3D beam monitoring for 12C radiotherapy by tracking of secondary ions using the timepix detector

Radiotherapy with narrow ¹²C ion beams enables treatment of tumors with high precision while sparing the surrounding healthy tissue. Unpredictable changes in the patient´s geometry can alter the ion range and result in changes in the dose distribution. Therefore, it is desirable to verify the actual dose delivery in the patient, preferably in real-time and in a non-invasive manner. Currently, the only technically feasible dose delivery monitoring method is based on tissue activation measurements by means of positron emission tomography (PET). As an alternative to PET-based measurements, beam range monitoring exploiting the detection of prompt secondary ions has been suggested. This modality is expected to allow for real-time monitoring, thereby reducing the influences of physiological signal wash-out known from PET-based techniques. In this contribution, the potential of monitoring narrow ¹²C ion beams in a head-sized PMMA phantom by tracking of secondary ions is investigated experimentally. Experiments with therapeutic carbon ion beams were performed at the Heidelberg Ion Beam Therapy Center (HIT), Germany. The Timepix detector was used to track secondary ions emerging from the irradiated phantom. Analysis of the secondary ion directions was used to monitor the range, width and position of pencil-like ¹²C ion beams in therapy relevant conditions. Clear dependences of the secondary ion track distribution on the investigated beam settings were found. Detectable were beam range differences down to about 2 mm and differences in the beam width of 1.4 mm. Furthermore, lateral shifts of the beam position by 1 mm were measurable. The presented experiments show the potential of secondary ion tracking for monitoring therapeutic carbon ion beams.