossing. The tails of the depth profiles are in good agreement, except for the comparison of CC RS with AAA ec for the 1x10 cm 3 field.

EP-1746 Influence of Cable Effect on Polarity Correction Factor of Micro Volume Ionization Chamber K. Sasaki 1 , Y. Shiota 2 , Y. Miura 2 1 Gunma Prefectural College of Health Sciences, Graduate School of Radiological Technology, Maebashi, Japan 2 Iwata City Hospital, Medical Physics, Iwata, Japan Purpose or Objective In absorbed dose measurement in radiation therapy, a micro volume ionization chamber has been used for small irradiation fields and high resolution measurements. In this study, the fundamental characteristics of micro volume ionization chamber dosimeter (PTW TM31022, Exradin A 26) with improved characteristics were investigated, compared with other ionization chamber dosimeters (PTW TM30013, PTW TM31014, PTW TM31016, Exradin A1SL) , and the influence of cable effect was investigated. Material and Methods In order to measure only the cable effect, the ionization chamber and stem were removed and the cut part was insulated with silicon. And the change in the collected electric charge with respect to the irradiated length of the cable was measured. The ionization chamber was installed at a depth of 10 cm in water. Three kinds (6 MV, 10 MV, 6 MV - SRS) of beams from Varian Novalis-Tx and 6 MV FFF beam from Tomotherapy were used. The applied voltage was varied from 50 V to 400 V and measured for the negative and positive voltages. To evaluate irradiation field size dependence, the polarity effect was measured at irradiation field size 3 cm × 3 cm to 30 cm × The cable structure of the two manufacturers was almost the same, but the change in the charge amount depending on the irradiated length was different. This is due to differences in insulation materials and coatings, and the effect of dose rate dependence. In TM 31016 and 31014, the ionization amount greatly changed depending on the polarity of the applied voltage, and at low voltage, the polarity effect was large. In A1SL, the change in polarity effect correction factor was small and was 1.005 or less. The polarity effect of TM31022 was stable in the range of 150 V to 400 V (1.000 to 1.002). The polarity effect of A 26 was stable in the range of 50 V to 300 V (0.998 to 1.001). It is thought that it depends on the structure, material and working accuracy of the ionization chamber. In TM30013, there was almost no dependence of polarity effect on irradiated field size. In A1SL, the influence of field size was small, 0.2% or less, and its change was small. In TM31022 and A26, the polarity effect correction factor increased as the irradiation field increased, 1.2% in the irradiation field size 30 cm × 30cm. This is because when the ionization volume of the ionization chamber is small, the influence ratio of the cable effect becomes large. Conclusion Changes in the amount of ionization due to the cable effect are small, but contributions of 1 to 2% are considered for micro volume ionization chamber. In the ionization chamber with a large polarity effect, sufficient consideration is also necessary for ion recombination correction, and it is not necessarily good that the high applied voltage is good. Improved micro volume ionization chamber is an excellent ionization chamber with small polarity effect and ion recombination, but it was suggested that attention should be paid to the influence of cable effect in large radiation field. 30 cm. Results

Conclusion For low energy and for lung densities ≥ 0,2 g/cm3, both CC RS calculations are in good agreement with AAAec and CC Pin predictions, as was confirmed by Aarup et al. For these conditions Raystation dose calculation algorithm can be safely introduced. Further investigation on higher energies need to be performed. (Aarup et al. Rad. Oncol 91 (2009))