First clinical prompt gamma measurement in pencil beam scanning proton therapy

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Researchers from the University of Pennsylvania and Ion Beam Applications (IBA)

Proton therapy offers a better dose conformity to the tumour volume and a lower overall dose to healthy tissue, compared to conventional photon therapy. However, the potential of protons cannot be fully exploited in clinical practice due to existence of the range uncertainties. Robust planning and beam-specific safety margins are tools to manage those range uncertainties coming from organ motion, patient position errors, and conversion from CT Hounsfield Units to proton stopping power. To fully exploit the potential of protons and to reduce the safety margins, researchers around the world are investigating different methods for an online range monitoring during the treatment.

Researchers from the University of Pennsylvania and Ion Beam Applications (IBA) have recently reported the first clinical prompt gamma (PG) measurement for range verification in actively scanned proton therapy (Int. J. Radiat. Oncol. Biol. Phys. doi: 10.1016/j.ijrobp.2017.04.027). Using the PG camera developed by IBA, the PG information of each pencil beam spot was obtained for all three fields in six treatment fractions for a curative brain tumour treatment. By comparing the measured data to simulations, the range variations could be determined for each individual spot. The mean range shift of each field varied between -1 mm and 2 mm with an uncertainty of around 1 mm (1 sigma). This is well below the distal safety margins of 5 mm and shows the potential of the PG camera for online range monitoring. 

The researchers concluded that the system accuracy is mainly limited by the positioning accuracy of the present implementation of the PG camera. Therefore, Johannes Petzoldt, one of the OMA fellows, is working on a more reproducible positioning system for the PG camera within the treatment room. After the installation of such a system, a higher number of patient treatments can be monitored with higher accuracy than before. This would foster the applicability of PG range verification and therefore improve proton therapy in general.