It is increasingly common for radiotherapy departments to encounter high density objects in patients being planned for radiotherapy. Many cases, such as artificial hip prostheses, are well documented. In our cancer center we have recently come across a new type of implant--a McGhan Style 133 Tissue Expander--implanted in patients who have undergone a mastectomy and will in future have breast reconstruction. This type of implant contains a small rare earth magnet encased in a titanium body, which induces significant perturbations in the dose distribution. These perturbations have been measured using a p-type semiconductor diode. Attenuation of up to 30% of local dose has been observed for a single beam. However, in more realistic clinical situations using tangential parallel opposed beams, it is estimated that part of the planning target volume maybe be underdosed by approximately 10%. Comparisons have been made between measured attenuation and that calculated by a treatment planning system, which demonstrates inadequate modeling of the dose perturbation caused by the implant in this case.