Room: Exhibit Hall
Purpose: Reduction of lateral penumbra with spot scanning proton therapy is desired to reduce dose to adjacent organs. Calculations and measurements were performed to determine the magnitude of lateral penumbra reduction with reduced air gap in a fixed nozzle, spot scanning proton therapy system. A comparison of measurement and calculation assessed the treatment planning systemâ€™s ability to model the effect.
Methods: Measurements at the center of a 4cm spread out Bragg peak (SOBP) for 5x5cm^2 fields with ranges of 8cm, 12cm, and 16cm were made at air gaps of 21cm, 11cm and 1 cm from the nozzle touch guard to the phantom surface. Patient treatment commonly occurs with air gaps of 10-20cm with isocentric setup. Radiochromic film was placed between solid water equivalent slabs for measurements and analyzed using a flatbed scanner. Penumbra and reductions achieved with smaller air gaps were compared to corresponding plan calculations using a Proton Convolution Superposition (PCS) algorithm. 80%-20% and 90%-10% penumbra were used for comparison.
Results: Lateral penumbra was consistently reduced with smaller air gaps. The magnitude of the reduction decreased with increasing range. The maximum measured reduction when reducing the air gap by 20cm was 1.7mm and 2.8mm for the 80%-20% and 90%-10% penumbra, respectively, for a range of 8cm. Reductions of 80%-20% and 90%-10% penumbra measured for of 12cm and 16cm were 1.4mm/2.2mm and 0.8mm/1.4mm, respectively. Measured penumbra reduction was larger than predicted by the dose calculation. Differences between measured and calculated penumbra reductions ranged from 0.3mm to 1.3mm, which is within the 2mm vendor-specified calculation uncertainty.
Conclusion: Reductions of lateral penumbra can be achieved by reducing the air gap between the patient and nozzle, particularly for smaller ranges. When compared to measurement, the PCS algorithm overestimates the size of the penumbra and underestimates the penumbra reduction achievable with reduced air gap.