Sun Nuclear’s Wide Field Calibration (WFC) allows users to calibrate their Sun Nuclear instrument without the need to return it to the factory. The WFC is precise, accurate, and efficient to perform.

Calibration files consist of sensitivity correction factors for each detector that can be applied to the measured output of a detector array so that the measurement of dose distribution is independent of the detector sensitivity. Historical methods of calibration have used a narrow field and a device to move the detector array in steps such that each detector occupies the central axis position while its response is measured. If the array detector’s response is energy and directional dependent, then the calibration value is valid only at the beam energy and orientation used during calibration. The severity of this limitation depends upon the magnitude of the energy response. These problems are eliminated in the WFC.

The WFC is an automated wizard within the Sun Nuclear instrument software. Animated steps guide the user through four to six beam exposures, depending on the instrument. Each exposure is followed by an instrument shift and/or rotation. Substitution of detector locations during these exposures yields a calibration file that makes no assumptions about beam shape; therefore the beam is not required to be within flatness and symmetry specifications to perform a calibration. There are two postulates required during the WFC exposures: 1) the beam shape is the same for all exposures, 2) the detectors’ response is the same during all exposures.

The benefit of the WFC is that the measured value for each detector is taken at or near the field location where it will be used to measure the field distribution. Any change in energy (including that caused by scatter in a wide field) or angle of incidence at adjacent regions in the field will be small which will minimize any error caused by energy or directional response. Furthermore, the relative sensitivity between adjacent detectors will be known to a higher precision because it is a simple ratio between two measured values, at the spatial location where they will be used. This will result in a higher precision in field profile measurement, which can reveal significant profile shapes otherwise obscured by measurement artifacts.

The WFC uncertainty depends upon conditions that satisfy the two postulates and the numerical precision of the measurement; in typical applications, the uncertainty will be less than 0.1%. WFC must be performed on a stable beam, but no assumptions about beam flatness, symmetry, or shape are made in the WFC.

The WFC may be used for all common therapy beam energies, both continuous and pulsed. For optimal results a WFC file will be created for each unique beam energy.

Most users will require approximately fifteen minutes to perform the WFC. First time users may require up to thirty minutes to gain familiarity.

Instruments using diode detectors will normally require calibration every one to two years. This is due to a diode sensitivity change of approximately 0.5% for every 100,000cGy at 6MV.