The International Journal of Engineering and Science (IJES) || Volume || 7 || Issue || 11 Ver.I || Pages || PP 17-21 || 2018 || ISSN (e): 2319 – 1813 ISSN (p): 23-19 – 1805 DOI:10.9790/1813-0711011721 www.theijes.com Page 17 Dosimetric Comparison of Flattening Filter-Free Energies for Lung SBRT Ismail Faruk Durmus 1 , Emine Dilara Atalay 2 1 Department of RadiationOncology, Yeni YuzyilUniversityGaziosmanpasaHospital,IstanbulTurkey 2 Department of Physics, CanakkaleOnsekiz Mart University, Canakkale, Turkey Corresponding Author: Emine Dilara Atalay -------------------------------------------------------------- ABSTRACT---------------------------------------------------------- Purpose: The purpose of this study is the dosimetric comparison of 6 MV flattening filter free (FFF) and 10 MV FFF energies in the lung stereotactic body radiotherapy (SBRT) plans. Materials and Methods: The treatment plans of 16 lung SBRT patients were prepared using the same fields and the same physical parameters for 6 MV FFF and 10 MV FFF energies. Critical organ doses, planning target volume doses, quality of plans (gradient index (GI), homogeneity index of International Commission on Radiation Units and Measurements (HI ICRU ), heterogeneity index (HI), and conformity index (CI)), and monitor unit (MU) values have been compared between the two plans. The high dose volume and low dose volume outside the target volume are compared according to the Radiation Therapy Oncology Group (RTOG) 0813/0915 protocols. The verification of the plans has been performed through 2D Array IBA® MatriXX Evolution Dosimetry System for each plan. Results: We have determined better CI and GI values with 6 MV FFF energy. A rapid decrease in dose in regions outside the target has been observed, when the GI value has been lower, which leads to enhancement in the protection of the healthy tissue. Lower MU values have been obtained with 10 MV FFF energy. The maximum doses of the heart and spinal cord have been similar for both energies. We have determined lower V5, V10, and V20 doses in the body and ipsilateral lung with 6 MV FFF plans. For both FFF energies and noncoplanar volumetric modulated arc therapy fields, high dose and low dose volumes are determined according to the RTOG criteria. High dose spillage, intermediate dose spillage, V5, V10, and V20 doses have been better with 6 MV FFF. Quality assurance (QA) is evaluated according to the gamma and average gamma indices in the plans prepared with both energies. Both plans that are prepared with 6 MV FFF and 10 MV FFF energies are suitable according to QA results. In addition, better gamma results have been obtained with 10 MV FFF than 6 MV FFF. Conclusions: Although 6 MV FFF and 10 MV FFF are suitable for lung SBRT, 6 MV FFF has some dosimetric advantages. KEYWORDS: FFF energies, Lung SBRT, Average Gama Index --------------------------------------------------------------------------------------------------------------------------------------- Date of Submission: 05-11-2018 Date of acceptance: 19-11-2018 --------------------------------------------------------------------------------------------------------------------------------------- I. INTRODUCTION Radiosurgery is a well‑established treatment modality used in the management of a wide variety of intracranial and extracranial lesions, in which a high dose is typically provided in a few fractions to a small and precisely localized target. [1,2] In stereotactic radiosurgery/stereotactic radiotherapy (SRS/SRT), small field sizes and increased number of beams are employed to create highly conformal dose distributions using rigidly attached stereotactic frames or a stereotactic image guidance system. This approach allows high doses to be delivered to the target in one or several fractions, while sparing critical surrounding structures. [3] Stereotactic body radiotherapy (SBRT) refers to the use of SRT on an extracranial region, and it is a successful treatment method with limited toxicity for primary and metastatic lung cancers. [4‑6] The first SBRT implementation to patients with lung cancer was reported in 1995 by Blomgrenet al. [7] Several studies have reported significantly improved local control and survival using SBRT in patients with Stage I lung cancer. [8‑10] SBRT administration achieves avoidance of normal tissue exposure to radiation during the planning process by providing for sharp fall‑off dose gradients outside the target. [11,12] The clinical use of flattening filter‑free (FFF) beams has initially been driven by the attempt to reduce the long delivery time required for SRS/SBRT treatments, as removing the flattening filter increases the dose rate by a factor of 2– 4. [13,14] Treatment time is reduced with high‑dose rates using FFF beams in SRS and SBRT. It also increases the efficacy and accuracy of the treatment through image‑guided radiotherapy systems, while providing fast and comfortable treatment for patients. Volumetric‑modulated arc therapy (VMAT) is a novel technique that delivers the dose, in which the linear accelerator rotates continuously around the patient. The dose rate, gantry rotation speed, and multileaf collimator (MLC) positions change dynamically during the treatment. [15] The aim of our work is to compare physically and dosimetrically the VMAT plans prepared with 6 MVFFF and 10 MVFFF. Parameters such as lung SBRT treatments, target dose distributions, and healthy organ doses were compared for two unfiltered energies [Figure 1]. II. MATERIALS AND METHOD All plans are calculated using the Monte Carlo algorithm at Elekta® Monaco 5.11 (Elekta, Crawley, England) Treatment Planning System (TPS). Monte Carlo dose calculation simulates transportation of millions of photons and