A reduction in the AAPM TG-36 reported peripheral dose distributions with tertiary multileaf collimation

Purpose: The American Association of Physicists in Medicine Task Group 36 ( AAPM TG-36) data can be used to estimate peripheral dose (PD) distributions outside the primary radiation field. However, the report data apply to lin ear accelerators not equipped with tertiary multileaf collimators (MLCs). P eripheral dose distributions consist of internal scatter, collimation scatt er, transmission through collimation, head leakage, and room scatter. Terti ary MLCs may significantly reduce the PD due to a reduction in collimation scatter, transmission through collimation, and head leakage. Measurements w ere performed on a multimodality linear accelerator, equipped with a tertia ry MLC, to determine PD distributions as a function of energy, field size, distance from the primary radiation field edge, MLC position, and collimato r orientation. Methods and Materials: Measurements were made using an ionization chamber e mbedded in a 20 x 40 x 120-cm(3) water-equivalent plastic phantom with the secondary collimator and MLC settings of 10 x 10, 15 x 15, 20 x 20, 25 x 25 cm(2), and with the MLC fully retracted. Data were taken along the longitu dinal axis of the machine for 6 and 18 MV photons. Peripheral dose distribu tions were evaluated with the collimator set to 180 and 90 degrees. Rotatio n of the collimator allowed measurements parallel and orthogonal to the dir ection of motion of the MLC. Results: For both photon energies, peripheral doses measured on a MLC machi ne were lower than the TG-36 data. When the collimator is rotated by 90 deg rees, placing the lower jaws and the MLC leaves along the plane of interest , PD was reduced by as much as a factor of three compared with PDs measured with the MLC fully retracted and the collimator rotated to 180 degrees. PD s measured with the MLC fully retracted and collimator rotated to 180 degre es were comparable to the TG-36 data. Measured PDs were lower when the MLC was used to shape the field than when the MLC was fully retracted. Conclusion: A strategic orientation of the collimator with a tertiary MLC c an reduce PD distributions by more than a factor of two. This decrease sign ificantly lessens or eliminates the need for external lead shielding to red uce the critical organ dose. This method can be used even when Lipowitz met al blocking (such as for mantle fields) is used, with the MLC leaves orient ed along the longitudinal plane. (C) 1999 Elsevier Science Inc.