Question
What are the magnetic field quality conversion factor kB,Q of IBA chambers?
Answer
The static magnetic field present in MR-guided radiotherapy systems can influence both dose deposition and the collection of charged particles in air-filled ionization chambers. The impact of the magnetic field on the ionization chamber response is characterized by the magnetic field quality conversion factor kB,Q [1].
Values of kB,Q have been determined for two MR-guided radiotherapy systems: the Elekta Unity, featuring a 1.5 T static magnetic field and a single-energy 7 MV FFF linear accelerator beam, and the ViewRay MRIdian, which operates with a 0.35 T static magnetic field and a 6 MV FFF beam. In both systems, the radiation beam is oriented perpendicular to the static magnetic field.
The table below summarizes the kB,Q values for IBA ionization chambers using ViewRay MRIdian and Elekta Unity MR-guided radiotherapy systems. All chambers were positioned parallel to the magnetic field (0° orientation). A combined uncertainty of 1.12% has been calculated among all chambers studied [1].
(For better readability, please copy the table into an Excel file)
kB,Q | |||
| Ion chamber model | Elekta Unity | ViewRay MRIdian | Ref. |
| IBA FC23-C | 0.9986 | 1.0005 | [1] |
| IBA FC65-G | 0.9943 | 0.9936 | [1] |
| IBA CC25 | 0.9992 | 0.9993 | [1] |
| IBA CC13 | 1.0012 | 1.0002 | [1] |
| IBA CC08 | 0.9982 | 1.0000 | [1] |
For the high-field 1.5 T Elekta Unity MR-linac, ionization chamber magnetic field quality conversion factors, kB,Q, were experimentally determined with the ionization chamber long axis positioned at the four cardinal angles relative to the magnetic field. The following table shows the resulting kB,Q values measured at 0°, 90°, 180°, and 270° orientations relative to the 1.5 T magnetic field.
| Ion chamber model | 0° | 90° | 180° | 270° | Ref. |
| IBA FC23-C | 1.002 | 0.983 (−1.9%) | 1.000 (−0.2%) | 0.999 (−0.3%) | [2] |
| IBA CC25 | 1.002 | 0.970 (−3.2%) | 1.002 (-) | 0.985 (−1.7%) | [2] |
| IBA CC08 | 1.002 | 0.976 (−2.6%) | 1.002 (-) | 1.028 (2.6%) | [2] |
Reference
[1] Sarfehnia A, Alvarez PE, Bellon MR, et al. AAPM task group report 351: Protocol for clinical reference dosimetry in external beam MR-guided radiotherapy. Med Phys. 2025; 52:e17884. https://doi.org/10.1002/mp.17884
[2] Orlando, Nathan, et al. "Experimental determination of magnetic field quality conversion factors for eleven ionization chambers in 1.5 T and 0.35 T MR‐linac systems." Medical physics 51.4 (2024): 2998-3009.