Investigating neutron activated contrast agent imaging for tumor localization in proton therapy: a feasibility study for proton neutron gamma-x detection (PNGXD)

Abstract

Proton neutron gamma-x detection (PNGXD) is a novel imaging concept being investigated for tumor localization during proton therapy that uses secondary neutron interactions with a gadolinium contrast agent (GDCA) to produce characteristic photons within the 40–200 keV energy region. The purpose of this study is to experimentally investigate the feasibility of implementing this procedure by performing experimental measurements on a passive double scattering proton treatment unit. Five experimental measurements were performed with varying concentrations and irradiation conditions. Photon spectra were measured with a 25 mm2, 1 mm thick uncollimated X-123 CdTe spectrometer. For a 10.4 Gy administration on a 100 ml volume phantom with 10 mg g−1 Gd solution placed in a water phantom, 1129 ± 184 K-shell Gd counts were detected. For an administered dose of 21 Gy and the same Gd solution measured in air, resulted in 3296 ± 256 counts. A total of 1094 ± 171, 421 ± 150 and 23 ± 141 K-shell Gd counts were measured for Gd concentrations of 10 mg g−1, 1 mg g−1 and 0 mg g−1 for 7 Gy dose in air. The signal to noise ratio for these five measurements were: 7, 15, 6, 3, and 0.2, respectively. The spectrum contained 43 keV Kα and 49 keV Kβ peaks, however a small amount of 79.5 and 181.9 keV prompt gamma rays were detected from gadolinium neutron capture. This discrepancy is due to a drop in the intrinsic detection efficiency of the CdTe spectrometer over this energy range. The measurements were compared with Monte-Carlo simulation to determine the contributions of Gd neutron capture from internal and external neutrons on a passive scattering proton therapy unit and to investigate the discrepancy in detected characteristic x-rays versus prompt gamma rays.

Neutron activation of gadolinium for ion therapy: a Monte Carlo study of charged particle beams

“…This dynamic tumor image displayed in coincidence to treatment can be fused with a CT or MRI image series and used as a method to localize tumor position. The visible tumor image can be used as a method to indicate the effectiveness of treatment with each subsequent fractionation and provide spatial information for adaptive therapy techniques 9 . Also, dose enhancement was not investigated within this study, however, it is worth further investigation to determine if Gd dose enhancement would provide an impactful benefit to therapeutic treatment from any of these 10 particles.…”

“…The application of novel dynamic-SPECT (D-SPECT) systems may allow a smaller treatment room presence with little reduction in imaging quality, however, further experimental investigation is needed. Experimental measurements performed on a 5 × 5 mm 2 cadmium telluride (CdTe) detector have documented signal-to-noise ratios as high as 15 9 . As proton range verification techniques such as positron emission tomography (PET) or prompt γ-ray emission also require the incorporation of a detection system, it may be possible to perform a PNGXD imaging procedure in-synchrony to a range verification technique 10 , 11 .…”

“…In a previous experimental study, the main photon spectrum for Gd was characterized for a passive scattering proton therapy beam 9 . In another study, gadolinium dose enhancement resulting from protons and carbon ion therapy was investigated via a MC simulation 7 .…”

“…The prospective applications of PNGXD includes the production of a dynamic tumor image to be fused with an anatomic image modality such as CT or MRI and used as a method to track tumor position. The localization of the tumor volume could also be used as a method to determine treatment response and allow the incorporation of additional treatment customizations such as, adaptive planning techniques 9 . In order to produce a PNGXD nuclear medicine image, the addition of a spectroscopic detection system would be included with treatment delivery.…”

“…These studies quantifying the RBE values, could take advantage of the recent advances made in MC simulation of biological nanostructures and microstructures 25 . In a previous experimental study, the main photon spectrum for Gd was characterized for a passive scattering proton therapy beam 9 . In another study, gadolinium dose enhancement resulting from protons and carbon ion therapy was investigated via a MC simulation 7 .…”