Abstract
BackgroundA range pull-back device, such as a machine-related range shifter (MRS) or a universal patient-related range shifter (UPRS), is needed in pencil beam scanning technique to treat shallow tumors.MethodsThree UPRS made by QFix (Avondale, PA, USA) allow treating targets across the body: U-shaped bolus (UB), anterior lateral bolus (ALB), and couch top bolus. Head-and-neck (HN) patients who used the UPRS were tested. The in-air spot sizes were measured and compared in this study at air gaps: 6 cm, 16 cm, and 26 cm. Measurements were performed in a solid water phantom using a single-field optimization pencil beam scanning field with the ALB placed at 0, 10, and 20 cm air gaps. The two-dimensional dose maps at the middle of the spread-out Bragg peak were measured using ion chamber array MatriXX PT (IBA-Dosimetry, Schwarzenbruck, Germany) located at isocenter and compared with the treatment planning system.ResultsA UPRS can be consistently placed close to the patient and maintains a relatively small spot size resulting in improved dose distributions. However, when a UPRS is non-removable (e.g. thick couch top), the quality of volumetric imaging is degraded due to their high Z material construction, hindering the value of Image-Guided Radiation Therapy (IGRT). Limitations of using UPRS with small air gaps include reduced couch weight limit, potential collision with patient or immobilization devices, and challenges using non-coplanar fields with certain UPRS. Our experience showed the combination of a U-shaped bolus exclusively for an HN target and an MRS as the complimentary device for head-and-neck targets as well as for all other treatment sites may be ideal to preserve the dosimetric advantages of pencil beam scanning proton treatments across the body.ConclusionWe have described how to implement UPRS and MRS for various clinical indications using the PBS technique, and comprehensively reviewed the advantage and disadvantages of UPRS and MRS. We recommend the removable UB only to be employed for the brain and HN treatments while an automated MRS is used for all proton beams that require RS but not convenient or feasible to use UB.
Highlights
A range pull-back device, such as a machine-related range shifter (MRS) or a universal patient-related range shifter (UPRS), is needed in pencil beam scanning technique to treat shallow tumors
The proton convolution superposition (PCS) in Eclipse Treatment planning system (TPS) (Varian Medical System, Palo Alto, CA, USA) underestimates the spot size distal to an range shifters (RS) because the spot geometric broadening along the air gap between the RS and the patient is not fully considered [7]
Due to low beam efficiency (< 1%), the proton pencil beam scanning (PBS) system runs with minimum proton energy of 100 MeV, which corresponds to a proton range of 7.71 cm water equivalent thickness (WET)
Summary
A range pull-back device, such as a machine-related range shifter (MRS) or a universal patient-related range shifter (UPRS), is needed in pencil beam scanning technique to treat shallow tumors. The proton convolution superposition (PCS) in Eclipse TPS (Varian Medical System, Palo Alto, CA, USA) underestimates the spot size distal to an RS because the spot geometric broadening along the air gap between the RS and the patient is not fully considered [7]. This introduces considerable inaccuracies, namely up to 30% errors on spot sizes when the position of RS differs more than 15 cm from the commissioning position [8]. Ding et al [9] reported their commissioning results of RS for PBS with RayStation TPS (RaySearch Laboratories, Stockholm, Sweden) and recommend using air gaps from 5–10 cm to maintain accurate dose calculations
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