ESTRO 37 Abstract book

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ESTRO 37

G(1,1) primarily failed in the low dose regions where Acuros overestimates the dose compared to MC, as seen on Fig 2. However, G(1,1) was always larger than 70% and in 60% of the patients G(1,1) ≥ 90%. G(2,1) > G(1,2), showing larger dependency on dose than DTA. In general for the volumes considered, the mean dose deviation was < 1% with SD < 2%.

Material and Methods Although the arm of the trUPoint ARCH can be changed from left to right and vice versa to place it in the opposite position to the injury, in bilateral multifocal metastasis this is not possible. The attenuation is evaluated in a real case, with a patient with bilateral metastasis where the most unfavorable one is selected for the study (Fig. 1 b). A treatment plan is created with the treatment planning system Eclipse v.11 from Varian Medical System (VMS; Palo Alto, USA) with three non- coplanar arcs of Volumetric Arc Therapy (VMAT) in a TrueBeam STx. The complete immobilizer is contoured independently in three sets of different structures (Fig. 1 c,d,e): 1) by considering it part of the structure “body”; 2) by contouring it as "Support" structure with 300 Hounsfield unit (HU) because these kind of structure are taken into account in the calculation but the user have to specify a constant value of HU for the entire structure ; 3) only the patient's body is contorted, leaving the immobilization support outside the calculation. In each of the sets of structures, a new plan is created, where the previously optimized fields are added, giving rise to three plans, identical to each other except for the structure of the immobilizer. A normalizing point is created inside PTV (ICRU point), where each of the plans is normalized. By using the plan that is not taken into a count the immobilizer as a reference, the percentage of extra monitor units needed to give the same dose is calculated. This percentage is equal to the dose error, because dose is proportional to UM.

Conclusion The Acuros dose calculation algorithm v.13.7 was benchmarked against the SciMoCa MC algorithm and an excellent agreement between the algorithms was found based on clinical treatment plans, which testifies for the high quality of both algorithms. EP-1833 Attenuation factor of the radiosurgery inmobilizer Trupoint Arch H. Perez-Garcia 1 , J. Castro Novais 1 , S. Ruiz Maqueda 1 , I.J. González Vecín 2 , M.Y. Molina López 1 1 Hospital Universitario QuirónSalud Madrid, Medical Physics, Madrid, Spain 2 Hospital Dr. Negrin, Medical Physics, Las Palmas, Spain Purpose or Objective The trUpoint ARCH ™ immobilization system of CIVCO Radiotherapy (Coralville, Iowa, USA) for Stereotactic RadioSurgery (SRS) (Fig.1 a) allows less than 1 mm of patient intrafraction movement according to the manufacturer's specifications, but in turn, it introduces new attenuating elements between the beam and the radiotherapy Planning Target Volume (PTV). In this work, we estimate how much can be the deviation in doses in the case of not considering this factor.

Results It is possible to commit an error around 2.5% of the dose if the immobilizer is not taking into account (table 1), so it is necessary to include the trUpoint ARCH immobilizer in the calculation of doses. No significant differences (around 0.2%) were found between including the immobilizer as a support structure or as part of the patient's body. For convenience, this second option should be the one chosen, as well as being easier, it is a more accurate calculation (it is made with the HU measured from the computational tomography (CT) instead of taking a constant value of HU). Conclusion