ESTRO 2021 Abstract Book

S1359

ESTRO 2021

longer irradiation times compared to the other applicators.

PO-1638 MLC modeling optimization for a TrueBeam linear accelerator in RayStation TPS based on TG119 P. Sánchez-Rubio 1 , A. Montes-Uruén 1 , M. Pinto-Monedero 2 , J. Martínez-Ortega 1 , M.Á. Arroyo-De La Cruz 1 , A. López-Corella 3 , M. López-Torres 1 1 hospital Universitario Puerta De Hierro Majadahonda, Medical Physics, Majadahonda, Spain; 2 hospital Universitario Puerta De Hierro Majadahonda, Medical Phyiscs, Majadahonda, Spain; 3 hospital San Pedro, Medical Physics, Logroño, Spain Purpose or Objective The quality assurance protocols for TPS propose simple geometries for the validation of the beam modeling. This procedure can lead to a model that doesn’t provide the dosimetric accuracy required for complex clinical treatments, especially in IMRT techniques, requiring additional tests to refine the model. The objective of this work is to show how the use of treatment plans representative of clinical practice allows optimizing the modeling of the MLC in the RayStation TPS. Materials and Methods A TrueBeam accelerator with energies 6 WFF and 10 WFF and 6 FFF, 120 HD-MLC, was modeled in RayStation vs 8.0 for collapsed cone algorithm. The configuration parameters of the MLC (transmission; rounded effect of the leaves (leaf-tip offset, gain and curvature), leaf-tip width, and tongue and Groove (T&G) were determined experimentally by adjusting the calculated profiles to the measured profiles for different field sizes collimated only by the MLC (1x1 cm 2 - 30x30 cm 2 ). The initial model, for each energy, was verified using the tests for photon beams indicated in the Practical Guide of medical physics 5.a5 of the AAPM. In addition, dIMRT plans were made in the three energies for the geometries and planning objectives proposed in the TG-1197 report: 7 beams for prostate and 9 beams for H&N, C-Shape easy, and C-Shape hard (fig1). Each plan was calculated for the initial model and the successive modifications of the same. VMAT was not used to rule out that the discrepancies between the calculated plans and the experimental measurements could be due to synchronism problems inherent to this technique (synchronization between MLC, gantry speed and dose rate). The absorbed dose calculated in each model and energy, both in PTV and in the relevant OAR, was compared with the absorbed dose measured with the pinpoint 3D 31016 (PTW) IC inside IMRT pelvic phantom (IBA).

Results With the initial MLC parameters (Table 1), a maximum discrepancy of -4.15% was obtained for PTV in a real prostate plan and -8.10% in spina cord for the H&N plan for 6 WFF. The large differences detected in the C- Shape hard plan are considered due to the strong gradient at the measurement point in the sparing zone, as can be seen in fig. 1. The T&G was not modified for any energy, while for 10 WFF, it was only necessary to modify the offset. However, at 6 FFF all parameters were changed, including gain. The choice of the final model required a compromise regarding the dosimetric precision to be obtained between PTV and OARs. The final configuration values achieved differences less than 1.5% (-0.03% - 1.63%) for PTVs and 2.5% (-1.18% - 2.44) for OARs.