Methods of verification and measurement of MLC-defined small field output factors for eight medical accelerators in Henan province

Objective: To investigate muli-leaf collimator (MLC)-defined small field output factors calculated by the treatment planning system (TPS), and to study the measuring method of small field output factors verified by 0.015 cc PinPoint ionization chamber. Methods: Eight medical accelerators for intensity-modulated radiation therapy (IMRT) were investigated in Henan province, and TPS-calculated output factors for various small fields (6 cm × 6 cm, 4 cm × 4 cm, 3 cm × 3 cm and 2 cm × 2 cm) were compared with published values recommended by IAEA. If the relative deviation was more than ±3% for the 2 cm × 2 cm field size and ±2% for the fields of 6 cm × 6 cm, 4 cm × 4 cm and 3 cm × 3 cm, which was beyond the scope of IAEA allowed, the output factors will be measured and verified using 0.015 cc PinPoint ionization chamber and Unidos electrometer. Results: TPS-calculated small field output factors for eight medical accelerators were compared with published values. The relative deviation of small field output factors for five pieces of equipment, which accounted for 62.5% of the total, met the IAEA's requirement, while the other three, which accounted for 37.5% of the total, did not. After measuring with PinPoint ionization chamber, the results from only three pieces of equipment met minimum IAEA's requirement. Conclusions: MLC-defined small field output factors calculated by TPS for some medical accelerators in Henan need to be measured and corrected using micro-ionization chamber, and the measured values could be taken as the basis of radiation treatment planning. (authors)