The changing roles of electronic portal imaging devices in radiotherapy

Historically, portal films were used in radiotherapy to verify treatment field placement and patient positioning. These films were placed orthogonal to the beam axis behind the patient. The films were exposed using the mega-voltage (MV) energy photons of the treatment beam and were subsequently analysed to ensure that the patient was being treated as planned by the treatment planning system (TPS).

The electronic portal image device (EPID) was originally designed to replace these portal films. The rationale was both the financial cost of the individual films and the cost in time needed to develop the films. The delay in processing and scanning films meant that the verification process took many minutes, whereas the EPID provides an instant digital image that can be quickly compared to a reference image from the TPS. Whereas portal films had been compared to films from a simulator, therefore requiring an extra simulation stage in the patient pathway, EPID images were compared to a digitally reconstructed radiograph (DRR) created in the TPS from the patient’s planning CT scan. From these DRRs the patient’s bony anatomy is outlined and used as a template to match the acquired EPID image.

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