Patient-specific online dose verification based on transmission detector measurements Johannes Thoelking a,⇑ , Jens Fleckenstein a , Yuvaraj Sekar a , Ramesh Boggula b , Frank Lohr a , Frederik Wenz a , Hansjoerg Wertz a a Department of Radiation Oncology, University Medical Center Mannheim, Medical Faculty Mannheim, University of Heidelberg, Germany; and b St George’s Cancer Care Center, Christchurch, New Zealand article info Article history: Received 11 September 2015 Received in revised form 23 March 2016 Accepted 2 April 2016 Available online xxxx Keywords: Quality assurance Transmission detector Online dose verification abstract Background and purpose: Since IMRT-techniques lead to an increasingly complicated environment, a patient specific IMRT-plan verification is recommended. Furthermore, verifications during patient irradi- ation and 3D dose reconstruction have the potential to improve treatment delivery, accuracy and safety. This study provides a detailed investigation of the new transmission detector (DTD) Dolphin (IBA Dosimetry, Germany) for online dosimetry. Materials and methods: The clinical performance of the DTD was tested by dosimetric plan verification in 2D and 3D for 18 IMRT-sequences. In 2D, DTD measurements were compared to a pre-treatment verification method and a treatment planning system by gamma index and dose difference evaluations. In 3D, dose– volume-histogram (DVH) indices and gamma analysis were evaluated. Furthermore, the error detection ability was tested with leaf position uncertainties and deviations in the linear accelerator (LINAC) output. Results: The DTD measurements were in excellent agreement to reference measurements in both 2D (c 3%,3mm = (99.7 ± 0.6)% <1, DD ±5% = (99.5 ± 0.5)%) and 3D. Only a small dose underestimation (<2%) within the target volume was observed when analyzing DVH-indices. Positional errors of the leaf banks larger than 1 mm and errors in LINAC output larger than 2% were identified with the DTD. Conclusions: The DTD measures the delivered dose with sufficient accuracy and is therefore suitable for clinical routine. Ó 2016 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology xxx (2016) xxx–xxx Intensity modulated radiotherapy (IMRT) requires a compre- hensive quality assurance (QA) program in general and puts considerable demands for the verification of dose delivery in par- ticular [1]. IMRT-techniques are increasingly forcing a sophisti- cated dose delivery sequence with various degrees of freedom in delivering high doses to the target. Furthermore, possible errors could result in serious consequences for patients. Thus, a patient specific IMRT-QA is recommended [2]. Depending on national guidelines the beam model in the treatment planning system (TPS) as well as the correct radiation delivery for individual patients is supposed to be verified in IMRT-QA. Therefore, 2D detector arrays equipped with ionization chambers or semicon- ductor detectors as well as EPID dosimetry play a major role to ensure that an IMRT-plan is accurately delivered. Different 3D dose reconstruction models have been discussed in the literature [3–5]. These methods can reconstruct the 3D dose distribution inside the CT-dataset of a patient based on machine log files (Mobius 3D, Mobius Medical Systems), radiochromic film, 2D array (Delta4, ScandiDos; ArcCheck/3DVH, Sun Nuclear), or EPID measurements. However, a major goal in modern radiotherapy is the integration of an adaptive radiotherapy approach whereby independent plan ver- ifications during patient irradiation are required. Therefore, various methods based on EPID dosimetry or transmission detectors placed in the beam between the treatment head and the patient are avail- able [5–9]. In this study, the clinical performance of a new verifica- tion platform based on transmission measurements (COMPASS and Dolphin, IBA Dosimetry, Germany) was investigated. Dosimetric plan verification in 2D and 3D was performed and the error detection ability of the new system was evaluated. Materials and methods COMPASS and Dolphin The verification platform COMPASS is a 3D anatomy based quality assurance system that provides different ways to verify treatment plans: (i) model-based dose computation, http://dx.doi.org/10.1016/j.radonc.2016.04.003 0167-8140/Ó 2016 Elsevier Ireland Ltd. All rights reserved. ⇑ Corresponding author at: Department of Radiation Oncology, Mannheim Medical Center, University of Heidelberg, Theodor-Kutzer-Ufer 1-3, 68167 Mannheim, Germany. E-mail address: johannes.thoelking@medma.uni-heidelberg.de (J. Thoelking). Radiotherapy and Oncology xxx (2016) xxx–xxx Contents lists available at ScienceDirect Radiotherapy and Oncology journal homepage: www.thegreenjournal.com Please cite this article in press as: Thoelking J et al. Patient-specific online dose verification based on transmission detector measurements. Radiother Oncol (2016), http://dx.doi.org/10.1016/j.radonc.2016.04.003