[P250] National survey of kilovoltage radiotherapy equipment in Denmark

Purpose The purpose of this study was to compare dosimetry specifications between kilovoltage (kV) radiotherapy units available at Danish radiotherapy departments. Methods A survey was conducted to gather information about the kV radiotherapy units in Danish radiotherapy departments, i.e. which dosimetry protocols each centre followed, the dosimeters used for calibration, kVp of the clinically used energies, measured half-value layer (HVL), dose rate, output factors, and depth dose measurements. Results Six of the seven Danish radiotherapy departments offer kV radiotherapy, and all six centres contributed data to the study. All kV units are from the same manufacturer, and all centres use clinical energies up to 100 kVp. Beam quality as measured by the HVL ranges from 0.9 to 4.2 mm aluminium, and all centres offer at least two different beam qualities. Dose rate output varies between 3.8 and 7.5 Gy/minute for a 4 cm diameter field, depending on the beam quality and kVp. Different protocols for the determination of absorbed dose are used, with 3 centres following the IPEMB protocol (Phys Med Biol 1996, pp2602-25), 2 centres following the IAEA TRS 277 protocol, and 1 centre following the AAPM protocol (Med Phys 2001, pp868-93). 4 centres use Farmer type cylindrical ion chambers for absolute dosimetry, while 2 centres use plane-parallel chambers. Comparing the data across the departments revealed no systematic differences based on the absorbed dose protocol each department followed. There may be a slight difference in the determination of relative output of the different applicators depending on the ion chamber used for output factor measurements, but with limited data available this remains inconclusive. The 90% and 85% dose to water range of x-rays agree between the departments within 1 mm for similar beam qualities, even though different methods were used to determine the depth dose deposition. Conclusions A national comparison of kV radiotherapy units was conducted, and overall the units were found very similar. No systematic differences were observed based on the calibration protocol, but the available data hint that the choice of ion chamber for calibration measurements may have a small effect on the determination of output factors.