Abstract
The purpose of this study was to model the metallic port in breast tissue expanders and to improve the accuracy of dose calculations in a commercial photon treatment planning system (TPS). The density of the model was determined by comparing TPS calculations and ion chamber (IC) measurements. The model was further validated and compared with two widely used clinical models by using a simplified anthropomorphic phantom and thermoluminescent dosimeters (TLD) measurements. Dose perturbations and target coverage for a single postmastectomy radiotherapy (PMRT) patient were also evaluated. The dimensions of the metallic port model were determined to be 1.75 cm in diameter and 5 mm in thickness. The density of the port was adjusted to be 7.5 g/cm3 which minimized the differences between IC measurements and TPS calculations. Using the simplified anthropomorphic phantom, we found the TPS calculated point doses based on the new model were in agreement with TLD measurements within 5.0% and were more accurate than doses calculated based on the clinical models. Based on the photon treatment plans for a real patient, we found that the metallic port has a negligible dosimetric impact on chest wall, while the port introduced significant dose shadow in skin area. The current clinical port models either overestimate or underestimate the attenuation from the metallic port, and the dose perturbation depends on the plan and the model in a complex way. TPS calculations based on our model of the metallic port showed good agreement with measurements for all cases. This new model could improve the accuracy of dose calculations for PMRT patients who have temporary tissue expanders implanted during radiotherapy and could potentially reduce the risk of complications after the treatment.
Highlights
It was found that the radiological diameter of the metallic port was 1.75 cm and the thickness was 2.5 mm, and these were smaller than the nominal diameter (2.1 cm) and thickness (3.5 mm) that included Ti shell
Using the determined dimensions from the film measurements, we found that the calculated doses in the treatment planning system (TPS) did not match the ion chamber (IC) measurements with the perpendicular setup when the density was adjusted for the parallel setup measurements
This could be attributed to the fact that the radiation beam was significantly attenuated by the metallic port with the perpendicular setup, while TPS could not handle this high density heterogeneity satisfactorily
Summary
More and more postmastectomy patients have immediate breast reconstructions mainly for cosmetic reasons.[1,2,3,4,5,6,7] A temporary tissue expander, which usually includes a high-density magnetic injection port, offers many advantages including relative simplicity, low morbidity, and good aesthetic results, over other types of breast reconstructions.[3,6] American Society of Plastic Surgeons reported that there were 74,694 breast reconstructions using tissue expander/implant in 2014 in the US and the number kept increasing.[8]Many patients will receive postmastectomy radiotherapy (PMRT) with the temporary tissue expander present, while the tissue expander could negatively impact the effectiveness of PMRT and increase the risk of complications.[9,10] Some physicians felt the reconstructions challenged their ability to deliver effective radiotherapy.[6]. It has been reported that failures in the breast reconstruction and complication rates were significantly higher for patients who received PMRT with the temporary tissue expanders than patients who received PMRT with permanent implants[11] or autologous tissue reconstruction.[12,13]
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