Feasibility of a dual detector system to perform transit dosimetry and MV imaging in-vivo

In this work, a dual detector system for simultaneous in-vivo dosimetry and MV imaging was investigated. The ability to measure real-time water equivalent dose post transit for a heterogeneous phantom was demonstrated, illustrating its advantage over non-water equivalent EPID dosimetry. Identification of incorrect MU delivery as well as patient misalignment can be identified in the dose profiles and confirmed through co-registration with EPID images. The dual detector comprising of a silicon array detector "Magic Plate" (MP) and an EPID was positioned beneath the patient couch at a source to detector distance of 150 cm. Two pseudo lung phantoms were utilised in this study; identical, except for the inclusion of a water equivalent hidden target mimicking a tumour. The dosimetric accuracy of the MP was evaluated with EBT3 film and TPS calculations. The presence and position of the hidden target was detected and accurately co-registered between the dose profiles measured with MP and the EPID image. The system measured a difference in post transit doses through the two phantoms of (5.0 ±1.6) cGy due to the presence of the target, in agreement with the TPS calculation of 5.16 cGy. The capability of the system for error detection was confirmed when a deliberate 7 mm lateral shift of the phantom was observed in both the dose profiles measured with MP and the EPID images. The MP as a dosimetric real time detector is lucent in MV photon fields and does not obstruct the EPID field of view.