ESTRO 36 Abstract Book

S965 ESTRO 36 _______________________________________________________________________________________________

Results As can be seen in Figure 1, the transmission through a closed lead endplate is comparable to that for the Cerrobend.

attempts to correctly position the shielding blocks was recorded for each beam.

Results We succeeded in positioning the shielding blocks from the first attempt in 10 out of 12 beams for the three patients. The position of the shielding blocks was adjusted only one time prior to treatment in 2 out of 12 beams. These results are compared to an average of 3 attempts per beam for each patient using the conventional technique of trial and error. The average time of a treatment session was 29 min with a maximum time of 41 min compared to an average of approximately 60 min in past treatments and a maximum of 120 min. Conclusion Most of TBI patients are pediatric patients and it is difficult to keep them immobilized for a long period of time. This new technique succeeded in reducing the length of the overall treatment session of the conventional TBI procedure and hence reduced patient discomfort while ensuring accurate shielding of the lungs. EP-1753 Determining the effect of using lead as electron cutout material compared to low melting point alloy M. Wanklyn 1 , S. Rizkalla 1 , T. Greener 1 1 Guy's and St.Thomas' Hospital NHS Foundation Trust, Radiotherapy Physics, LONDON, United Kingdom Purpose or Objective The aim of this investigation was to determine whether lead cut-outs are suitable for delivering MeV electron treatments on a Varian TrueBeam which have been planned using the eMC algorithm in Eclipse. Due to the eMC algorithm beam data being configured using Cerrobend low melting point alloy as the cut-out material it is important to assess the dosimetric differences between the lead and Cerrobend cut-outs. Material and Methods Unlike the Cerrobend cut-outs which are 1.5cm thick, the lead cut-outs were made to 1cm thickness. This was done to minimise the cost of lead. Lead versions of all the standard Varian cut-outs were made in house (6x6, 10x10, 6x10, 15x15, 20x20 & 25x25cm 2 ). Two regular cut-outs were also made, a 4x8 cm 2 cut-out for the 10x10 cm 2 applicator and a 10x14cm 2 cut-out in a 15x15 cm 2 applicator to determine the out-of- field transmission. Transmission factors through a 10x10 cm 2 closed end plate were calculated for the lead and Cerrobend materials for a range of energies (6, 9, 12, 16, and 18MeV) PDDs in water at 100cm SSD and output factors in solid water at d max at 100cm SSD were measured for the standard applicators with both the lead and Cerrobend inserts for all energies. Cross line and inline profiles at d max were taken in water at 100cm SSD for all energies using the two regular cut- outs.

Figure 1: Transmission through a closed endplate in a 10x10cm^2 applicator for both materials. There is higher transmission through the lead endplate compared to the Cerrobend endplate for all energies but even at the highest energy the difference is only 0.74%. The measured PDDs agree with each other to within 1.2mm for all energies. The discrepancies were observed near the surface of the PDD curves. The output factors measured in solid water using the lead inserts agreed with the commissioning values obtained with the Cerrobend inserts to within ±1% apart from the 6e 10x6cm 2 applicator in which a 1.7% difference in output factor was observed between the lead and Cerrobend. This difference could be due to inaccuracies in the solid water setup as the effective point of measurement of the NACP was estimated to be 1mm. The profiles measured agreed very well, with the largest discrepancies occurring out of field for the higher energies both crossplane and inplane. This is due to the higher transmission through the lead cut-out at higher energies. Conclusion As there was very good agreement between the lead and Cerrobend inserts and cut-outs for all the tests performed, it can be concluded that using the lead cut-outs is dosimetrically similar to the Cerrobend inserts with which the eMC algorithm was configured. EP-1754 Isocentric accuracy of Elekta VersaHD linear accelerators E. Kouwenhoven 1 , J. Van Egmond 1 , J. Van Wingerden 1 , M. De Goede 1 , M. Mast 1 , J. Van Santvoort 1 1 Haaglanden Medical Centre Location Antoniushove, Radiation therapy, The Hague, The Netherlands Purpose or Objective The demands on isocentric accuracy are high when accelerators are used for stereotactic treatments. The determination of the optical or mechanical isocenter is inadequate for this purpose, and instead we aim for a procedure to find the megavoltage isocenter. The radius of the smallest sphere through which all rotation axes pass when various collimator, gantry and table angles are applied, is what we use as a measure to quantify the isocentric accuracy. The purpose of the present study is to (1) give an accurate measure of the size of the region containing the megavoltage isocenter, and (2) establish the distance between the rotation axes of table and collimator, respectively. We developed a highly accurate method to determine the isocentric accuracy of Elekta VersaHD linear accelerators. Material and Methods The tests were performed on all 4 Elekta VersaHD accelerators in our institute. We applied a modified Winston-Lutz test, using a phantom containing a radio- opaque ball, and imaged this phantom onto the EPID using a 10 x 10 cm 2 field. Using the information of a considerable