Clinical PET imaging for in-vivo 3D activation verification in proton therapy suffers from two major drawbacks: minimal activation near the Bragg Peak region and biological washout. Here, we investigate the possibility of using 18O-enriched water (18-W) for in-vivo range verification in a clinical scenario. PET images are acquired with a novel in-beam system that can detect and process on-the-fly PET activity produced during and after irradiation.
Head and neck cancer cells were grown as xenografts in an in-vivo chick embryo chorioallantoic membrane (CAM) model. Three tumours were infused with 18-W, in vivo and irradiated with a clinical 80-MeV proton beam at the Quironsalud proton therapy center. Activity was recorded in two separated periods, during irradiation (beam-on) and decay (beam-off) with a specific PET setup consisting of 6 phoswich detector blocks with 338 pixels each, with of 1.55x1.55 x LYSO (7mm)+GSO (8mm) . The system was coupled to a fast data acquisition system able to sustain rates up to 10 Msingles/sec.
Dynamic 3D maps of the activity in the eggs were reconstructed from coincidences during irradiation, showing the spot structure of the treatment plan. Measured decay curves were correlated with dynamic PET images and 3D activity maps and fitted to a sum of exponentials including the decay of 11C, 13N, 15O and 18F, to obtain the individual contribution for each isotope.
3D PET activity maps during irradiation can be reconstructed in a clinical scenario with sub-ms and sub-mm resolutions. Decay analysis shows specific activity of 18F close to the tumour area several minutes after irradiation. This validates the experimental setup for its use for in-beam on-the-fly reconstruction of the 3D activity spatial distribution as well as late specific 18F activity entrapped in the tumour.