Parametrization of in-air spot size as a function of energy and air gap for the ProteusPLUS pencil beam scanning proton
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Parametrization of in‑air spot size as a function of energy and air gap for the ProteusPLUS pencil beam scanning proton therapy system Suresh Rana1,2,3 · Anatoly B. Rosenfeld4 Received: 24 July 2020 / Revised: 24 September 2020 / Accepted: 26 September 2020 © Japanese Society of Radiological Technology and Japan Society of Medical Physics 2020
Abstract The purpose of this study was to parametrize the in-air one sigma spot size for various energies and air gaps in pencil beam scanning (PBS) proton therapy. The current study included range shifters with a water equivalent thickness (WET) of 40 mm (RS40) and 75 mm (RS75). For RS40, the spot sizes were measured for energies ranging from 80 to 225 MeV in increments of 2.5 MeV, whereas the air gap was varied from 5 to 25 cm in increments of 2.5 cm. For RS75, the spot sizes were measured for energies ranging from 120 to 225 MeV in increments of 2.5 MeV, whereas the air gap was varied from 5 to 35 cm in increments of 2.5 cm. For both RS40 and RS75, all measurements (n = 1090) were acquired at the isocenter using a Lynx 2D scintillation detector. For RS40, the spot sizes increased from 3.1 mm to 10.4 mm, whereas the variation in spot sizes for RS75 ranged from 3.3 mm to 13.1 mm. For each range shifter, an analytical equation demonstrating the relationship of the spot size with the proton energy and air gap was obtained. The best parametrization results were obtained with the 3rd degree polynomial fits of the energy and air gap parameters. The average difference between the modeled and measured spot sizes was 0.0 ± 0.1 mm (range, − 0.24–0.21 mm) for RS40, and 0.0 ± 0.1 mm (range, − 0.23–0.15 mm) for RS75. In conclusion, the analytical model agrees within ± 0.25 mm of the measured spot sizes on a ProteusPLUS PBS proton system with a PBS dedicated nozzle. Keywords Analytical model · Spot size · Range shifter · Air gap · Proton therapy
1 Introduction Pencil beam scanning (PBS) proton therapy is gaining broader attention in the radiotherapy community. Advancements in proton therapy technology have continued to grow over the last decade [1]. One such improvement is the reduction of the in-air spot size (hereafter referred to as the spot size) of a pencil beam delivered by a proton machine [2–4]. The spot size is one of the fundamental parameters of the * Suresh Rana [email protected] 1
Department of Radiation Oncology, Miami Cancer Institute, Baptist Health South Florida, Miami, FL, USA
2
Department of Radiation Oncology, Herbert Wertheim College of Medicine, Florida International University, Miami, FL, USA
3
Department of Medical Physics, The Oklahoma Proton Center, Oklahoma City, OK, USA
4
Centre for Medical Radiation Physics (CMRP), University of Wollongong, Wollongong, NSW, Australia
proton beam model. The accuracy of the proton beam model/ treatment planning system (TPS) in predicting spot sizes at various energies is critical for ensuring dose homogeneity within the target volume [3, 4]. Beam modifying devices, such as range shifters/preabsorbers,
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