Influence of the Integral Quality Monitor transmission detector on high energy photon beams: A multi-centre study.
OBJECTIVE
The influence of the Integral Quality Monitor (IQM) transmission detector on photon beam properties was evaluated in a preclinical phase, using data from nine participating centres: (i) the change of beam quality (beam hardening), (ii) the influence on surface dose, and (iii) the attenuation of the IQM detector.


METHODS
For 6 different nominal photon energies (4 standard, 2 FFF) and square field sizes from 1×1cm2to 20×20cm2, the effect of IQM on beam quality was assessed from the PDD20,10values obtained from the percentage dose depth (PDD) curves, measured with and without IQM in the beam path. The change in surface dose with/without IQM was assessed for all available energies and field sizes from 4×4cm2to 20×20cm2. The transmission factor was calculated by means of measured absorbed dose at 10cm depth for all available energies and field sizes.


RESULTS
(i) A small (0.11-0.53%) yet statistically significant beam hardening effect was observed, depending on photon beam energy. (ii) The increase in surface dose correlated with field size (p<0.01) for all photon energies except for 18MV. The change in surface dose was smaller than 3.3% in all cases except for the 20×20cm2field and 10MV FFF beam, where it reached 8.1%. (iii) For standard beams, transmission of the IQM showed a weak dependence on the field size, and a pronounced dependence on the beam energy (0.9412 for 6MV to 0.9578 for 18MV and 0.9440 for 6MV FFF; 0.9533 for 10MV FFF).


CONCLUSIONS
The effects of the IQM detector on photon beam properties were found to be small yet statistically significant. The magnitudes of changes which were found justify treating IQM either as tray factors within the treatment planning system (TPS) for a particular energy or alternatively as modified outputs for specific beam energy of linear accelerators, which eases the introduction of the IQM into clinical practice.
Department of Radiation Physics, Institute of Oncology, Ljubljana, Slovenia. Electronic address: bcasar@onko-i.si.
Bozidar
B
Casar
Lake Constance Radiation Oncology Centre, Singen & Friedrichshafen, Germany.
Marlies
M
Pasler
Department of Radiation Oncology, University of Würzburg, Würzburg, Germany.
Sonja
S
Wegener
Department of Radiation Oncology, UC Davis Comprehensive Cancer Center, Sacramento, USA.
David
D
Hoffman
University of Florence, Department of Biomedical Experimental and Clinical Science "M. Serio", Azienda Ospedaliera Universitaria Careggi, Florence, Italy.
Cinzia
C
Talamonti
Department of Radiation Oncology & Molecular Radiation Sciences, Johns Hopkins University School of Medicine, Baltimore, USA.
Jianguo
J
Qian
Department of Radiation Physics, Institute of Oncology, Ljubljana, Slovenia.
Ignasi
I
Mendez
Department of Radiation Physics, Institute of Oncology, Ljubljana, Slovenia.
Denis
D
Brojan
Christie Medical Physics & Engineering, The Christie NHS Foundation Trust, Withington, Manchester, United Kingdom.
Bruce
B
Perrin
Department of Radiation Oncology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands.
Martijn
M
Kusters
Department of Radiation Oncology, Radboud University Nijmegen Medical Centre, Nijmegen, The Netherlands.
Richard
R
Canters
University of Florence, Department of Biomedical Experimental and Clinical Science "M. Serio", Azienda Ospedaliera Universitaria Careggi, Florence, Italy.
Stefania
S
Pallotta
Department of Radiation Physics, Institute of Oncology, Ljubljana, Slovenia.
Primoz
P
Peterlin
D002985:Clinical Protocols; D004566:Electrodes; D006801:Humans; D010315:Particle Accelerators / Q000592:standards*; D019047:Phantoms, Imaging; D017785:Photons / Q000627:therapeutic use*; D011785:Quality Assurance, Health Care; D011874:Radiometry
IQM; Linearbeschleuniger; On-line-Dosisüberwachung; Qualitätssicherung; Strahlaufhärtung; Transmission detector; Transmissionsdetektor; beam hardening; linear accelerator; on-line dose monitoring; quality assurance