ESTRO 36 Abstract Book

S790 ESTRO 36 _______________________________________________________________________________________________

Purpose or Objective The aim of this study was to model a beam modulator linear accelerator in Pinnacle v.14.0 treatment planning system for intracranial stereotactic radiosurgery and radiotherapy. Material and Methods Depth dose, beam profile and total scatter correction factor data were collected for 6 MV photons of Elekta Synergy Beam Modulator TM linear accelerator with 80 leaves each of 4 mm leaf pitch using unshielded IBA stereotactic field diode for field sizes ranging from 0.8x0.8 cm 2 to 4x4 cm 2 and field sizes above 4x4 cm 2 up to 16x21 cm 2 using IBA CC04 pinpoint chamber. The measured data were imported in to the photon physics module of Pinnacle v.14.0 and physical accelerator head specific data such as primary collimator, flattening filter, MLC were input in addition to beam data measurements. The auto modeler of Pinnacle TPS was iteratively used to adjust parameters such as photon beam energy spectrum, Gaussian height and width of the photon source that affect various regions of the depth doses and beam profiles to match measured data. Dose grids of 1 mm and 2.5 mm were used for beam modelling of fields from 0.8x0.8 cm 2 up to an equivalent square field of 6.7 cm 2 and above 6.7 cm 2 respectively. A common photon energy spectrum did not prove sufficient to achieve the required agreement between Pinnacle calculated and measured depth doses and beam profiles for the whole range of field sizes. This was overcome by a split field model that employs field size specific beam energy spectra, with higher relative weights of low energy bins and lower relative weights of high energy bins for small fields and vice-versa for field sizes larger than 6.7 cm 2 . The validity of the model was tested independently using a Standard Imaging Exradin A26 chamber in LUCY phantom for field sizes ranging from 0.8x0.8 cm 2 by comparing calculated and measured absolute doses and relative output factors. Results Optimization of photon beam energy spectrum specific to small field sizes improved the agreement of depth doses both in and beyond build-up region for the small fields. Measured versus calculated absolute planned doses were found to be within 1% for field sizes larger than 1.6x1.6 cm 2 and less than 2.5% for 0.8x0.8 cm 2 field. The agreement between the measured and calculated relative output factors were within 2% for field sizes larger than 1.6x1.6 cm 2 and less than 3.5% for 0.8x0.8 cm 2 fields.

Conclusion A split field model was generated for the whole range of field sizes of a beam modulator linear accelerator from 0.8x0.8 cm 2 to 16x21 cm 2 using field size dependent photon beam energy spectra. The model was successfully validated independently and was found have a good agreement with measured doses and relative output factors. EP-1479 Gamma 3D analysis for VMAT treatments using two detector arrays E.M. Ambroa Rey 1 , D. Navarro Jiménez 1 , A. Ramirez Muñoz 1 , R. Gómez Pardos 1 , D. Amat de los Angeles 1 , A. Gibert Serrano 1 , A. López Muñoz 1 , M. Parcerisa Torné 1 , M. Colomer Truyols 1 1 Consorci Sanitari de Terrassa, Medical Physics Unit- Radiation Oncology Department, Terrassa, Spain Purpose or Objective The development of advanced radiation therapy techniques, such as volumetric modulated arc therapy (VMAT), requires a patient-specific pre-treatment quality assurance (QA). Two-dimensional array detectors are widely used for dose distribution verifications and the 3D gamma index is one of the metrics which have been extensively used for clinical routine patient specific QA. The aim of this study is to evaluate the 3D gamma index for different VMAT plans, such as head and neck (H&N) and prostate, with the Octavius 4D system using two 2D-arrays (PTW Octavius4D 1500 and PTW Octavius4D 729).