ESTRO 36 Abstract Book

S780 ESTRO 36 2017 _______________________________________________________________________________________________

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.

patient’s -5° gantry error was deemed unacceptable and subsequently measured, patient 2 with this error detected (gamma pass rate of 68.8%). Results The results for 10 patients are shown in Figure 1.

Figure 1. Gamma pass rate (%) for non-error (NE) plans and for deliberately introduced errors (including the Collimator (C), MLC shift (S), and MLC Field Size (F) error), where the latter are selected as exceeding dose tolerances by the smallest magnitude. *Patient 1 F error of -5 was not deliverable due to machine tolerances, hence an F error of -2 is utilised here. The global 3%/3mm gamma pass is able to detect the majority of unacceptable plans, however some MLC field size plans still pass. Decreasing the MLC field size by 1mm can result in significantly reduced dose to the PTV, which affects tumour control e.g. patient 3 MLC FS-1 passed, however this plan under-doses the PTV63Gy by -5.4% relative to the original non-error plan. Conclusion Not all deliberately introduced clinically significant errors were discovered for VMAT plans using a typical 3%/3mm (10% threshold with correction off) gamma pass rate. EP-1478 A split field beam model of Beam Modulator linear accelerator in Pinnacle treatment planning system M. Chandrasekaran 1 , S. Worrall 1 , M.K.H. Chan 1 , N. Khater 1 , C. Birch 1 1 University Hospital Southampton NHS Foundation Trust, Radiotherapy Physics, Southampton, United Kingdom 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