Field output factors for small fields: A large multicentre study

Abstract

The determination of output factors in small field dosimetry is a crucial point, especially when implementing stereotactic radiotherapy (SRT). Herein, a working group of the French medical physicist society (SFPM) was created to collect small field output factors. The objective was to gather and disseminate information on small field output factors based on different detectors for various clinical SRT equipment and measurement configurations. Method: Participants were surveyed for information about their SRT equipment, including the type of linear particle accelerator (linac), collimator settings, measurement conditions for the output factors and the detectors used. Participants had to report both the ratio of detector readings and the correction factors applied as described in the IAEA TRS-483 code of practice for nominal field sizes smaller or equal to 3 cm. Mean field output factors and their associated standard deviations were calculated when data from at least 3 linacs were available. Results: 23 centres were enrolled in the project. Standard deviations of the mean field output factors were systematically smaller than 1.5% for field sizes larger or equal to 1 cm and reached 5% for the smallest field size (0.5 cm). Deviations with published data were smaller than 2% except for the 0.5 cm circular fixed aperture collimator of the CyberKnife where it reached 3.5%. Conclusion:These field output factor values obtained via a large multicentre study can be considered as an external cross verification for any radiotherapy centre starting a SRT program and should help minimize systematic errors when determining small field output factors.

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

“…A deviation of SFOFs measured for various linacs of 5% to up to 17% were found by other groups. 11,13,20 If the linac-type-curves derived in this work are used as benchmarks for SFOF measurements, an uncertainty with a coverage factor of two of at least 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, needs to be considered. This tolerance was calculated based on the residuals of the fits calculated for the linac-typecurves.…”

“…This code of practice (TRS-483) provides a guideline on the determination of absorbed dose in small photon fields as well as detector and field size-specific output correction factors for relative dosimetry. 7,8 Since then, a few studies on the application of TRS-483 for determination of SFOFs for standard LINACs [10][11][12][13][14] as well as correction factors for detectors or detector orientations that are not available in TRS-483 have been published. [15][16][17] In 2015, the International Atomic Energy Agency initiated a coordinated research project (Testing of Code of Practice on Small Field Dosimetry-E24021) to test the consistency of the procedures and correction factors published in TRS-483 to find pitfalls, if any, and to guide Member States with its implementation.…”

Evaluation of Acuros® XB accuracy for static small fields dose calculations based on the IAEA/AAPM TRS-483 recommendation

“…The algorithm's accuracy using these jaw openings not only has an impact on static fields but also may impact the dose calculation in modulated fields [10]. Moreover, the dose calculation accuracy of Table 4 Comparison of the average field output factors from this work and those reported by Dufreneix et al [16] and Huq et al [9] for flattened and unflattened beams. The data from this work and from Huq et al were interpolated to the nominal field sizes for comparison.…”

“…Table 4 shows the differences between field output factors measured in this work and those reported in the literature for flattened and unflattened beams as a function of the nominal field size. The data from Dufreneix et al [16] were not interpolated because the S clin values were not reported; possibly explaining the difference found for the 1 cm × 1 cm field. For field sizes ⩾1.0 cm, the differences were less than 1.5% for both beam qualities.…”

Small static radiosurgery field dosimetry with small volume ionization chambers

“…In this study we focused on 6 MV and 10 MV FFF, 5 mm SRS fields dosimetry with four ion chambers with active volumes ranging between 0.003 and 0.016 cm 3 . Accurate small field measurements are essential for a proper commissioning of Treatment Planning Systems devoted to SRS and SBRT or Highly Modulated radiotherapy where narrow beams are employed [21,22].…”