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

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 used FBP and OSEM (STIR 6.2) with attenuation correction. 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 artifacts, 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 favorable 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.