First application of the Prompt Gamma-Ray Timing method for proton treatment verification on anthropomorphic head phantom under clinical irradiation

The Prompt Gamma-Ray (PG) Timing technique (PGT) is a promising candidate for online proton treatment verification as it is small, light-weight, gantry-integrable, and it introduces no additional dose to patients. We report on the first evaluation of the PGT system under clinically relevant conditions.
To this end, the CIRS Proton Therapy Dosimetry Head phantom was irradiated with clinically realistic glioblastoma treatment plans. Time-of-flight distributions of PG were acquired with inorganic scintillators of different sizes (Ø1″×1″, Ø2″×1″, Ø2″×2″). Fast dedicated plug-on spectrometers with throughput rates up to 1×10⁶ s⁻¹ and integrated pile-up rejection were used to enable high-resolution time and energy spectroscopy.
While maximizing the detection efficiency, the 2″-detectors show the expected reduced time resolution and worse gain stability compared to the 1″×1″ crystals. Doubling the acceptance of stabilized voltage dividers from 4 to 8 TeV/s improves the gain stability and significantly reduces the gain drift caused by extreme load changes during pencil beam scanning. With active pile-up rejection, the overall count rate was reduced from 700×10³ s⁻¹ to a maximum of 450×10³ s⁻¹. The limited number of processable PG with less than 100 events per treatment spot per detector was identified as the main factor inhibiting clinical application. These findings imply that further development of the PGT setup is necessary for a successful translation into clinical application. We propose strategies for such a development, including the use of larger crystals, or higher segmentation, small ring collimators, and spot aggregation, and report on the first results acquired by applying these methods.