Temporary breast tissue expanders contain a metal port that can vary in position throughout the course of radiation treatments. In this study, the magnitude and the dosimetric effect of the interfractional positional variations of the metal port was quantified. The robustness of the three most common external beam treatment techniques (tangential 3DCRT, VMAT and helical Tomotherapy) was evaluated against the measured positional errors. Results showed that interfractional positional errors were generally small, but errors larger than 1.5 cm were also observed. The interfractional positional variations arise from a combination of internal port errors and patient registration errors. Patient registration errors were shown to have larger dosimetric effect on target coverage and nearby organs than internal port errors. VMAT and Tomotherapy treatments are more robust when patient registration errors are minimized. The misalignment of the metal port during patient registration is acceptable when a more optimal anatomy match can be achieved.