Radiotherapy QA phantoms and dosimeters (Sun Nuclear, IBA Dosimetry, PTW)
The measuring instruments that prove a radiotherapy machine gives the dose it claims: ionisation chambers, detector arrays, water tanks and plastic stand-in patients, checked every day, month and year and before every complex plan is delivered.
Overview
Every radiotherapy dose rests on a chain of measurement. Calibrated ionisation chambers (the Farmer chamber design dates from the 1950s) in a water tank fix the machine's output against national standards; three-dimensional scanning tanks map beam profiles at commissioning; daily QA devices check output, flatness, symmetry and lasers before the first patient; and two-dimensional or cylindrical detector arrays (Sun Nuclear's MapCHECK and ArcCHECK, IBA's MatriXX, PTW's OCTAVIUS) measure each patient's IMRT or VMAT plan before it is delivered and compare it with the calculation, the patient-specific QA that AAPM Task Group 218 standardised. Anthropomorphic phantoms with inserts for film, thermoluminescent or optically stimulated dosimeters stand in for the patient in end-to-end tests and in the credentialing that the IROC Houston centre requires before a department may enter a clinical trial. Log-file analysis and independent dose calculation software (RadCalc, Sun Nuclear's SunCHECK, Radformation's ClearCheck) increasingly complement or replace measurement.
The vendors are few and specialised: Sun Nuclear and CIRS are now part of Mirion Medical, IBA Dosimetry belongs to the proton company IBA, and PTW in Freiburg has made chambers since the 1920s. The limits are the time these measurements take from machines and physicists, the sensitivity of array-based QA to the passing criteria chosen, and the difficulty of measuring at the ultra-high dose rates of FLASH or inside the magnetic field of an MR-linac.
- Linac head + MLC
- Target (PTV)
- Gantry arc
- Cone-beam CT guidance
How it works
Calibrated ionisation chambers, diode and scintillator arrays, film and luminescent dosimeters in water or tissue-equivalent phantoms measure absorbed dose and its distribution, traceable to primary standards, to verify machine output and each patient's plan.
- Independent proof that the machine delivers the planned dose
- Catches planning and delivery errors before treatment
- Standardised through AAPM and IAEA protocols
- Consumes machine and physicist time
- Array QA sensitivity depends on chosen criteria
- New regimes (FLASH, MR-linac) outrun existing detectors
Latest papers
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