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Multicenter Study
. 2022 Aug;49(8):5537-5550.
doi: 10.1002/mp.15797. Epub 2022 Jul 8.

A multi-institutional evaluation of small field output factor determination following the recommendations of IAEA/AAPM TRS-483

Wolfgang Lechner  1 , Rodolfo Alfonso  2 , Mehenna Arib  3 , M Saiful Huq  4 , Anas Ismail  5 , Rajesh Kinhikar  6 , José M Lárraga-Gutiérrez  7 , Karthick Raj Mani  8 , Nkosingiphile Maphumulo  9 , Otto A Sauer  10 , Shaima Shoeir  11 , Sivalee Suriyapee  12 , Karen Christaki  13
Affiliations
Multicenter Study

A multi-institutional evaluation of small field output factor determination following the recommendations of IAEA/AAPM TRS-483

Wolfgang Lechner et al. Med Phys. 2022 Aug.

Abstract

Purpose: The aim of this work was to test the implementation of small field dosimetry following TRS-483 and to develop quality assurance procedures for the experimental determination of small field output factors (SFOFs).

Materials and methods: Twelve different centers provided SFOFs determined with various detectors. Various linac models using the beam qualities 6 MV and 10 MV with flattening filter and without flattening filter were utilized to generate square fields down to a nominal field size of 0.5 cm × 0.5 cm. The detectors were positioned at 10 cm depth in water. Depending on the local situation, the source-to-surface distance was either set to 90 cm or 100 cm. The SFOFs were normalized to the output of the 10 cm × 10 cm field. The spread of SFOFs measured with different detectors was investigated for each individual linac beam quality and field size. Additionally, linac-type specific SFOF curves were determined for each beam quality and the SFOFs determined using individual detectors were compared to these curves. Example uncertainty budgets were established for a solid state detector and a micro ionization chamber.

Results: The spread of SFOFs for each linac and field was below 5% for all field sizes. With the exception of one linac-type, the SFOFs of all investigated detectors agreed within 10% with the respective linac-type SFOF curve, indicating a potential inter-detector and inter-linac variability.

Conclusion: Quality assurance on the SFOF measurements can be done by investigation of the spread of SFOFs measured with multiple detectors and by comparison to linac-type specific SFOFs. A follow-up of a measurement session should be conducted if the spread of SFOFs is larger than 5%, 3%, and 2% for field sizes of 0.5 cm × 0.5 cm, 1 cm × 1 cm, and field sizes larger than 2 cm × 2 cm, respectively. Additionally, deviations of measured SFOFs to the linac-type-curves of more than 7%, 3%, and 2% for field sizes 0.5 cm × 0.5 cm, 1 cm × 1 cm, and field sizes larger than 1 cm × 1 cm, respectively, should be followed up.

Keywords: TRS-483; small field dosimetry; small field output factors.

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Conflict of interest statement

The authors have no conflicts to disclose.

Figures

FIGURE 1
FIGURE 1
SFOFs for nominal field sizes ranging from 0.5 cm x 0.5 cm to 2 cm x 2 cm for a nominal beam energy of 6 MV WFF for three different types of linacs: Elekta Precise, Elekta Versa HD, and Siemens Primus. The linac‐type‐curves are depicted as dashed lines
FIGURE 2
FIGURE 2
SFOFs for nominal field sizes ranging from 0.5 cm x 0.5 cm to 2 cm x 2 cm for a nominal beam energy of 6 MV WFF for three different types of linacs: Varian 21EX, Varian TrueBeam, and Varian TrueBeam Stx. The linac‐type‐curves are depicted as dashed lines
FIGURE 3
FIGURE 3
SFOFs for nominal field sizes ranging from 0.5 cm x 0.5 cm to 2 cm x 2 cm for a nominal beam energy of 6 MV FFF for three different types of linacs: Elekta Versa HD, Varian TrueBeam, and Varian TrueBeam Stx. The linac‐type‐curves are depicted as dashed lines
FIGURE 4
FIGURE 4
SFOFs for nominal field sizes ranging from 0.5 cm x 0.5 cm to 2 cm x 2 cm for a nominal beam energy of 10 MV WFF for three different types of linacs: Elekta Versa HD, Varian TrueBeam, and Varian TrueBEAM Stx. The linac‐type‐curves are depicted as dashed lines
FIGURE 5
FIGURE 5
SFOFs for nominal field sizes ranging from 0.5 cm x 0.5 cm to 2 cm x 2 cm for a nominal beam energy of 10 MV FFF for three different types of linacs: Elekta Versa HD, Varian TrueBEAM, and Varian TrueBEAM Stx. The linac‐type‐curves are depicted as dashed lines
FIGURE 6
FIGURE 6
Spread plots of all investigated beam energies. Each data point represents the spread of the SFOFs of one linac for a given energy and nominal field size measured using two or three different detectors. Thereby this plot is not influenced by the inter‐linac deviation of the SFOFs. This relative spread of each detector combination is depicted for the following energies 6 MV WFF, 6 MV FFF, 10 MV WFF, and 10 MV FFF. The detector combination IBA/Wellhöfer CC01–IBA EFD–IBA PFD was of particular interest as the same combination was used by two different centers (Nr. 2 and 11). The spread measured by Centre 11 was systematically larger compared to the spread measured by Centre 2 for the 10 MV WFF and FFF beams
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