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Abstract
The electronic portal imaging device (EPID) has the potential to be used for in vivo dosimetry during radiation therapy as an additional dose delivery check. In this study we have extended a method developed by A. Piermattei and colleagues in 2006 that made use of EPID transit images (acquired during treatment) to calculate dose in the isocenter point. The extension allows calculation of two‐dimensional dose maps of the entire radiation field at the depth of isocenter. We quantified the variability of the ratio of EPID signal to dose in the isocenter plane in Solid Water phantoms of various thicknesses and with various field sizes, and designed a field edge dose calculation correction. To validate the method, we designed three realistic conventional radiation therapy treatment plans on a thorax and head anthropomorphic phantom (whole brain, brain primary, lung tumor). Using CT data, EPID transit images, EPID signal‐to‐dose correlation, and our edge correction, we calculated dose in the isocenter plane and compared it with the treatment planning system's prediction. Gamma evaluation (3%, 3 mm) showed good agreement (Pγ<1 ≥ 96.5%) for all fields of the whole brain and brain primary plans. In the presence of lung, however, our algorithm overestimated dose by 7%–9%. This 2D EPID‐based in vivo dosimetry method can be used for posttreatment dose verification, thereby improving the safety and quality of patient treatments. With future work, it may be extended to measure dose in real time and to prevent harmful delivery errors.PACS numbers: 87.55.km, 87.55.Qr, 87.55.T‐
Highlights
The goal of radiation therapy (RT) is to deliver the prescribed dose to the target while sparing surrounding tissues
The in vivo dosimetry method we propose is a two-dimensional extension of previous work by Piermattei et al[28] which has successfully implemented transit electronic portal imaging device (EPID) dosimetry to calculate absorbed dose at the isocenter to within 3% of the value predicted by the treatment planning system (TPS)
EPID signal to dose correlation ratios The correlation ratios FCAX and the displacement factors ƒ were measured by irradiating Solid Water phantoms (Gammex, Middleton, WI) while acquiring transit EPID images, as described by Piermattei et al[28] In this work, FCAX values relate central-axis EPID signal to the dose in the midpoint of the phantom when centered at isocenter (Fig.2(a)), and were determined as the ratio of the signal from the EPID’s central 20 × 20 pixels (SCAX) to the dose at isocenter (DCAX) given by the TPS (Eclipse 8.9; Varian Medical Systems, Palo Alto, CA), for five Solid Water phantom thicknesses (w = 6, 12, 16, 20, 26 cm) and seven square fields (l = 5, 7.5, 10, 12.5, 15, 17.5, 20 cm)
Summary
The goal of radiation therapy (RT) is to deliver the prescribed dose to the target while sparing surrounding tissues. To this aim, the great majority of cancer centers rely on pretreatment quality assurance of the plan calculation by the treatment planning system (TPS) and of the dose distribution in homogeneous phantoms. The presence of any device on the patient’s skin may modify the surrounding dose distribution. Another tool for in vivo dosimetry, which has been largely investigated but used clinically only in few select sites,(1) is the electronic portal imaging device, or EPID. The EPID can be run in continuous acquisition or cine mode, and has potential to provide dose measurement in real time.[8]
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