PubMed · 806722
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1975. [High LET radiotherapy].. https://pubmed.ncbi.nlm.nih.gov/806722/
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The constancy of radiation quality of therapeutically employed particle accelerators has to be checked at weekly intervals. Any change in radiation quality may have considerable therapeutic effects owing to its influence on dose distribution. It can be recommended to make measurements instead of, or in addition to, the axial reference-point measurement at 5 and 15 cm depth in the phantom, at 5 cm depth in the beam axis and at a reference-point about 1 cm within the geometric edge of the field, for checking the constancy of the radiation quality of bremsstrahlung. Only then, if routine checks carried out for the axial and the lateral reference-point dose ratios do not show any deviations greater than e.g. +/- 2%, radiation quality is deemed to have remained sufficiently constant for radiotherapeutical applications.
An accessory beam flattening filter (brass disk with critical angles) attaches to the shadow tray of the clinac-4 linear accelerator, intercepts the x-ray beam, and eliminates higher dose areas at the edges of large fields near the skin surface. This improves the x-ray beam dose distribution within the patient and results in a reduction of dose to normal tissues near the surface.
An auxiliary flatness filter for large field treatment on 4 MV linear accelerator provides an isodose distribution which does not vary more than 6% over the central 80% of the axis of a 32 x 32-cm field at 80 cm in air, and at a 2.5, 6, and 12-cm depth in water. This lead disc filter does not affect the central axis depth dose values, and results in only a 16% reduction in dose rate on the central field axis.