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It is difficult to determine dosimetric characteristics for small field photon beams since such small fields do not achieve complete lateral electronic equilibrium and have steep dose gradients. Dosimetric characteristics of small field 4, 6, and 10 MeV photon beams have been measured in water with a diamond detector and compared to measurements using small volume cylindrical and plane parallel ionization chambers. Percent depth dose (PDD) and beam profiles for 6 and 10 MeV photon beams were measured with diamond detector and cylindrical ion chamber for small fields ranging from 1×1 to 4×4 ㎠. Total scatter factors(S_(c,p)) for 4, 6, and 10 MeV photon beams were measured with diamond detector, cylindrical and plane parallel ion chambers for small fields ranging from 1×1 to 4×4 ㎠. The S_(c,p) factors obtained with three detectors for 4, 6, and 10 MeV photon beams agreed well (±1.2%) for field sizes greater than 2×2, 2.5×2.5, and 3×3 ㎠, respectively. For smaller field sizes, the cylindrical and plane ´parallel ionization chambers measure a smaller S_(c,p) factor, as a result of the steep dose gradients across their sensitive volumes. The PDD values obtained with diamond detector and cylindrical ionization chamber for 6 and 10 MeV photon beams agreed well (±1.5%) for field sizes greater than 4×4 ㎠ For smaller field sizes, diamond detector produced a depth-dose curve which had a significantly shallower falloff than that obtained from the measurements of relative depth-dose with a cylindrical ionization chamber. For the measurements of beam profiles, a distortion in terms of broadened penumbra was observed with a cylindrical ionization chamber since diamond detector exhibited higher spatial resolution. The diamond detector with small sensitive volume, near water equivalent, and high spatial resolution is suitable detector compared to ionization chambers for the measurements of small field photon beams.
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