Modelling of silicone breast implants for radiotherapy treatment planning

There has been no published work characterizing the attenuation of silicone breast implants in MV energy photon beams. As a result of systematic out of tolerance in-vivo dosimetry results, this report investigates whether the CT Hounsfield Units to electron density curve provides an accurate estimate of attenuation in silicone implants. A CT scan of a silicone breast implant centered on top of WT1 blocks was acquired with simple 6 MV and 10 MV plans created. Dose was calculated using the CT and a collapsed cone algorithm. The predicted dose was compared to doses measured with ionization chamber at 2 points downstream of the implant. Predicted dose from the treatment planning system was 0.9-1.7% lower than measured. The use of a density override on the implant of water (1 g/cm³) improved agreement to less than 1% for all energies and measurement depths. We conclude that the use of CT Hounsfield Units for silicone breast implants leads to an under-estimation of dose in MV photon fields. Dose accuracy has been shown to be improved in the treatment planning system when silicone breast implants have a density override of water.