Extrafocal radiation: A unified approach to the prediction of beam penumbra and output factors for megavoltage x-ray beams

摘要: An extrafocal source model has been developed to explain the dependence of head scatter and beam penumbra on field size. In this model, x‐ray a medicallinear accelerator is described by two components: small but intense focal component; broadly distributed component low intensity. The so large that it can be ‘‘eclipsed’’ field‐defining collimators. Extrafocal radiation was found account for 12% energy fluence central axis 6 MV from Varian Clinac 2100c accelerator. Head factors were calculated ‘‘in‐air’’ symmetric, asymmetric, half‐blocked fields. Calculations agreed with measured values better than 0.5%, average. However, asymmetric fields underestimated 1.2% when one dimensions reduced 4 cm (the minimum jaw setting tested). combined convolution/superposition dose calculation algorithm calculate dose‐per‐monitor‐unit calibration (output) profiles in water. These calculations predict degradation edge as function size, output within most extreme case width, 2%. Dose are predicted without aid an empirical fit data. will particularly useful defined independent multileaf collimation systems.