I Jornadas RSEF / IFIMED de Física Médica

Commissioning of a Siemens 160-MLC GEANT4 geometry model constructed automatically with an inhouse algorithm

No programado

Parque Científico. Salon de Actos Edificio de Cabecera (IFIC-Valencia)

Descripción

We present new features of the code developed with the GEANT4 [1] Monte Carlo (MC) toolkit, for the MC simulation of the Siemens Oncor linac that belongs to the Hospital Universitario Virgen Macarena. We have created an algorithm which allows the automatic construction of the geometry model of the multileaf collimator (MLC) 160-MLCTM [2], Siemens Medical Solutions, according to customizable parameters; hence our purpose is to show the goodness of the algorithm from a dosimetric point of view. With this purpose we compare the MC calculations, carried out with version 10.1.p01 of GEANT4, with experimental measurements through dosimetric tests which enable to optimize the following parameters: (1) tilt angle, (2- 3) intra- and interleaf leakage profile, (4) distance between opposite leaves for a closed MLC disposition and (5) leaf edge. One of the characteristic of the 160-MLC TM design is that the leaf ends follow a pattern where areas have alternated concave and convex curvatures. To our knowledge, the 160-MLC TM had only been simulated in GEANT4 thanks to a CAD software that stored the exactly position of the vertices of each leaf [3]. However our algorithm uses the information of the manufacturer directly for that intention, doing a discretization of the edge according to a specific angular tolerance. The algorithm also parametrizes the position and inclination of each leaf according to its position and aperture projected to the isocenter plane, requiring the number of leaves for each bank, the positions of the upper and lower ends and the positions of the leaf corners for a closed configuration. The experimental measurements were done with radiochromic films, placing the films at a depth of 1.5 cm in water solid phantom. The films were scanned using the Epson 10000 XL scan and the dosimetric calibration was made at a distance of 100 cm from the source, following the indications described by Arráns et al [4]. The dosimetric tests were the following: (I) Picket Fence. Dose distribution created by the superposition of two fields that alterns the positions of the odd and even leaves of a specific bank, allowing the study of the inclination of each leaf with respect of the divergent ray emerging from the source (tilt angle). (II) Transmission Field. With a setup defined by closed MLC and fully opened jaws we can evaluate the intra- and interleaf radiation leakage profiles and the distance between opposite leaves. (III) Edge Field. With a 0.5x10 cm2 field, we verify the sensitivity against the angular tolerance of the leaf end modeling. The results show a good agreement between experimental and MC data, demonstrating the validity of the algorithm to build a geometry model of the 160-MLC TM from a dosimetric point of view. This evidence displays the precision and accuracy of the algorithm for the aim of constructing any type of MLC model, including those with a great level of complexity like the 160-MLC TM. Thus, we have presented a powerful tool for the dosimetric study of small fields, that are very common in radiotherapy treatments of novel radiotherapy techniques, since it can reproduce the singularities of the MLC design in the dose distribution. Furthermore thanks to the accuracy achieved in these dose calculations, we can extend this study from homogeneous to heterogeneous mediums, with the purpose of knowing the dose uncertainties present in treatment planning systems (TPS) for that kind of situations.