Determination of the appropriate physical density of internal metallic ports in temporary tissue expanders for the treatment planning of post-mastectomy radiation therapy

Some patients undergoing breast reconstruction require post-mastectomy radiation therapy, but the metallic ports used in temporary tissue expanders attenuate the X-rays. In this study, we evaluated by the film method, the attenuation of 4 MV and 6 MV X-rays after passing through a metallic port, with the aim of identifying a useful method for determining the appropriate density to use in the radiation treatment planning system (RTPS), taking into account the distance between the metallic port and the targets. Radiochromic film was used to measure depth doses after the X-rays passed through the metallic port. The physical density allotted to the metal port portion was varied on the RTPS within the range 1–16 g/cm(3), and the physical density values were calculated that best reproduced the depth–dose distribution extrapolated from the film method. When the metallic port was orientated perpendicularly, the attenuation of the X-rays peaked at ~7% at both 4 MV and 6 MV. In the parallel orientation, the X-rays were attenuated by up to ~40% at 4 MV and by up to ~30% at 6 MV. We estimated the optimum physical density to be 9.8 g/cm(3), which yielded the best fit with the actual measurements. We demonstrated the most likely range for the target depth from the CT images of actual patients and, within this range, we identified the optimum physical density at which the measured and calculated values were most consistent with each other.