Question
What are the small field output correction factors of IBA ionization chambers?
Answer
In small field dosimetry, the detector-specific output correction factor k(fclin,fmsr)(Qclin,Qmsr), is applied to ensure accurate dose measurements. This factor accounts for challenges such as loss of lateral charged particle equilibrium, volume averaging, and detector perturbation effects, all of which are significant in small fields.
The correction factor can be determined experimentally or through Monte Carlo simulations, following the formalism and definitions outlined in [1].
The table below lists the small field output correction factors k(fclin,fmsr)(Qclin,Qmsr) 1σ Type A (statistical) uncertainty for a 6 MV Varian Clinac, as calculated via Monte Carlo [2].
For optimal readability, copy the table into an Excel spreadsheet.
| K(fclin,fmsr)(Qclin,Qmsr) | ||||
| Field size [cm] | CC08 | CC04 | Razor™ Chamber | Razor™ Nano Chamber |
| 6 | 0.9990(6) | 0.9992(7) | 1.0003(17) | 1.0000(13) |
| 4 | 0.9988(6) | 0.9983(7) | 1.0023(17) | 0.9983(20) |
| 3 | 1.0002(8) | 0.9987(9) | 1.0006(19) | 1.0005(18) |
| 2.5 | 0.9997(8) | 0.9975(12) | 1.0017(18) | 0.9981(21) |
| 2 | 1.0021(7) | 0.9995(7) | 1.0009(17) | 0.9992(14) |
| 1.5 | 1.0116(6) | 1.0022(8) | 0.9996(17) | 0.9937(14) |
| 1.2 | 1.0295(6) | 1.0118(7) | 0.9990(17) | 0.9908(12) |
| 1 | 1.0566(7) | 1.0221(8) | 0.9991(17) | 0.9848(12) |
| 0.8 | 1.1175(8) | 1.0484(9) | 0.9983(18) | 0.9709(16) |
| 0.6 | 1.2635(9) | 1.1091(9) | 1.0006(18) | 0.9407(14) |
| 0.5 | 1.3884(24) | 1.1587(20) | 1.0040(22) | 0.9146(18) |
| 0.4 | 1.5574(12) | 1.2260(11) | 1.0032(21) | 0.8713(13) |
Reference
[1] Hugo Palmans, et al. Dosimetry of small static fields used in external photon beam radiotherapy: Summary of TRS-483, the IAEA–AAPM international Code of Practice for reference and relative dose determination. Medical Physics, 45(11):e1123–e1145, 2018.
[2] D. Czarnecki, internal report (March 2025).