Purpose: Sun Nuclear Corporation (SNC) Edge Detector sensors are often picked as detectors of choice for measurements of small fields output factors (OF) due to their small measurement volume and high sensitivity. However these detectors show increased response for the measurement of larger field output factors. Here we review the limitations of edge detectors for large field output measurements.
Methods: 4 Edge Detectors were chosen to evaluate TrueBeam output factors for fields in the range of 1cmx1cm to 40cmx40cm. The measurements were done using PTW water tank at 5cm depth and 95cm SSD. Each detector was used to measure output factors for 4 TrueBeam energies, 6x, 6xFFF, 10x and 15x. The field output measurements were initially normalized to 10cmx10cm field output for each of the detectors. Similarly, CC13 mini ion chamber (PTW) was used to measure the outputs for fields 3cmx3cm and higher. All the measured data was compared to output factors measured using Standard Imaging A19 ion chamber. Statistical uncertainty was evaluated for Edge Detector sample group for each field size measurement and the average output factor compared to ion chamber measurement.
Results: PTW CC13 and SI A19 chambers showed excellent agreement for field sizes 5cmx5cm and larger. Edge Detectors matched the ion chamber output factors in the range of 5cmx5cm-10cmx10cm for low X energies 6x and 6xFFF and between 5cmx5cm and 15cmx15cm for 10x and 15x energies. For larger fields, edge detectors overresponded, and relative output factors exceeded ion chamber measurements with effect more pronounced for larger fields. The largest deviation noted was 2.3%(6x), 2.2%(6xFFF), 1.5%(10x), 1.2%(15x) for 40cmx40cm field.
Conclusion: While being a good tool for measurement of small field dosimetry, edge detector measurements for fields 15cmx15cm and larger should be treated with care. Normalization of OF should be done to output at 5x5-10x10cm field.