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Adapting a practical EPR dosimetry protocol to measure output factors in small fields with alanine

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2023

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Liebig, Pauline
Fix, Michael K.
Zwicker, Felix

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Journal of Applied Clinical Medical Physics. Wiley. 2023, 24(12), e14191. eISSN 1526-9914. Available under: doi: 10.1002/acm2.14191

Zusammenfassung

Purpose
Modern radiotherapy techniques often deliver small radiation fields. In this work, a practical Electron Paramagnetic Resonance (EPR) dosimetry protocol is adapted and applied to measure output factors (OF) in small fields of a 6 MV radiotherapy system. Correction factors and uncertainties are presented and OFs are compared to the values obtained by following TRS-483 using an ionization chamber (IC).

Methods
Irradiations were performed at 10 cm depth inside a water phantom positioned at 90 cm source to surface distance with a 6 MV flattening filter free photon beam of a Halcyon radiotherapy system. OFs for different nominal field sizes (1 × 1, 2 × 2, 3 × 3, 4 × 4, normalized to 10 × 10 cm2) were determined with a PinPoint 3D (PTW 31022) IC following TRS-483 as well as with alanine pellets with a diameter of 4 mm and a height of 2.4 mm. EPR readout was performed with a benchtop X-band spectrometer. Correction factors due to volume averaging and due to positional uncertainties were derived from 2D film measurements.

Results
OFs obtained from both dosimeter types agreed within 0.7% after applying corrections for the volume averaging effect. For the used alanine pellets, volume averaging correction factors of 1.030(2) for the 1 × 1 cm2 field and <1.002 for the larger field sizes were determined. The correction factor for positional uncertainties of 1 mm was in the order of 1.018 for the 1 × 1 cm2 field. Combined relative standard uncertainties uc for the OFs resulting from alanine measurements were estimated to be below 1.5% for all field sizes. For IC measurements, uc was estimated to be below 1.0%.

Conclusions
A practical EPR dosimetry protocol is adaptable for precisely measuring OFs in small fields down to 1 × 1 cm2. It is recommended to consider the effect of positional uncertainties for field sizes <2 × 2 cm2.

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Fachgebiet (DDC)
540 Chemie

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alanine, density effect, EPR dosimetry, output factor, small field, volume effect

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ISO 690HÖFEL, Sebastian, Pauline LIEBIG, Michael K. FIX, Malte DRESCHER, Felix ZWICKER, 2023. Adapting a practical EPR dosimetry protocol to measure output factors in small fields with alanine. In: Journal of Applied Clinical Medical Physics. Wiley. 2023, 24(12), e14191. eISSN 1526-9914. Available under: doi: 10.1002/acm2.14191
BibTex
@article{Hofel2023Adapt-68650,
  year={2023},
  doi={10.1002/acm2.14191},
  title={Adapting a practical EPR dosimetry protocol to measure output factors in small fields with alanine},
  number={12},
  volume={24},
  journal={Journal of Applied Clinical Medical Physics},
  author={Höfel, Sebastian and Liebig, Pauline and Fix, Michael K. and Drescher, Malte and Zwicker, Felix},
  note={Article Number: e14191}
}
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    <dcterms:abstract>Purpose&lt;br /&gt;Modern radiotherapy techniques often deliver small radiation fields. In this work, a practical Electron Paramagnetic Resonance (EPR) dosimetry protocol is adapted and applied to measure output factors (OF) in small fields of a 6 MV radiotherapy system. Correction factors and uncertainties are presented and OFs are compared to the values obtained by following TRS-483 using an ionization chamber (IC).&lt;br /&gt;&lt;br /&gt;Methods&lt;br /&gt;Irradiations were performed at 10 cm depth inside a water phantom positioned at 90 cm source to surface distance with a 6 MV flattening filter free photon beam of a Halcyon radiotherapy system. OFs for different nominal field sizes (1 × 1, 2 × 2, 3 × 3, 4 × 4, normalized to 10 × 10 cm&lt;sup&gt;2&lt;/sup&gt;) were determined with a PinPoint 3D (PTW 31022) IC following TRS-483 as well as with alanine pellets with a diameter of 4 mm and a height of 2.4 mm. EPR readout was performed with a benchtop X-band spectrometer. Correction factors due to volume averaging and due to positional uncertainties were derived from 2D film measurements.&lt;br /&gt;&lt;br /&gt;Results&lt;br /&gt;OFs obtained from both dosimeter types agreed within 0.7% after applying corrections for the volume averaging effect. For the used alanine pellets, volume averaging correction factors of 1.030(2) for the 1 × 1 cm&lt;sup&gt;2&lt;/sup&gt; field and &lt;1.002 for the larger field sizes were determined. The correction factor for positional uncertainties of 1 mm was in the order of 1.018 for the 1 × 1 cm&lt;sup&gt;2&lt;/sup&gt; field. Combined relative standard uncertainties u&lt;sub&gt;c&lt;/sub&gt; for the OFs resulting from alanine measurements were estimated to be below 1.5% for all field sizes. For IC measurements, uc was estimated to be below 1.0%.&lt;br /&gt;&lt;br /&gt;Conclusions&lt;br /&gt;A practical EPR dosimetry protocol is adaptable for precisely measuring OFs in small fields down to 1 × 1 cm&lt;sup&gt;2&lt;/sup&gt;. It is recommended to consider the effect of positional uncertainties for field sizes &lt;2 × 2 cm&lt;sup&gt;2&lt;/sup&gt;.</dcterms:abstract>
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