3D EPID-based in vivo dosimetry for IMRT and VMAT

In this paper the various approaches of EPID-based in vivo IMRT and VMAT dose verification, and their clinical implementation, are described. It will be shown that EPID-based in vivo dosimetry plays an important role in the total chain of verification procedures in a radiotherapy department. EPID-based dosimetry, in combination with in-room imaging, is a fast and accurate tool for 3D in vivo verification of VMAT delivery. EPID-based in vivo dosimetry provides clinically more useful information and is less time consuming than patientspecific pre-treatment dose verification. In addition to accurate 3D dose verification, in vivo EPID-based dosimetry will also detect major errors in the dose received by individual patients, and provides a safety net for advanced treatments such as IMRT and VMAT. 1. Introduction The implementation of IMRT and VMAT has increased the need for a high accuracy in the dose delivery to patients. For that purpose comprehensive quality assurance (QA) programs have been introduced to verify the correct functioning of all components in the radiotherapy treatment planning and delivery process. In addition to these QA programs of the separate components required for a patient treatment, often additional pre-treatment verification checks for individual patients are performed using a variety of phantoms in combination with ionisation chamber or diode arrays. With these different QA programmes in place, one may question the necessity for additional in vivo dose measurements during the actual treatment of an individual patient. The clinical use of in vivo dosimetry (IVD) in external beam radiotherapy has been addressed in a large number of studies, which were mainly related to the use of point detectors. Some review papers, for instance the recent IAEA Human Health Report Nr. 8 [1], identified a number of treatment errors by means of in vivo entrance and/or exit dosimetry during conventional radiotherapy. Also by means of EPID-based in vivo dosimetry a number of serious errors during 3D conformal radiotherapy [2] and IMRT delivery [3] were observed that could not have been detected by other QA checks using pre-treatment measurements. Although able to detect major errors, the main application of IVD is to assess all clinically relevant differences between planned and delivered dose. Another important aspect of IVD is that it also provides a record of the actual dose received by individual patients and fulfils legal requirements in some countries. In this paper we will elucidate the current experience with EPID-based in vivo dosimetry for IMRT and VMAT dose verification, and the additional information that can be obtained compared to other patient-specific dose verification measurements. 7th International Conference on 3D Radiation Dosimetry (IC3DDose) IOP Publishing Journal of Physics: Conference Series 444 (2013) 012011 doi:10.1088/1742-6596/444/1/012011 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Published under licence by IOP Publishing Ltd 1 2. EPID-based in vivo IMRT dose verification 2.1 IVD during IMRT The use of EPID dosimetry has proliferated since several groups have demonstrated its unique possibilities for QA of IMRT, including IVD applications. Two approaches have been reported for using EPID-based in vivo dosimetry. In the first approach, a portal dose image with a patient in the beam at the position of the EPID is predicted using the planning CT data of that patient, which is then compared with a portal dose image measured with the EPID [e.g., 4]. A limitation of this forward approach is that it is not always clear how dose differences in the plane of the EPID are related to dose differences in the patient. Several groups have therefore explored back-projection methods for the derivation of the patient dose distribution from a measured portal dose image. These models require the primary dose component at the position of the EPID, which is obtained by correcting the EPID response for the scattered component inside the EPID, and the radiation scattered from the patient/phantom. This primary radiation component is then back-projected to a point inside the patient/phantom and the scattered dose at that position is added (see figure 1). Figure 1: Schematic presentation of the various steps involved in the reconstruction of the dose distribution inside a patient/phantom from an EPID measurement using a back-projection model. Nijsten et al [2] correlated the dose measured with an EPID on the central beam axis with dose values at 5 cm depth using a back-projection model having a (semi-) empirical relationship between these two quantities. Piermattei et al [5] reported a simple method for the in vivo determination of the midplane dose along the central beam axis using a transmitted signal measured by the central pixels of an a-Si type of EPID. Recently the first results of a national Italian project using this method have been published for photon beams generated by linacs of different manufacturers equipped with a-Si EPIDs [6]. The various back-projection models based on transit dosimetry have been discussed in a review article on EPID-based dosimetry by van Elmpt et al [7]. 2.2 Clinical implementation of EPID-based in vivo dose verification of IMRT at NKI-AVL In the Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital (NKI-AVL), back-projection algorithms have been implemented for the 2D and 3D dose verification of IMRT using a-Si EPIDs [8, 9]. For in vivo dosimetry, the reconstructed dose is generally compared for each IMRT field with the patient plan in a plane parallel to the EPID perpendicular to the beam intersecting the isocentre, i.e. by means of a multiple 2D approach. For some sites (e.g., breast) the isocentre is not always very relevant for in vivo dose verification and a new reference point and other planes are chosen for the 2D γevaluation. As an example, the results of a verification of a 6-field IMRT plan for oesophagus cancer treatment are shown in figure 2 to illustrate how EPID-based in vivo dosimetry is clinically implemented. The comparison of the EPID-reconstructed and planned dose distribution is done by a 2D γ-evaluation method (3%/3mm dose-difference and distance-to-agreement) using the mean γ 7th International Conference on 3D Radiation Dosimetry (IC3DDose) IOP Publishing Journal of Physics: Conference Series 444 (2013) 012011 doi:10.1088/1742-6596/444/1/012011