Measurements of output factors for small photon fields up to 10 cm x 10 cm

Field output factors (OF) for photon beams from a 6 MV medical accelerator
were measured using five different detectors in a scanning water phantom. The
measurements were taken for square field sizes of integral widths ranging from 1 cm to
10 cm for two reference source-to-surface distances (SSD) and depths in water. For the
diode detectors, square field widths as small as 2.5 mm were also studied. The photon
beams were collimated by using either the jaws or the multileaf collimators. Measured
OFs are found to depend upon the field size, SSD, depth and also upon the type of beam
collimation, size and type of detector used. For field sizes larger than 3 cm x 3 cm, the
OF measurements agree to within 1% or less. The largest variation in OF occurs for jawsshaped field of size 1 cm x 1cm, where a difference of more than 18% is observed.