Dose Quantification in Neutron Capture Enhanced Particle Therapy

The main aim of radiotherapy is to target tumour cells while limiting dose to healthy tissue. Particle therapy is ideal for this purpose, with a low entrance dose and a characteristic Bragg Peak at the end of the ion path for high dose inside the target.

Charged particles can interact in matter through a variety of ways, including via nuclear inelastic collisions with atoms in the beam path; secondary particle generation in this process leads to the production of fragments such as neutrons. These neutrons, through the tumour uptake of high neutron capture cross-section isotopes such as ¹⁰B or ¹⁵⁷Gd, can be captured to deliver additional dose to the target. This is the main principle of Neutron Capture Enhanced Particle Therapy (NCEPT). However, the implementation of such a modality requires extensive research into methods of accurate dose quantification, for both the primary ion dose and additional neutron capture dose.

One possibility is through the detection of characteristic photons which are emitted as a product of the neutron capture interactions; for ¹⁰B this occurs at 478 keV, while a range of photons are produced for ¹⁵⁷Gd including the higher energy photons at 6.75 MeV and 7.94 MeV. As these photons are only emitted during neutron capture, their detection indicates that the interaction has occurred and hence there is additional dose deposition.

The detection of photons due to neutron capture is limited by false positive counts that occur within the same energy and temporal windows, but were created by other processes (including Compton scattered photons). The discrimination of neutron capture photons from these other detected counts is essential for the application of a prompt gamma detection based system for NCEPT; this can be through various methods, including the implementation of energy and timing windows, or they may be reduced through shielding.

This Thesis details the initial stages of the development of a dose quantification system in NCEPT through both simulation and experimental methods. The characteristics of photon and neutron emission following ¹²C and ⁴He ion therapy are studied, with timing windows for the detection of prompt gamma and neutron capture photons identified for potential application in neutron capture discrimination. A window of 12 ns - 10⁴ ns is recommended, with prompt gamma photons arriving at the detector prior to this time and positron annihilation photons arriving after 10⁴ ns.

The BeNEdiCTE detection module is investigated for its feasibility as a prototype scintillation system and its ability to distinguish an increase in the 478 keV signal with the addition of ¹⁰B. This was identified for ¹²C and ⁴He ion beams, with the first experimental demonstration of neutron capture photon detector during heavy ion therapy detailed here. For comparison to the heavier ion beams and consideration of its feasibility, an experimental study was also conducted with a proton beam.

Additionally, the ability of the detection system to measure differences in the signal with variation in position and concentration was considered. The simulated detector was able to detect a change in concentration down to the order of 100 ppm, however for concentrations below this, a multi-head detection system is suggested.

Finally, a shielding optimisation study was performed in which it was determined that in a low-background environment it is preferable to avoid shielding of the detector crystal. Alternatively, for high neutron background environments or in the presence of boroncontaining electronics (such as printed circuit boards) during ¹⁰B NCEPT, a small layer of shielding such as 1 mm Gd₂O₃ is suggested.

Overall, it has been demonstrated that dose quantification during NCEPT using a neutron capture detection system is both feasible and essential for the clinical implementation of this modality.