Monte carlo simulation of different source configurations for a new design of rotating Gamma Knife system

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Abstracts / Physica Medica 32 (2016) 251–273

INVESTIGATION OF THE AGFA HIGH DOSE CASSETTE FOR QUALITY ASSURANCE IN RADIOTHERAPY I.A. Tsalafoutas *, P. Birmbakos, K. Kourinou, P. Georgolopoulou Medical Physics Department, General Anticancer Oncology Hospital of Athens ‘Agios Savvas’, 171 Alexandras Avenue, 115 22 Athens, Greece ⇑ Corresponding author. Introduction. The Agfa CR system is comprised by the CR 35-X digitizer, the NX workstation and the CR RT1.5 high dose cassette (HDC). The HDC has a nominal tolerance of about 400 MU and is designated for portal imaging in radiotherapy, during irradiation of the patient with the full field dose. Purpose. To investigate the characteristics of the HDC and its applicability to radiotherapy quality assurance (QA) tests. Materials and methods. The HDC was irradiated with doses in the range of 0.02–4.1 Gy, using different: (a) field sizes, (b) irradiation geometries and, (c) beam energies (6 MV, 15 MV, wedged fields), in order to determine the dependence of its response on the above conditions. Results. An exponential equation was found to accurately reproduce dose from pixel values, for single irradiated 10  10 cm2 fields. However, this equation was not accurate for irradiations made using other field sizes, multiple and complex field irradiations. Typical errors were within ±20% but larger errors were observed in some extreme irradiation conditions. The main problems identified were: the increased response of the HDC to lower energy scattered photons and the non-uniformities observed across its area. However, concerning geometry, the accuracy of distance measurements was better than ±0.2 mm. The HDC was also proven useful for testing the dynamic multileaf collimator and the enhanced dynamic wedges. Conclusion. The HDC is very useful as a first level QA tool, mainly for geometrical tests but also for dosimetry tests, when relative rather than absolute dosimetry issues are under investigation. http://dx.doi.org/10.1016/j.ejmp.2016.07.596

CRANIO-SPINAL IRRADIATION OF PEDIATRIC PATIENTS USING VOLUMETRIC MODULATED ARC THERAPY C. Paraskevopoulou *, M. Synodinou, G. Kollias, N. Yakoumakis, E. Koutsouveli, P. Karaiskos, C. Dardoufas Radiation Oncology Center, Hygeia Hospital, Er. Stavrou 4 and Kifisias, 151 23 Athens, Greece ⇑ Corresponding author. Introduction. Cranio-spinal irradiation (CSI) used for the management of certain primary brain tumours is one of the most challenging processes in radiation planning and delivery. Volumetric modulated arc therapy (VMAT), a recent advantage of radiotherapy technology can provide highly conformal radiation dose distributions around complex target volumes, while minimizing the dose to adjacent organs at risk (OARs), in an efficient way (i.e. reasonable treatment delivery times comparable to 3D conformal radiotherapy). Purpose. To report the treatment of cranio-spinal irradiation with volumetric modulated arc therapy in comparison to the commonly used conventional 3D conformal radiotherapy (3D-CRT). Materials and methods. Five children were treated with CSI using VMAT on an Elekta Versa linear accelerator. Multiple arc arrangements with two isocentres were used. Supine position was utilized for all patients. Plan quality and DVH-indices used for plan evaluation and approval were compared to corresponding values derived using 3D conformal plans. Results. VMAT plans were found superior in all criteria clinically used for plan evaluation and acceptance including dose distributions, plan quality indices such as target coverage and dose homoge-

neity as well as doses to OARs and thus VMAT was used for patient treatments. Conclusion. The use of VMAT provides excellent target coverage and sparing of the adjacent OARs, with absorbed doses well within the published pediatric dose constraints, in relatively short treatment time (less than 5 min) and thus being superior in treating patients requiring CSI compared to the currently used 3D-CRT. http://dx.doi.org/10.1016/j.ejmp.2016.07.597

