Separation of scatter from small MV beams and its effect on detector response.

PURPOSE: Separating the scatter from the primary component of a MV beam to study detector response separately in each case for a better understanding of the role of different effects influencing the response in nonstandard fields.

METHODS: Detector response in three different experimental setups was investigated for a variety of different types (diamond, shielded and unshielded diodes, ionization chamber and film): (a). Detectors positioned in water under a thin steel pole blocking the central part of the beam, yielding only the response to the scatter part of the beam. (b). Detectors positioned in air under a PMMA cap to approximate the contribution of the primary beam without scatter. (c). Detectors positioned in water in the standard open field configuration to obtain a superposition of both.

RESULTS: Detector differences became more clearly observable when the primary beam was blocked and detector behavior heavily depended on the construction type. It was possible to calculate the response in the open fields from the values measured in the blocked configuration with 1% accuracy for all studied field sizes between 0.8 and 10 cm and for all detectors.

CONCLUSIONS: The limitations of clinically used detectors in nonstandard situations were illustrated in the extreme situation of just scattered radiation reaching the detector. By experimentally separating scatter from the primary beam, the roles of different effects on the detector response were observed.