Head scatter factors for small MV photon fields. Part II: the effects of source size and detector

Small field dosimetry continues to be problematic for the planning and delivery of both circular stereotactic radiotherapy (SRT) fields and intensity modulated radiotherapy (IMRT) fields. Although the separation of head and phantom scatter is primarily of interest in IMRT fields, measurements can often been difficult. However, the set-up in fields formed by stereotactic collimators is more precise and total scatter and head scatter factors can be more easily measured. Phantom scatter factors calculated from these measurements can be extrapolated from the stereotactic situation to the IMRT one. However, the problem of measuring small field head scatter factors must be overcome first. In this work, four solid-state detectors (photon, electron, stereotactic diodes and a diamond) were examined in the measurement of small (0.5-4 cm width) field S(c), to determine whether differences in S(c) values were due to real differences in scatter and source size, or due to apparent differences caused by the detector and experimental set-up.

Measurements were carried out on the 6 MV beams of three different linacs (Varian 600 CD, BBC CH6 and CH20), in open fields produced by the movable linac collimators and in fields produced by stereotactic collimators and the MLC. A small plastic top, equal in thickness to build-up and 5 cm deep and equal in diameter to that of each detector, was used in all experiments. In-air profiles were measured for each measurement situation and these were used to remove the effects of volume averaging in each detector.

Real changes in S(c) were found to be due to changes in the amount of scatter produced in the linac head and in the air, with particular reference to the amount of the opening at the top of the primary collimator 'seen' from the point of measurement. Apparent changes were found to be caused by the experimental set-up, in particular the positional accuracy, the detector size (sensitive volume and size of outer casing) and the width of phantom used.

Each of the four solid-state detectors investigated was found to be suitable for measurements in fields 1cm width, when used with the build-up top and when volume averaging is accounted for. However, phantom diameter is a problem in fields <1cm width. It is not experimentally accurate to measure S(c) in fields <1cm width with any of the above detectors, without further modification of experimental technique.