SU-E-T-298: Small Field Total Scatter Factors Using a Commercial Scintillator Detector: Calibration Parameters Are Not Independent of Field Size




To evaluate the accuracy in total scatter factor (Sc,p) determination for small fields using commercial plastic scintillator detector (PSD). The manufacturer's spectral discrimination method to subtract Cerenkov light from the signal is discussed.


Sc,p for field sizes ranging from 0.5 to 10 cm were measured using PSD Exradin (Standard Imaging) connected to two channel electrometer measuring the signals in two different spectral regions to subtract the Cerenkov signal from the PSD signal. A Pinpoint ionisation chamber 31006 (PTW) and a non-shielded semiconductor detector EFD (Scanditronix) were used for comparison. Measures were performed for a 6 MV X-ray beam. The Sc,p are measured at 10 cm depth in water for a SSD=100 cm and normalized to a 10' 10 cm2 field size at the isocenter. All detectors were placed with their symmetry axis parallel to the beam axis.We followed the manufacturer's recommended calibration methodology to subtract the Cerenkov contribution to the signal as well as a modified method using smaller field sizes. The Sc,p calculated by using both calibration methodologies were compared.


Sc,p measured with the semiconductor and the PinPoint detectors agree, within 1.5%, for field sizes between 10' 10 and 1' 1 cm2. Sc,p measured with the PSD using the manufacturer's calibration methodology were systematically 4% higher than those measured with the semiconductor detector for field sizes smaller than 5' 5 cm2. By using a modified calibration methodology for smalls fields and keeping the manufacturer calibration methodology for fields larger than 5' 5cm2 field Sc,p matched semiconductor results within 2% field sizes larger than 1.5 cm.


The calibration methodology proposed by the manufacturer is not appropriate for dose measurements in small fields. The calibration parameters are not independent of the incident radiation spectrum for this PSD. This work was partially financed by grant 2012 of Barcelona board of the AECC.