J Med Phys Close
 

Figure 7: (a) A comparison of out-of-field dose from the (vertical waveguide) Varian 600C with that from the (multi-mode) Varian Trilogy. Note that for the latter linac, the doses are consistently higher than the single-energy mode linac, despite both being operated at the same energy of 6 MV. The differences in out-of-field dose may be attributed to the significant differences in treatment head design; in particular, unlike the 600C, the Trilogy possesses a bending magnet. Data adapted from Taylor et al.[51] (b) An indication of the decreasing influence of field size on out-of-field dose (from a Varian Trilogy) with increasing distance from the isocenter. Taking mean organ doses from a study on pediatric radiotherapy out-of-field dose by Taylor et al.,[51] it is clear that while differences between large (5 × 5 cm2) and small (0.5 × 0.5 cm2) fields are significant close to the primary field, further away the field-size-dependent contributions to out-of-field dose (i.e., collimator scatter and patient-scatter) are reduced. In both (a) and (b), the mean values of the published doses are presented, and the horizontal scale is the distance from isocenter

Figure 7: (a) A comparison of out-of-field dose from the (vertical waveguide) Varian 600C with that from the (multi-mode) Varian Trilogy. Note that for the latter linac, the doses are consistently higher than the single-energy mode linac, despite both being operated at the same energy of 6 MV. The differences in out-of-field dose may be attributed to the significant differences in treatment head design; in particular, unlike the 600C, the Trilogy possesses a bending magnet. Data adapted from Taylor <i>et al.</i><sup>[51]</sup> (b) An indication of the decreasing influence of field size on out-of-field dose (from a Varian Trilogy) with increasing distance from the isocenter. Taking mean organ doses from a study on pediatric radiotherapy out-of-field dose by Taylor <i>et al.</i>,<sup>[51]</sup> it is clear that while differences between large (5 × 5 cm<sup>2</sup>) and small (0.5 × 0.5 cm<sup>2</sup>) fields are significant close to the primary field, further away the field-size-dependent contributions to out-of-field dose (i.e., collimator scatter and patient-scatter) are reduced. In both (a) and (b), the mean values of the published doses are presented, and the horizontal scale is the distance from isocenter