Dosimetric effect of internal metallic ports in temporary tissue expanders on postmastectomy radiation therapy: a Monte Carlo study.

To investigate the dosimetric effect of the internal metallic port (IMP) in a tissue expander (TE) on the dose distribution of postmastectomy radiation therapy (PMRT). A total of 10 patients who have received PMRT with a TE were selected retrospectively. For each patient, the dose distributions of treatment plans with a 10 MV photon beam were calculated using the Monte Carlo (MC) method with CT images. The dose distributions without the TE were also calculated by designating the mass densities of the TE including the IMP as those of tissue. From the MC calculations, the dose-volumetric parameters were calculated and analyzed for several structures: the planning target volume (PTV) including the TE, the PTV excluding the TE (PTVreal), the TE alone, heart, and lungs. For the PTV and PTVreal, dose-volumetric parameters did not appear to depend on the IMP. Within the TE volume, the maximum dose and D 1% were higher with the IMP than without the IMP (62.8  ±  1.4 Gy versus 57.9  ±  1.3 Gy with p  <  0.001 and 58.6  ±  1.6 Gy versus 57.0  ±  1.2 Gy with p  =  0.035). The values of V 100% and V 95% were lower with the IMP than without the IMP (77.9%  ±  7.6% versus 87.2%  ±  5.3% with p  =  0.008 and 89.5%  ±  5.6% versus 94.6%  ±  2.9% with p  =  0.027). The IMP did not affect dose-volumetric parameters of heart and lungs. Dosimetric changes due to the IMP occurred mainly within the TE, and not in the target volume, heart, and lungs.