Room: ePoster Forums
Purpose: To commission the extended range shifter (ERS) and evaluate improvement in beam scattering over the standard range shifter (RS).
Methods: A 4.5cm thick ABS resin slab, the ERS, positioned 30cm upstream from treatment isocenter was used in place of the standard 4cm range shifter (RS) that is located 42.5cm from treatment isocenter. The treatment nozzle (Hitachi Probeat-V) was simulated in the TOPAS Monte Carlo code with the ERS and RS geometry. The beam at the nozzle entrance was modeled as a convergent beam. The Monte Carlo model was validated with the commissioning measurements of integrated depth doses (IDDs) in a water phantom and lateral spot profiles in the air at various distances from the isocenter. The Monte Carlo data were used to create Eclipse TPS models. The test fields with extended and standard range shifters were created and compared.
Results: Therapeutic ranges (R90) computed with Monte Carlo simulation were within 0.15 mm with respect to measurements for both ERS and RS. The ERS produces smaller spot sizes than the standard RS at all distances from the isocenter. For the same nominal beam energies, the difference in spot size between the ERS and RS was as large as 3mm at treatment isocenter and as small as 2mm distal to the treatment isocenter. The smaller beam size is associated with the smaller distances from the range shifter to the patient.
Conclusion: The extended range shifter produces smaller spots that would be beneficial for reduced penumbra in the treatment of tumors at shallow depths.