CORRECTION FACTORS FOR THE IBA RAZOR DIODE FOR OUTPUT FACTOR MEASUREMENTS IN PERFEXION GAMMA KNIFE SYSTEM N. Yakoumakis *, G. Kollias, C. Paraskevopoulou, E. Koutsouveli, P. Karaiskos Medical Physics Department, Hygeia Hospital, Er. Stavrou 4 and Kifisias, 151 23 Athens, Greece ⇑ Corresponding author. Introduction. A new formalism has been recently proposed for the dosimetry of small and non-standard fields. In terms of output factor (OF) measurements, a correction factor k(Q(clin),Q(msr)) (f (clin),f(msr)) is introduced, that accounts for differences in the detectors response in a given clinical field, f(clin), with regard to the machine-specific reference field, f(msr). Purpose. To derive these correction factors for the 4 mm and 8 mm collimators of Perfexion Leksell gamma knife (LGK) system for the new Razor diode detector providing by IBA for small filed dosimetry. Materials and methods. In Perfexion system, f(msr) is defined by the 16 mm collimator. OF measurements were performed for the 4 mm and 8 mm collimators (f(clin)), using the solid water, 16 cm in diameter LGK dosimetry phantom, along with a custom made insert facilitating accurate detector positioning. The k(Q(clin),Q (msr)) (f(clin),f(msr)) factor of the Razor diode detector was determined, by comparing the ratio of detector readings in f(clin) (4 mm and 8 mm collimators) and f(msr) (16 mm collimator) to corresponding MC reference OF data. Results. Razor diode was found to slightly overestimate both f (clin) OFs with overestimation being greater for the smaller 4 mm collimator reaching up to 4% and almost negligible (<0.6%) for the 8 mm collimator. Conclusion. IBA Razor diode can be safely used for OF measurements in gamma knife perfexion system. However, a correction of the order of 4% must be used for accurate determination of the smallest 4 mm collimator. http://dx.doi.org/10.1016/j.ejmp.2016.07.598

MONTE CARLO SIMULATION OF DIFFERENT SOURCE CONFIGURATIONS FOR A NEW DESIGN OF ROTATING GAMMA KNIFE SYSTEM G. Mora a,*, O. Chivani b, J. Li b, C.M. Ma b a Institute of Biophysics and Biomedical Engineering, Faculty of Sciences, Lisbon University, Portugal b Radiation Oncology Department, Fox Chase Cancer Center, Philadelphia, USA ⇑ Corresponding author.

Introduction. A new design of Gamma Knife consist of 30 60C0 source capsules, two circular primary collimators (diameter of 6.60 mm and 6.10 mm) and four different changeable collimators. The sources (diameter of 2.8 mm) are distributed in six groups of five sources in the spherical geometry. Each source is individually colli-

Abstracts / Physica Medica 32 (2016) 251–273

mated to obtain four different circular fields at the isocenter (3 mm, 3.5 mm, 6 mm and 8 mm). Purpose. To determine the characteristics of the Co60 beam emerging from a new design of Gamma Knife system and to calculate dose distributions at the isocenter distance for different source configurations and collimator openings Materials and methods. We have used the BEAM-Monte Carlo code to realistically model the geometry design, including source capsules, primary and secondary collimators. The shielding of the head was also simulated. The dose distributions at the isocenter distance are calculated using GEPTS in a previous designed spherical component module for the circular field sizes studied. Results. The spectra of particles emerging from each sourcecollimator configuration is calculated.The radial photon fluence does not vary significantly inside the collimator openings. The spectra of particles from different source groups are compared. Conclusion. The 60Co beam emerging from each group source configuration was characterized but our preliminary results do not allow to properly determine the variation of spectra of particles at isocenter as function of source group position. Further investigations are needed. http://dx.doi.org/10.1016/j.ejmp.2016.07.599

SECONDARY RADIATION DOSES IN IMRT TECHNIQUES Kinga Polaczek-Grelik a,b,*, Jakub Derus a, Agnieszka Giłka a, Aneta Kawa-Iwanicka a, Marta Stefan´czyk a a

