158. Modification of the penumbra of the 6 MV X Rays beams by individual blocks

PurposeTo assess the influence of individual blocks on penumbra of the photon beams in order to improve the dose homogeneity for photon and electron matching.Materials and methodsFive individual blocks with side wall angles of 2.2o;4.3 o;5.4 o;6.5 o and 9.0 o were maid. Beam profiles (BP) at 1,2,3 and 4 cm depth were measured for rectangular half blocked 15x20 cm, 6 MV X Rays beams. Blocks were placed in the beam contrarily to the beam divergence. BP for typical electron field size used for the inverse hockey stick technique of 6, 9, 12 and 15 MeV energies at 1,2,3 and 4 cm depth were measured and for each field the penumbra was calculated. Dose distribution for matching electron and photon beams were calculated. Distance between the edges of the photon and electron beams was search to get the most homogenous dose distribution in the therapeutic region (TR).ResultsPenumbra of the open photon beam was 5 mm. For photon modified beam with the block of the angle 2,2o;4,3o; 5,4 o; 6,5 o and 9.0 o at 2 cm depth were 5.1, 7.3, 8.6, 9.9 and 12.2 mm respectively. Irrespective of the energy at maximum electron field penumbra was about 12 mm. The best dose distribution for matched beams were obtained for photon beam modified by the 9o block and for gap 6 mm. In the match region the differences between maximum dose and minimum dose to the TR was always less than 25%. This is of about 10% smaller than for matching of open photon beam and electron beam.Conclusions1.Individual blocks placed contrarily to the beam divergence allow for changing the penumbra of photon beams. 2.By means of the individual block application the dose homogeneity in the photon and electron matchting region may be considerably improved