Verification of representative data for output factors of SRS cones utilizing IAEA TRS 483 recommendations

Abstract

Measurements of small fields continue to be a clinical challenge despite the recent work done to identify their characteristics. Due to this challenge, many physicists use representative data supplied by their vendors to verify their own measurements for small field output factors. However, with recent guidelines being released in IAEA TRS 483, the question remains if this representative data provides an accurate representation for small field dosimetry. A Sun Nuclear EDGE detector, PTW 60012 stereotactic diode, and GafChromic EBT3 films were used to measure the output factor for a set of Varian SRS cones (4 mm-17.5 mm diameters) on a TrueBeam linear accelerator. The measured output factors were then compared to the Varian provided SRS representative data. IAEA TRS 483 recommendations for measuring small field output factors were applied and the impact of those recommendations were examined. The EDGE detector showed good agreement with the representative data when correction factors were not applied (0.01%-1.64% difference) but the PTW 60012 diode showed larger deviation (0.61%-3.35% difference). The EBT3 film showed the largest difference with the representative data (0.66%-9.19%). After application of IAEA TRS 483 detector specific correction factors the output factors measured by the diodes showed good agreement with the EBT3 film for 6MV (<1.8% difference) but showed a large deviation with the representative data (up to 9% difference). The 6FFF energy output factors agreed between the EDGE, the PTW 60012, and EBT3 Film. This work shows that the use of uncorrected representative data on the Truebeam can lead to a significant over estimation of the SRS cone output factors.

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

“…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.…”

Monte Carlo simulation of conical collimators for stereotactic radiosurgery with a 6 MV flattening‐filter‐free photon beam

“…6,7 Many publications reported experimental dosimetric data of conical collimators for stereotactic radiosurgery. [8][9][10][11][12][13][14][15][16][17][18] However, in most of those publications, the reported dosimetric data were acquired in a diversity of setups, which makes difficult to establish comparisons. Moreover, in the light of the current knowledge, these reports have limitations such as: (i) OF are uncorrected in some publications.…”

“…In some cases, no well‐established output correction factors were available for the detectors used. In other works, the available output correction factors were not compatible with the experimental setups used, which were chosen to satisfy the requirements of treatment planning systems; and (ii) most of the published data correspond to flattened beams and few data exist on the dosimetry of conical collimators for flattening‐filter‐free (FFF) beams available in modern linac models 13,14,16,19 …”

“…In other works, the available output correction factors were not compatible with the experimental setups used, which were chosen to satisfy the requirements of treatment planning systems; and (ii) most of the published data correspond to flattened beams and few data exist on the dosimetry of conical collimators for flattening-filter-free (FFF) beams available in modern linac models. 13,14,16,19 There are several published works devoted to study the dosimetry of conical collimators using Monte Carlo simulations. 9,10,13,15,18,20 From those, and to the best of our knowledge, the work by Cheng et al 13 is the only one addressing the dosimetry of cones with a FFF beam.…”

“…One of the most critical parameters is the output factor (OF), which is especially sensitive to the detector used and to the field size 6,7 . Many publications reported experimental dosimetric data of conical collimators for stereotactic radiosurgery 8–18 . However, in most of those publications, the reported dosimetric data were acquired in a diversity of setups, which makes difficult to establish comparisons.…”