Accuracy of dose under narrow shielding blocks

Today shielding blocks are widely used to reduce the dose to organ at risk. The dose under a block depends on transmitted primary dose, scatter within the patient and external scatter due to head geometry, blocking tray and field size. Absorbed dose to shielded organ is expected to change also with block width, depth of organ and tissue inhomogeneity. Algorithms used in treatment planning systems take into account of these factors only partially. Therefore dose under shielding blocks is known with some inaccuracy. Dose measurements were made in a water phantom under low melting alloy blocks in a beam of 6 MV X rays. Dose distributions were calculated by treatment planning system with the same irradiation geometry and compared with values in water phantom. Results show that changing block width from 0.9 cm to 1.6 cm, dose under block, at 5 cm in water, ranges from 14% to 7% of the maximum dose on the central axis for an unshielded field of the same size. In the same conditions, calculated dose agree with experimental value only for 1.6 cm block. Size of shielded area doesn't change greatly with depth, so great care must be taken in comparison with optical shadow, dosimetric data and portal film image of the block