Dose calculation of Acuros XB and Anisotropic Analytical Algorithm in lung stereotactic body radiotherapy treatment with flattening filter free beams and the potential role of calculation grid size

Huang et al. Radiation Oncology Dose calculation of Acuros XB and Anisotropic Analytical Algorithm in lung stereotactic body radiotherapy treatment with flattening filter free beams and the potential role of calculation grid size Baotian Huang 0 Lili Wu 0 Peixian Lin 1 Chuangzhen Chen 0 0 Department of Radiation Oncology, Cancer Hospital of Shantou University Medical College , 7 Raoping Road, Shantou 515031 , China 1 Department of Nosocomial Infection Management, The Second Affiliated Hospital of Shantou University Medical College , 69 North Dongxia Road, Shantou 515041 , China Background: The study aimed to appraise the dose differences between Acuros XB (AXB) and Anisotropic Analytical Algorithm (AAA) in stereotactic body radiotherapy (SBRT) treatment for lung cancer with flattening filter free (FFF) beams. Additionally, the potential role of the calculation grid size (CGS) on the dose differences between the two algorithms was also investigated. Methods: SBRT plans with 6X and 10X FFF beams produced from the CT scan data of 10 patients suffering from stage I lung cancer were enrolled in this study. Clinically acceptable treatment plans with AAA were recalculated using AXB with the same monitor units (MU) and identical multileaf collimator (MLC) settings. Furthermore, different CGS (2.5 mm and 1 mm) in the two algorithms was also employed to investigate their dosimetric impact. Dose to planning target volumes (PTV) and organs at risk (OARs) between the two algorithms were compared. PTV was separated into PTV_soft (density in soft-tissue range) and PTV_lung (density in lung range) for comparison. Results: The dose to PTV_lung predicted by AXB was found to be 1.33 1.12% (6XFFF beam with 2.5 mm CGS), 2.33 1.37% (6XFFF beam with 1 mm CGS), 2.81 2.33% (10XFFF beam with 2.5 mm CGS) and 3.34 1.76% (10XFFF beam with 1 mm CGS) lower compared with that by AAA, respectively. However, the dose directed to PTV_soft was comparable. For OARs, AXB predicted a slightly lower dose to the aorta, chest wall, spinal cord and esophagus, regardless of whether the 6XFFF or 10XFFF beam was utilized. Exceptionally, dose to the ipsilateral lung was significantly higher with AXB. Conclusions: AXB principally predicts lower dose to PTV_lung compared to AAA and the CGS contributes to the relative dose difference between the two algorithms. Dose difference; Acuros XB; Anisotropic analytical algorithm; Stereotactic body radiotherapy; Calculation grid size - Introduction The Anisotropic Analytical Algorithm (AAA), a convolution/superposition method, is widely utilized for dose calculation in Eclipse treatment planning system [1,2]. However, AAA was reported to significantly overestimate the dose near air-tissue interfaces [3]. Recently, a new dose calculation algorithm named Acuros XB (AXB) has been implemented for clinical use. This new algorithm explicitly solves the linear Boltzmann transport equation that describes the macroscopic behavior of radiation particles as they travel through and interact with matters [4]. AXB was found to provide good agreement with Monte Carlo (MC) algorithm [5-7] and achieved more accurate dose prediction than AAA in heterogeneous phantom [2,6-10]. The dose difference between the two algorithms has became an issue of concern in the past few years and several publications have investigated the dosimetric impact of AXB in breast cancer, non-small cell lung cancer (NSCLC) and nasopharyngeal carcinoma with AAA as a benchmark [4,11,12]. The dose difference between the two algorithms was reported to be closely related to the beam energy, field size and the density of the materials [6]. However, they failed to consider the potential impact of the calculation grid size (CGS), which was reported to be associated with dose variation [13,14]. So far, the dose difference between AAA and AXB due to the use of different CGS in lung stereotactic body radiotherapy (SBRT) treatment remains unknown and the effect of it requires further investigation. On the other hand, although two studies have evaluated the dosimetric impact of AXB in SBRT plan for lung cancer [15,16], little information has been focused on the dose difference generated from flattening filter free (FFF) beams, which was widely used in lung SBRT treatment for its improved treatment efficiency [17,18]. Because FFF beams provide softer spectrum than flattened beams [19], investigation of their impact is also essential. Therefore, the aim of this study is to quantitatively determine the dose differences between AXB and AAA in SBRT treatment for lung cancer with FFF beams. Furthermore, the potential role of the CGS on the dose differences between the two algorithms was also discussed. Methods Beam configuration The same set of beam data (including percentage depth dose curve, profiles and output factors) used by AAA and measured in a three-dimensional water scanning system (PTW, Germany) for field sizes from 3 3 to 40 40 cm2 were imported in Eclipse treatment planning system (Version 10.0, Varian Medical Systems, Palo Alto, CA) for the configuration of AXB. All data presented in this study were collected from a commissioned Varian Truebeam accelerator equipped with a Millennium 120 multileaf collimator (MLC, with spatial resolution of 5 and 10 mm for the central and outer 20 cm, respectively. CT scanning and contouring of organs at risk (OARs) This retrospective study was approved by the ethics committee at the Cancer Hospital of Shantou University Medical College. Informed consent was obtained from all subjects prior to the study. During March 2012 and May 2013, 10 patients suffering from stage I NSCLC at our hospital were enrolled in this study. The CT datasets were acquired using a 16-slice CT scanner (The Philips Brilliance CT Big Bore Oncology Configuration, Cleveland, OH, USA) for all patients. Four dimensional computed tomography (4DCT) scanning and conventional enhanced 3D scanning with intravenous (IV) contrast was performed for 3 and 7 patients, respectively. Scanning was acquired at a 3 mm slice thickness for both 3D and 4DCT. CT images were then transferred to Eclipse treatment planning system. For patients with conventional enhanced scanning, gross tumor volume (GTV) was contoured by an experienced radiation oncologist under the CT pulmonary windows, and the planning target volume (PTV) was acquired according to the tumor motion under fluoroscopic examination with the aid of a simulator. For patients with 4DCT scanning, GTV accounting for tumor motion on all 10 phases of the 4DCT were contoured in the same way. These 10 phases of the GTV were then combined to form the internal target volume (ITV). To account for set-up uncertainties and potential baseline tumor shift, PTV was expanded with a uniform 5 mm margin from ITV. The PTV was then split into two substructures: PTV_soft, with a density 3 in the soft-tissue range (0.590-0.985 g/cm ), and PTV_lung with a density in the lung range (0.011 (...truncated)