Construction and characterisation of a coded-mask gamma camera for beam range monitoring in proton therapy

Abstract

The major advantage of proton therapy over conventional radiotherapy is the dose deposition pattern, enabling precise coverage of the tumour volume while sparing nearby healthy tissues. However, accurate control of the proton beam range online during patient irradiation is still considered a challenge. Thus, there are extensive efforts to develop a detector capable of in vivo beam range monitoring for proton therapy, among them, the coded-mask gamma camera developed by the SiFi-CC group. The assembly of this detector, as well as its first test under clinical conditions, are the subject of this thesis. The detector consists of a tungsten collimator and a stack of thin scintillation fibres, read out at both ends by SiPMs. Such a design allows for efficient detection of prompt gammas (PG) and reconstruction of a PG depth profile, which is strongly correlated with the proton beam range. The readout system was selected in an extensive comparison study of five different systems. After assembly, the detector was tested under clinical conditions: a phantom was irradiated with proton beams of energies from 70.51 MeV to 108.15 MeV, the detector response was processed with low-level reconstruction software, and the MLEM algorithm was applied to reconstruct the PG depth profiles (images). The distal falloff positions of these profiles were determined and compared with the calculated proton ranges. The image reconstruction parameters were optimised in terms of accuracy of proton range shift determination. The results obtained were checked against a Geant4 simulation, showing good consistency. The statistical precision was found to be 1.7 mm for $10^8$ protons. The result is comparable to similar systems investigated by other research groups. However, simulation results suggest that certain hardware amendments to the detector can increase that precision about four times.