Feasibility study of a dual detector configuration concept for simultaneous megavoltage imaging and dose verification in radiotherapy
Purpose: To test the feasibility of a dual detector concept for comprehensive verification of external beam radiotherapy. Specifically, the authors test the hypothesis that a portal imaging device coupled to a 2D dosimeter provides a system capable of simultaneous imaging and dose verification, and that the presence of each device does not significantly detract from the performance of the other. Methods: The dual detector configuration comprised of a standard radiotherapy electronic portal imaging device (EPID) positioned directly on top of an ionization-chamber array (ICA) with 2 cm solid water buildup material (between EPID and ICA) and 5 cm solid backscatter material. The dose response characteristics of the ICA and the imaging performance of the EPID in the dual detector configuration were compared to the performance in their respective reference clinical configurations. The reference clinical configurations were 6 cm solid water buildup material, an ICA, and 5 cm solid water backscatter material as the reference dosimetry configuration, and an EPID with no additional buildup or solid backscatter material as the reference imaging configuration. The dose response of the ICA was evaluated by measuring the detectors response with respect to off-axis position, field size, and transit object thickness. Clinical dosimetry performance was evaluated by measuring a range of clinical intensity-modulated radiation therapy (IMRT) beams in transit and nontransit geometries. The imaging performance of the EPID was evaluated quantitatively by measuring the contrast-to-noise ratio (CNR) and spatial resolution. Images of an anthropomorphic phantom were also used for qualitative assessment. Results: The measured off-axis and field size response with the ICA in both transit and nontransit geometries for both dual detector configuration and reference dosimetry configuration agreed to within 1%. Transit dose response as a function of object thickness agreed to within 0.5%. All IMRT test patterns and clinical IMRT beams had gamma pass rates of ≥98% at 2%/2 mm criteria. In terms of imaging performance, the measured CNR and spatial resolution ( f50) were 263.23±24.85 and 0.4025±1.25×10?3 for dual detector configuration and 324±26.65 and 0.4141±1.14×10?3 for reference imaging configuration, respectively. The CNR and spatial resolution were quantitatively worse in the dual detector configuration due to the additional backscatter. The difference in imaging performance was not visible in qualitative assessment of phantom images. Conclusions: Combining a commercially available ICA dosimetry device with a conventional EPID did not significantly detract from the performance of either device. Further improvements in imaging performance may be achieved with an optimized design. This study demonstrates the feasibility of a dual detector concept for simultaneous imaging and dosimetry in radiation therapy.