Optimization of selected SPECT system parameters for proton range verification: a Monte Carlo study.

Abstract

This Monte Carlo study investigated a dual-head SPECT-based system for prompt-gamma (PG) range verification in proton therapy, focusing on energy-window and collimator septal thickness effects on PG emission profiles and distal dose fall-off. Using GATE, correlations between PG emissions (4.44, 5.24, 6.13 MeV) and proton depth-dose distributions were evaluated for 90-180 MeV beams in a PMMA phantom. Narrow (4.2-4.6, 5.0-5.4, 5.9-6.3 MeV) and broad (4.2-6.3 MeV) windows were studied. A dual-head SPECT with parallel-hole collimator (1-5 mm septa) was simulated. Image reconstruction was performed with FBP and with OSEM including attenuation correction, using STIR 6.2. The 5.9-6.3 MeV window showed the highest spatial correlation with proton range (Δ ≈1 mm), but its low yield limited applicability. The 4.2-4.6 and 4.2-6.3 MeV windows balanced detection yield and accuracy (Δ ≈2-3 mm). Septal thicknesses ⩾4 mm produced shadowing artefacts, whereas 3 mm gave the most accurate PG localization for the 90 MeV proton beam studied. For the 4.2-4.6 and 4.2-6.3 MeV windows, the PG localization errors (relative to the true PG emission profile) were 1.5 ± 0.5 and 1.4 ± 0.4 mm with FBP, and 1.9 ± 0.3 and 1.7 ± 0.4 mm with OSEM, despite ∼20% lower PG yield than the 1 mm septum. Overall, appropriate window and septal thickness provided PG localization accuracy of 1-2 mm. A 3 mm septum with either the 4.2-4.6 or 4.2-6.3 MeV window offers a favourable trade-off between spatial accuracy and detection efficiency under the 90 MeV condition investigated here, highlighting the potential of optimized SPECT systems for proton range monitoring at this energy.