ESTRO 38 Abstract book

S1168 ESTRO 38

Purpose or Objective To evaluate the benefits of Apex micromultileaf collimator (mMLC) attached to the Agility multileaf collimator (MLC) against the same treatment without it attached when planning using Dynamic Conformal Arc (DCA) technique. Material and Methods Three brain metastases were prescribed to 18Gy, first one located in the central lobe, away from any critical structure. Second one located behind the optic nerves over the chiasm, and the third one in the right temporal lobe and showing irregular shape. The three locations are shown in Figure 1. Treatment was planned with Elekta Monaco treatment planning system (v. 5.11.02), and optimized using biological and physical based cost functions for isocentric DCA SRS treatment on an Elekta Synergy linear accelerator equipped with a 160-Agility MLC with of 5 mm nominal width leaves at the isocenter and an Elekta Apex mMLC attached, which has 56 pairs of tungsten alloy leaves with nominal width of 2.45 mm at the isocenter. Five non coplanar partial arcs were used, maintaining the same constraints when Apex was attached and when it wasn’t. Additionally, a VMAT treatment was planned using only Agility MLC with another constraints, as with this technique a better modulation is possible. Conformation index (CI), conformation number (CN), Heterogeneity index (HI), Homogen

Results In the first month of clinical use, the daily output of the MR-linac, as measured by the EPID pixel factor, had a coefficient of variation (standard deviation/mean) of 0.22%. The mean RMS of the daily MR to MV alignment registration compared to the baseline registration was below 0.1mm. The results are summarised in Table 2, the MLC and Jaw positions reflected by the field size measurements were all within tolerance. All MR image quality tests passed Philips thresholds and were consistent. Further results will be reported including the weekly and monthly tests.

eity index (HmI), Target coverage (TC), Monitor units MU, Modulation (UM/cGy) and Minimun Target Dose have been compared, as well as doses to organs at risk (OAR). Results Considering the TC, the Apex mMLC improves the treatment compared with the same DCA planning realized only with the Agility MLC, as it is shown in Table 1. Nevertheless VMAT planning achieves better TC at the cost of more MU. This better TC with Apex and DCA causes that some maximum doses are slightly higher, being the number of MU very similar. So as Apex is nearer to the patient, its penumbra is lower, and TC can be higher without increasing much the maximum doses to OAR. Conformation indexes are similar, being almost equal for Apex DCA and VMAT treatments.

Conclusion A comprehensive QA programme has been developed for the MR-linac, drawing on both Linear Accelerator and MRI methods. As more system data is collected, the programme will be optimised with respect to test frequency and methods, in order to reduce the overall time. The MR-linac performance in its first month of clinical use has been dosimetrically stable, geometrically stable with respect to the system alignment, with consistent MR image quality. EP-2111 Apex micromultileaf SRS Dynamic Conformal Arc treatment comparison with Agility multileaf collimator C. Ferrer 1 , C. Huertas 1 , R. Plaza 1 , R. Simón de Blas 1 , F. Sánchez 1 , E. Corredoira 1 1 Hospital universitaria La Paz, Radiofísica y Radioprotección, Madrid, Spain

Conclusion The results achieved with Apex attached in SRS DCA treatments are superior then the same treatments are planned only with Agility. However VMAT treatments attain better results, at the cost of more MU. We will have to wait to be able to perform treatments with VMAT and APEX to have a full comparison.

EP-2112 Automation of consistency and integrity checks in external radiotherapy plans