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Ion Recombination Effect in Proton Therapy with Pencil Beam Scanning (PBS): A Comparison of Three 2-Voltage Correction Methods

M Gao

M Gao*, H Ramirez , B Kreydick , M Pankuch , Northwestern Medicine Chicago Proton Center, Warrenville, IL


SU-I-GPD-T-148 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: In PBS, proton beam is aimed at a defined spot until all MUs are delivered. Instantaneous dose rate at a spot is much higher in PBS than in passive scattering. Hence, effect of ion recombination can be quite significant, which is typically corrected using 2-Voltage measurements. We compared three popular methods to compute the recombination correction factor for two representative types of chambers.

Methods: Six PBS plans were created to deliver a uniform dose to a 6x6x6 cm³ volume centered at 20 cm depth. The dose was 0.5, 1.0, 3.0, 5.0, 6.9 and 9.9 Gy. Plans were delivered via IBA Proteus Plus with universal nozzle. Doses were measured with 2 chambers: 0.6 cc cylindrical farmer chamber (FC65) & plate-parallel chamber (PPC05) and at 2 biases: -300V & -150V. Recombination correction factor was computed using three formulas recommended in ICRU78: Pulsed, Pulsed-Scanned, and Continuous radiation.

Results: Without correction, charge collected per Gy delivered decreased from 1.000 (normalized to lowest dose rate) to 0.898 for FC65 chamber and to 0.990 for PPC05 chamber. For FC65, corrected readings from lowest to highest dose rate were 1.000, 1.003, 0.953, 0.945, 0.949, 0.947 (Pulsed); 1.000, 1.002, 0.952, 0.945, 0.949, 0.948 (Pulsed-Scanned); 1.000, 0.986, 0.930, 0.918, 0.917, 0.913 (Continuous) respectively. For PPC05, corrected readings were 1.000, 1.002, 1.002, 1.003, 0.998, 0.996 (Pulsed); 1.000, 1.002, 1.001, 1.002, 0.997, 0.995 (Pulsed-Scanned); 1.000, 1.001, 0.999, 0.998, 0.993, 0.992 (Continuous) respectively.

Conclusion: FC65 chamber can only be used in low dose rate measurements as ion recombination effect cannot be adequately corrected by any methods when dose rate is high. In the calculation of recombination correction, PBS cannot be treated as continuous radiation. PPC05 chamber’s charge collection efficiency drops slightly with dose rate, but can be corrected to be within 0.5% accuracy, where Pulsed or Pulsed-Scanned formula gives similar results.

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