Dosimetric characteristics of brass mesh as bolus under megavoltage photon irradiation.

OBJECTIVE: This article presents a set of dosimetric measurements describing the properties of brass mesh (Whiting and Davis, Attleboro Falls, MA) under megavoltage photon irradiation conditions, with particular relevance to its use in breast radiotherapy. METHODS: The effectiveness of brass mesh as a bolus material was investigated using 6-, 15- and 6-MV flattening filter-free photon beams. The effect on dose build-up at the entrance surface, build-down at the beam-exit surface, dose with surface entrance obliquity, beam profiles, penumbra and percentage depth doses were investigated. RESULTS: One layer of the brass mesh produces a build-up effect equivalent to 1.1 mm of water at 6 MV and 1.9 mm at 15 MV. The brass generates a backscattered component of dose, if the photon beam exits through it. Percentage depth-dose curves are largely unaffected by the mesh and are shown to be equivalent to plain-field data. Beam penumbra and profiles are unchanged by the brass except within the first millimetre (mm) of phantom, where a periodic pattern of dose enhancement is seen. CONCLUSION: The data presented demonstrate that one layer of brass mesh provides a similar dose build-up effect equivalent to only a few millimetres of water. However, backscatter from the high atomic number (Z) mesh, at the beam exit, contributes appreciably to the overall dose surface enhancement. This dosimetric consequence cannot be neglected and indeed should be considered and accounted for, when determining the bolus effect of the brass mesh in the case of tangential breast irradiation. Advance in knowledge: This article provides dosimetric data necessary for the introduction of brass mesh bolus into the clinical setting for external-beam breast radiotherapy.