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Secondary neutron dosimetry for conformal FLASH proton therapy.

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Title: Secondary neutron dosimetry for conformal FLASH proton therapy.
Authors: Chen, Dixin1 (AUTHOR), Motlagh, Seyyedeh Azar Oliaei1 (AUTHOR), Stappen, François Vander2 (AUTHOR), Labarbe, Rudi2 (AUTHOR), Bell, Beryl2 (AUTHOR), Kim, Michele1 (AUTHOR), Teo, Boon‐Keng Kevin1 (AUTHOR), Dong, Lei1 (AUTHOR), Zou, Wei1 (AUTHOR), Diffenderfer, Eric Stanton1 (AUTHOR) Eric.Diffenderfer@pennmedicine.upenn.edu
Source: Medical Physics. Jul2024, Vol. 51 Issue 7, p5081-5093. 13p.
Subjects: International Electrotechnical Commission, Neutron irradiation, Proton therapy, Neutrons, Neutron measurement, Ionization chambers
Geographic Terms: Los Alamos (N.M.)
Abstract: Background: Cyclotron‐based proton therapy systems utilize the highest proton energies to achieve an ultra‐high dose rate (UHDR) for FLASH radiotherapy. The deep‐penetrating range associated with this high energy can be modulated by inserting a uniform plate of proton‐stopping material, known as a range shifter, in the beam path at the nozzle to bring the Bragg peak within the target while ensuring high proton transport efficiency for UHDR. Aluminum has been recently proposed as a range shifter material mainly due to its high compactness and its mechanical properties. A possible drawback lies in the fact that aluminum has a larger cross‐section of producing secondary neutrons compared to conventional plastic range shifters. Accordingly, an increase in secondary neutron contamination was expected during the delivery of range‐modulated FLASH proton therapy, potentially heightening neutron‐induced carcinogenic risks to the patient. Purpose: We conducted neutron dosimetry using simulations and measurements to evaluate excess dose due to neutron exposure during UHDR proton irradiation with aluminum range shifters compared to plastic range shifters. Methods: Monte Carlo simulations in TOPAS were performed to investigate the secondary neutron production characteristics with aluminum range shifter during 225 MeV single‐spot proton irradiation. The computational results were validated against measurements with a pair of ionization chambers in an out‐of‐field region (≤$\le$ 30 cm) and with a Proton Recoil Scintillator‐Los Alamos rem meter in a far‐out‐of‐field region (0.5–2.5 m). The assessments were repeated with solid water slabs as a surrogate for the conventional range shifter material to evaluate the impact of aluminum on neutron yield. The results were compared with the International Electrotechnical Commission (IEC) standards to evaluate the clinical acceptance of the secondary neutron yield. Results: For a range modulation up to 26 cm in water, the maximum simulated and measured values of out‐of‐field secondary neutron dose equivalent per therapeutic dose with aluminum range shifter were found to be (0.57±0.02)mSv/Gy$(0.57\pm 0.02)\ \text{mSv/Gy}$ and (0.46±0.04)mSv/Gy$(0.46\pm 0.04)\ \text{mSv/Gy}$, respectively, overall higher than the solid water cases (simulation: (0.332±0.003)mSv/Gy$(0.332\pm 0.003)\ \text{mSv/Gy}$; measurement: (0.33±0.03)mSv/Gy$(0.33\pm 0.03)\ \text{mSv/Gy}$). The maximum far out‐of‐field secondary neutron dose equivalent was found to be (8.8±0.5$8.8 \pm 0.5$) μSv/Gy$\umu {\rm Sv/Gy}$ and (1.62±0.02$1.62 \pm 0.02$) μSv/Gy$\umu {\rm Sv/Gy}$ for the simulations and rem meter measurements, respectively, also higher than the solid water counterparts (simulation: (3.3±0.3$3.3 \pm 0.3$) μSv/Gy$\umu {\rm Sv/Gy}$; measurement: (0.63±0.03$0.63 \pm 0.03$) μSv/Gy$\umu {\rm Sv/Gy}$). Conclusions: We conducted simulations and measurements of secondary neutron production under proton irradiation at FLASH energy with range shifters. We found that the secondary neutron yield increased when using aluminum range shifters compared to conventional materials while remaining well below the non‐primary radiation limit constrained by the IEC regulations. [ABSTRACT FROM AUTHOR]
Copyright of Medical Physics is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)
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Items – Name: Title
  Label: Title
  Group: Ti
  Data: Secondary neutron dosimetry for conformal FLASH proton therapy.
– Name: Author
  Label: Authors
  Group: Au
  Data: <searchLink fieldCode="AR" term="%22Chen%2C+Dixin%22">Chen, Dixin</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Motlagh%2C+Seyyedeh+Azar+Oliaei%22">Motlagh, Seyyedeh Azar Oliaei</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Stappen%2C+François+Vander%22">Stappen, François Vander</searchLink><relatesTo>2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Labarbe%2C+Rudi%22">Labarbe, Rudi</searchLink><relatesTo>2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Bell%2C+Beryl%22">Bell, Beryl</searchLink><relatesTo>2</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Kim%2C+Michele%22">Kim, Michele</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Teo%2C+Boon‐Keng+Kevin%22">Teo, Boon‐Keng Kevin</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Dong%2C+Lei%22">Dong, Lei</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Zou%2C+Wei%22">Zou, Wei</searchLink><relatesTo>1</relatesTo> (AUTHOR)<br /><searchLink fieldCode="AR" term="%22Diffenderfer%2C+Eric+Stanton%22">Diffenderfer, Eric Stanton</searchLink><relatesTo>1</relatesTo> (AUTHOR)<i> Eric.Diffenderfer@pennmedicine.upenn.edu</i>
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  Data: <searchLink fieldCode="JN" term="%22Medical+Physics%22">Medical Physics</searchLink>. Jul2024, Vol. 51 Issue 7, p5081-5093. 13p.
