Detector comparison for small field output factor measurements in flattening filter free photon beams
Wolfgang Lechner1, Hugo Palmans, Lukas Sölkner
1Department of Radiation Oncology, Medical University of Vienna/AKH Wien, Vienna, Austria; Christian Doppler Laboratory for Medical Radiation Research for Radiation Oncology, Medical University of Vienna, Vienna, Austria.
Purpose:
The applicability of various detectors for small field dosimetry and whether there are differences in the detector response when irradiated with FF- and FFF-beams was investigated.
Materials And Methods:
Output factors of 6 and 10 MV FF- and FFF-beams were measured with 14 different online detectors using field sizes between 10 × 10 and 0.6 × 0.6 cm(2) at a depth of 5 cm of water in isocentric conditions. Alanine pellets with a diameter of 5 and 2.5mm were used as reference dosimeters for field sizes down to 1.2 × 1.2 and 0.6 × 0.6 cm(2), respectively. The ratio of the relative output measured with the online detectors to the relative output measured with alanine was evaluated (referred to as dose response ratio).
Results:
The dose response ratios of two different shielded diodes measured with 10 MV FF-beams deviated substantially by 2-3% compared to FFF-beams at a field size of 0.6 × 0.6 cm(2). This difference was less pronounced for 6 MV FF- and FFF-beams. For all other detectors the dose response ratios of FF- and FFF-beams showed no significant difference.
Conclusion:
The dose response ratios of the majority of the detectors agreed within the measurement uncertainty when irradiated with FF- and FFF-beams. Of all investigated detectors, the microDiamond and the unshielded diodes would require only small corrections which make them suitable candidates for small field dosimetry in FF- and FFF-beams.
