Abstract Book

S1003

ESTRO 37

Table 1. Measured and calculated OF doses (cGy) determined at 50, 100 and 200mm distance from the beam CA for Ø of 50mm IRIS collimator.

collimation helmet with different aperture sizes, depending on the treatment requirements. The validation of the model was performed by comparing MC estimations and measurements of different dosimetric parameters for a treatment delivered with 201 open sources to the head of a simulated pseudo- anthropomorphic phantom (dose prescription of 120Gy). In particular, both, absorbed dose and photon spectrum were calculated at several points of the phantom. Dose measurements were carried out with pairs of TLD-100 dosimeters placed at the same points. The TLD readings were converted into absorbed dose by means of a calibration factor (obtained at 6MV) corrected by an energy response function accounting for the change in response of the dosimeters from the 6 MV calibration beam to the softened MC beam spectrum at the measuring points. Second primary cancer risks we estimated with the calculated doses for the modelled treatment and the risk coefficient parameters from BEIR VII report. Results MC simulations reproduced the expected FWHM of the profiles at isocenter. Mean energy (from the energy spectrum) at calculation points in the phantom ranged from 1.17MeV at the treatment isocenter to 0.14MeV at points farer than 75 cm ( see figure). An absorbed dose of 200mGy was obtained for a point inside the head placed at 3cm of the isocenter. For points at mid thorax (50 cm far from isocenter), dose decreased by a factor of 10. We estimated risks of 0.3% and 0.8% for thyroid and lung cancers, respectively.

Figure 1. A) Film and RPL dosimeters attached to film holder in water phantom. B) Film calibration curve. C) OF dose measured for Ø of 50mm IRIS collimator. D) OF dose measured for Ø of 15mm IRIS collimator. Conclusion OF doses determined with Gafchromic EBT3 films were in line with IC and RPL dosimeter measurements. A great care is needed when RPL dosimeters are used near the high dose gradients. Present results demonstrate that EBT3 film dosimetry is a feasible method for measuring OF doses in RT. References 1. Peet SC et al. Medical Physics 54, 2016 EP-1858 Monte Carlo modelling of peripheral dose associated to treatments with a GammaKnife B. Sanchez-Nieto 1 , E. Doerner 1 , A. Cardona 1 1 Pontificia U-dad Catolica de Chile, Insitute of Physics, Santiago, Chile Purpose or Objective In external beam radiotherapy (RT), dose deposited outside the treatment field is associated to second primary cancer induction [1] . This is an issue of growing concern due to the larger life expectancy of cancer patients (> 5y). TPS are not designed to provide doses to peripheral organs and alternative methodologies are needed for that. Analytical models and/or set of measurements for out-of-field doses associated to external beam RT have been reported [2,3] . However, this is not the case for GammaKnife which is indeed used to treat benign brain lesions such as AVMs and Parkinson’s disease (with doses above 100Gy). The aim of this work was to create a Monte Carlo (MC) model of a GammaKnife unit in order to estimate the dose deposited along the body of an anthropomorphic phantom. Material and Methods A Leksell Gamma Knife 4C radiosurgery equipment was modelled using the egs_chamber user code, part of the EGSnrc C++ class library. The model considers the entire set of 201 Cobalt-60 sources, along with the collimation system. The collimation system includes a stationary collimator, housed on the Gamma Knife unit and a

Conclusion The use of the EGSnrc C++ class library allowed us to model accurately the GammaKnife unit, being this essential for the peripheral dose calculation. Estimated cancer risks were found not to be negligible. Therefore, peripheral doses associated to each treatment should be estimated. [1] Radiother Oncol 2012;10(5):122-126 [2] Phys. Med. Biol. 57 (2012) 6167–6191 [3] Biomed. Phys. Eng. Express 1 (2015) 045205 EP-1859 How useful are Dose Reference Levels in Radiotherapy? A regional treatment planning CT dose audit H. McCallum 1 , A. Lecomber 2 , R. Tulip 3 , A. Greenhalgh 4 , K. Pilling 1 , H. Taylor 3 , H. Bayles 4 , C.L. Chapple 2 , K. Robson 2 , J. Willis 2 , J. Byrne 1 1 The Newcastle Upon Tyne Hospitals NHS Foundation Trust, Northern Centre for Cancer Care, Newcastle upon Tyne, United Kingdom 2 The Newcastle Upon Tyne Hospitals NHS Foundation Trust, Northern Medical Physics and Clinical Engineering, Newcastle upon Tyne, United Kingdom 3 North Cumbria University Hospitals, Medical Physics Department, Carlisle, United Kingdom 4 James Cook University Hospital, Medical Physics Department, Middlesbrough, United Kingdom