Using field size factors to characterize the in-air fluence of a proton machine with a range shifter

Abstract

Introduction: The range shifter (RS) is used to treat shallow tumors for a proton pencil beam scanning system (PBS). Adding RS certainly complicates the commissioning of the treatment planning system (TPS) because the spot sizes are significantly enlarged with RS. In this work, we present an efficient method to configure a commercial TPS for a PBS system with a fixed RS. Methods: By combining a spiral delivery with customized control points, we were able to significantly improve measurement efficiency and obtain 250 field size factors (FSF) within three hours. The measured FSFs were used to characterize the proton fluence and fit the parameters for the double-Gaussian fluence model used in the TPS. Extensive validation was performed using FSFs measured in air and in water, absolute doses of spread-out Bragg peak (SOBP) fields, and the dose measurements carried out for patient-specific quality assurance (QA). Results: The measured in-air FSFs agreed with the model's prediction within 3% for all 250 FSFs, and within 2 for 94% of the FSFs. The agreement between model's prediction and measurement was within 2% for the in-air and in-water FSFs and the absolute doses for SOBP beams. The patient-specific QA of 113 fields showed an excellent gamma passing rates (96.95 ± 2.51%) for the absolute dose comparisons with gamma criteria of 2 mm and 2%. Conclusion: The excellent agreement between the model's prediction and measurements proved the efficiency and accuracy of the proposed method of using FSFs to characterize the proton fluence and configure the TPS for a PBS system with fixed RS.

Nuclear halo measurements for accurate prediction of field size factor in a Varian ProBeam proton PBS system

“…As mentioned above, previous investigators have shown that a double Gaussian can be used to model the nuclear halo effect both in air and in medium . Shen et al and Lin et al both showed that inclusion of a double Gaussian spot profile can more accurately model FSF even for Monte Carlo dose calculation . Based on the results presented in this work, in addition to the work done by Shen et al, inclusion of this double Gaussian in the commercial TPS could help mitigate the presented disagreements.…”

“…Additionally, many shallow treatment fields are treated with range shifters, which are expected to exacerbate the halo effect. Future investigations could include measurement of FSFs with range shifters in order to more accurately quantify these discrepancies over independent dose calculations outside of TPS …”

Validation and clinical implementation of an accurate Monte Carlo code for pencil beam scanning proton therapy

“…Shen8 has proposed an efficient and accurate method of using multiple FSF measurements to characterize the proton profile with fixed range shifter and therefore configure the TPS. We believe that with a well validated Monte Carlo platform, use of Monte Carlo calculated dose profile can provide an alternative and efficient method for TPS/range shifter commissioning.…”

“…To minimize the spot broadening, the air gap between the range shifter and patient should be as small as possible, though the potential for collision with the patient often requires a gap that is larger than physically optimal. Due to the generation of secondary products as well as the particle transport within the air gap, it is difficult to model the dose calculation with a range shifter analytically given a limited measured data set,5 thus an approach such as MC is desirable; the broader MC generated dataset is valuable for analytically approximating the low‐dose halo5 using multi‐Gaussian lookup tables in water or in air after a range shifter given the magnification of potential MCS and halo calculation inaccuracies by various range shifter thicknesses and air gaps 7, 8…”

“…The major advantage is that this does not require knowledge of beam line or nozzle components and material compositions, and hence significantly reduces computing time without the need to model the nozzle. As the halo caused by the range shifter is intrinsically different from a halo in vacuum, a proper characterization of the halo component of the beam, below a factor of 10 −4 of the central axis dose is necessary,12 as is a double‐Gaussian fluence model,8 to avoid dose inaccuracies. Hence, the range shifter in this paper is included as part of the simulated area as described by Grevillot10 rather than creating an additional source model.…”