Cancer Diagnosis and Treatment Center Katowice, Poland University of Silesia in Katowice, Department of Medical Physics, Poland ⇑ Corresponding author. b

Introduction. The probability for developing the secondary cancer related to low peripheral doses during teleradiotherapy is nowadays of increasing interest. Among the components of the secondary dose photoneutrons and activated radionuclides should be discussed. Purpose. Investigation of the secondary neutron and induced gamma radiation doses during intensity modulated radiation therapy employing Step&Shoot, sliding window and VMAT techniques. Materials and methods. Elekta Synergy linacs with Agility MLC, working in 10 MV and 15 MV modes were used. Six plans with a 15 Gy fractional dose to PTV in mediastinum were studied and realized at anthropomorphic male phantom. Neutron flux was measured by the activation of indium 115In foils. Neutron dosemeter LB 123N was also used. The induced gamma-activity of linac head was investigated using high-purity germanium spectrometer. Doses were assessed using fluence-to dose conversion coefficients. Results. The dominant activation dose comes from 28Al, 56Mn and 187 W isotopes and may reach 2 lSv/h 10 cm from the linac head. Neutron flux densities are of the order of 103–104 cm 2 s 1 for slow and 104–105 cm 2 s 1 for fast neutrons. Additional doses connected with neutrons are 0.5 mSv/MU for 10 MV beam and within the

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range of 0.8–1.7 mSv/MU for 15 MV beam. Variation of neutron doses across the patient body is within 13%, for studied plans. Conclusion. The neutron flux can be even 3.5 times lower whereas gamma-ray doses 5 times lower for 10 MV than for 15 MV beams in IMRT techniques. Disclosure. No conflict of interest. http://dx.doi.org/10.1016/j.ejmp.2016.07.600

THE DOSIMETRIC COMPARISON OF TRUEBEAM AND CYBERKNIFE TREATMENT PLANNING SYSTEMS DOSE CALCULATION ACCURACY FOR BRAIN SRS TREATMENT ON RANDOPHANTOM H. Mabhouti *, E. Sanli, M.S. Cebe, G. Codel, P. Pacaci, E. Serin, M. Doyuran, E. Kucukmorkoc, A. Altinok, D. Canoglu, N. Kucuk, H. Acar, H. Caglar Istanbul Medipol University, Radiation Oncology Department, Turkey Corresponding author.

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Purpose. Brain stereotactic radiosurgery (SRS) involves the use of precisely directed, single session radiation to create a desired radiobiologic response within the brain target with acceptable minimal effects on surrounding structures or tissues. In this study, the dosimetric comparison of target and OAR doses in Truebeam 6 FFF and Cyberknife M6 machines were made. Methods. Treatment planning were done using 2 full arc VMAT technique with 6 FFF beam on the CT scan of Randophantom simulating the treatment of sterotactic treatments for one brain metastasis. The dose distribution were calculated using Eclipse treatment planning system (TPS) with Acuros 13 algorithm. In addition treatment planning for same target volume were also done for Cyberknife M6 machine using Multiplan TPS with Monte Carlo algorithm. Target and brainstem, chiasm, optic nerve, lense median doses were measured using OSL dosimeters. The measured and calculated doses were compared. Results. The calibration of OSL dosimetry were done at first. Target dose measurements made using the OSL dosimetry for both Cyberknife and Truebeam within the measurement uncertainty of 3.1%. The max differences between OSL measured and Eclipse calculated OARs doses were 3.3%,4.5%,5.6% and 6.2% for brainstem, chiasm, optic nerve, lense doses respectively. For cyberknife treatment, the max differences between OSL measured and Monte Carlo calculated OARs doses were 0.9%,1.2%,1.6% and 2.2% for brainstem, chiasm, optic nerve, lense doses respectively. Conclusion. The study shows that dosimetrically comparable plans are achievable Cyberknife and Linac. However, a better conformity, target coverage, less OAR dose and insteeper dose fall off away from the target is achieved with Cyberknife. Monte carlo calculation algorithm predicts the OARs doses more accurately than Acuros algorithm. http://dx.doi.org/10.1016/j.ejmp.2016.07.601