Enhancing the capability of MAPS for radiotherapy verification

We have developed a thin, upstream device for real time 2D verification of intensity modulated radiotherapy based on the LASSENA monolithic active pixel sensor. In intensity modulated radiotherapy, the beam is shaped by a stack of thin moving collimators to spare as much of the healthy tissue as possible. The device is very thin and can therefore be placed between the beam and the patient as its attenuation is less than 1%. The device reports the position of each of the collimators in real time with unprecedented precision (our position resolution is 146 micron using the data generated in only one second of treatment) and an integrated signal, which gives a crude estimate of the beam intensity. Thus the treatment plan can be verified in real time without prior measurements.

One of the trends in radiotherapy is to move to shorter but more intense treatments. One strand of this is Flattening Filter Free (FFF) treatments. These treatments provide a much higher dose rate and therefore it is even more important to monitor the treatment in real time as errors can have grave consequences.

Our current LASSENA-based system is not suitable for FFF treatments. A new sensor has been developed, the Athena. In this proposal we want to characterise the Athena in a clinical setting and demonstrate that it works at least as well as the LASSENA, even for FFF treatments. In normal clinical operation our device would receive an integrated radiation dose of around 5kGy per year. We want to perform a radiation test of the Athena to find its clinical lifetime to ensure that the Athena based device is viable as a product. The LASSENA is not able to measure the dose administrated to the patient. This is because the signal in the device is the result of signal generated by the therapeutic photons and contamination electrons. The latter are generated by interactions of the photons with air and materials in the radiotherapy machine. The electron signal is dependent on many and partly unknown parameters and thus cannot be removed in a simple way. We have invented a way to measure simultaneously the electron and the photon contribution separately by patterning the substrate of the sensor. We successfully completed proof of principle measurements and showed that the method works. A patent application has been made. In this project we want to optimise the design and determine the dose measurement precision. The LASSENA system is able to report the collimator positions in real time. The whole algorithm was implemented in the firmware for the DAQ system for a predecessor of the LASSENA sensor, the Achilles. In this project we will adapt the firmware from the Achilles system to the Athena system.

After completion of the project, we will have produced a system that can monitor the collimator positions and the dose administered to the patient in real time. This significantly enhances patient safety as the beam can be stopped within seconds if the measured treatment is not conform the planned treatment. Such a system would also be of great benefit for the NHS as there is no need for measurements (phantom runs) before the actual treatment. Hence, more patients can be safely treated in the same time.