– Name: Subject
  Label: Subjects
  Group: Su
  Data: <searchLink fieldCode="DE" term="%22International+Electrotechnical+Commission%22">International Electrotechnical Commission</searchLink><br /><searchLink fieldCode="DE" term="%22Neutron+irradiation%22">Neutron irradiation</searchLink><br /><searchLink fieldCode="DE" term="%22Proton+therapy%22">Proton therapy</searchLink><br /><searchLink fieldCode="DE" term="%22Neutrons%22">Neutrons</searchLink><br /><searchLink fieldCode="DE" term="%22Neutron+measurement%22">Neutron measurement</searchLink><br /><searchLink fieldCode="DE" term="%22Ionization+chambers%22">Ionization chambers</searchLink>
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  Label: Geographic Terms
  Group: Su
  Data: <searchLink fieldCode="DE" term="%22Los+Alamos+%28N%2EM%2E%29%22">Los Alamos (N.M.)</searchLink>
– Name: Abstract
  Label: Abstract
  Group: Ab
  Data: Background: Cyclotron‐based proton therapy systems utilize the highest proton energies to achieve an ultra‐high dose rate (UHDR) for FLASH radiotherapy. The deep‐penetrating range associated with this high energy can be modulated by inserting a uniform plate of proton‐stopping material, known as a range shifter, in the beam path at the nozzle to bring the Bragg peak within the target while ensuring high proton transport efficiency for UHDR. Aluminum has been recently proposed as a range shifter material mainly due to its high compactness and its mechanical properties. A possible drawback lies in the fact that aluminum has a larger cross‐section of producing secondary neutrons compared to conventional plastic range shifters. Accordingly, an increase in secondary neutron contamination was expected during the delivery of range‐modulated FLASH proton therapy, potentially heightening neutron‐induced carcinogenic risks to the patient. Purpose: We conducted neutron dosimetry using simulations and measurements to evaluate excess dose due to neutron exposure during UHDR proton irradiation with aluminum range shifters compared to plastic range shifters. Methods: Monte Carlo simulations in TOPAS were performed to investigate the secondary neutron production characteristics with aluminum range shifter during 225 MeV single‐spot proton irradiation. The computational results were validated against measurements with a pair of ionization chambers in an out‐of‐field region (≤$\le$ 30 cm) and with a Proton Recoil Scintillator‐Los Alamos rem meter in a far‐out‐of‐field region (0.5–2.5 m). The assessments were repeated with solid water slabs as a surrogate for the conventional range shifter material to evaluate the impact of aluminum on neutron yield. The results were compared with the International Electrotechnical Commission (IEC) standards to evaluate the clinical acceptance of the secondary neutron yield. Results: For a range modulation up to 26 cm in water, the maximum simulated and measured values of out‐of‐field secondary neutron dose equivalent per therapeutic dose with aluminum range shifter were found to be (0.57±0.02)mSv/Gy$(0.57\pm 0.02)\ \text{mSv/Gy}$ and (0.46±0.04)mSv/Gy$(0.46\pm 0.04)\ \text{mSv/Gy}$, respectively, overall higher than the solid water cases (simulation: (0.332±0.003)mSv/Gy$(0.332\pm 0.003)\ \text{mSv/Gy}$; measurement: (0.33±0.03)mSv/Gy$(0.33\pm 0.03)\ \text{mSv/Gy}$). The maximum far out‐of‐field secondary neutron dose equivalent was found to be (8.8±0.5$8.8 \pm 0.5$) μSv/Gy$\umu {\rm Sv/Gy}$ and (1.62±0.02$1.62 \pm 0.02$) μSv/Gy$\umu {\rm Sv/Gy}$ for the simulations and rem meter measurements, respectively, also higher than the solid water counterparts (simulation: (3.3±0.3$3.3 \pm 0.3$) μSv/Gy$\umu {\rm Sv/Gy}$; measurement: (0.63±0.03$0.63 \pm 0.03$) μSv/Gy$\umu {\rm Sv/Gy}$). Conclusions: We conducted simulations and measurements of secondary neutron production under proton irradiation at FLASH energy with range shifters. We found that the secondary neutron yield increased when using aluminum range shifters compared to conventional materials while remaining well below the non‐primary radiation limit constrained by the IEC regulations. [ABSTRACT FROM AUTHOR]
– Name: AbstractSuppliedCopyright
  Label:
  Group: Ab
  Data: <i>Copyright of Medical Physics is the property of Wiley-Blackwell and its content may not be copied or emailed to multiple sites without the copyright holder's express written permission. Additionally, content may not be used with any artificial intelligence tools or machine learning technologies. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract.</i> (Copyright applies to all Abstracts.)
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        Value: 10.1002/mp.17050
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        Text: English
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      – SubjectFull: International Electrotechnical Commission
        Type: general
      – SubjectFull: Neutron irradiation
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      – SubjectFull: Proton therapy
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      – SubjectFull: Ionization chambers
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      – SubjectFull: Los Alamos (N.M.)
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      – TitleFull: Secondary neutron dosimetry for conformal FLASH proton therapy.
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              Text: Jul2024
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