Session Item

Gas-filled temporary tissue expanders are increasingly being used during breast cancer treatments, due to improved patient comfort and convenience, compared to older saline-filled systems. However the implantation of a silicone bladder containing a  large volume of gas and a metallic gas reservoir creates obvious challenges for radiotherapy treatment planning, especially regarding dose calculation accuracy. In this study, 3D-printed phantoms were designed and fabricated specifically for use in evaluating the accuracy of dose calculations for two different types of dynamic photon radiotherapy treatments of breast cancer.

Dose calculations provided by the Varian Eclipse treatment planning system, for volumetric modulated arc therapy (VMAT) treatments, the Tomotherapy Hi-Art treatment planning system, for helical tomotherapy (HT) treatments, and the Tomotherapy Planned Adaptive dose calculation and review system, for HT adaptive radiotherapy (HT-ART), were evaluated using radiochromic film measurements made in and around two purpose-designed 3D printed phantoms. One of these phantoms was designed to be as anatomically-realistic as possible, including overlying breast tissue, bolus, and contralateral breast tissue, for placement on a humanoid thorax phantom, with a sample tissue expander. The other phantom was designed as a simple rectilinear box, with sides with approximately anatomically-realistic thicknesses and densities and with a simple CO2 canister used to model the reservoir, for use in providing a clear and unequivocal comparison of the HT and HT-ART dose calculations. Both phantoms were printed using PLA using an Ultimaker 2plus 3D-printer.

Measurements showed that the metal reservoir increased the uncertainty of downstream dose calculations, for all three systems, although the HT-ART system was least affected by this component. By contrast, the VMAT and HT treatment planning systems provided more accurate dose calculations throughout the gas volume within the implant, and therefore provided more accurate dose-volume histograms (DVHs) when the gas volume was included in the target (PTV). Generally, all three systems provided comparatively accurate calculations of dose in the thin layer of tissue surrounding the implant, in regions not affected by the reservoir, as well as accurate calculations of skin dose in areas covered by bolus.

The use of gas-filled temporary tissue expanders during breast radiotherapy treatments leads to unavoidable uncertainties in treatment planning dose calculations, including potential under-dosing downstream of the metal reservoir and inaccurate dose calculations within the gas volume. Although a PTV that includes the implant may be needed for plan optimisation, the creation of a separate PTV that excludes the implant is advisable, so that DVH evaluation refers only to tissue, where the accuracy of the dose calculation is truly important.