Tissue Assignment and Dosimetric Influence for Breast Radiotherapy With Metallic Port in Breast Tissue Expander Using Acurosxb Algorithms.

Purpose/Objective(s) Breast cancer patients receiving postmastectomy radiotherapy (PMRT) often present with temporary tissue expanders. These expanders aid in postmastectomy breast reconstruction, but cause challenges for radiation treatment planning. The metallic injection port within the tissue expander can lead to dosimetric uncertainty due to generation of metal artifact on CT imaging, and the potential for incorrect contouring and density assignment of the port. In this study, the dosimetric uncertainty resulting from variation of the port definition method was investigated when computing dose with a material-based calculation algorithm, AcurosXB (AXB). Materials/Methods 10 anonymized CT-datasets for breast patients were used for this study. Iterative metal artifact reduction (iMAR) was utilized to minimize artifact. The metallic port consists of a magnetic disc (density = 7.4 g/cm3) and a titanium shell (density = 4.2 g/cm3). Thus, an extended CT Hounsfield Unit (HU) table up to maximum value as 3071 in a TPS was used to more accurately delineate metals within the port. The port was contoured using three methods. # 1: the port was contoured using the bone automatic CT window/level setting. # 2: HU thresholding with tight and high HU range (3000-Max.) was used to contour the port automatically. # 3: The same method as # 2 but the wider and medium HU range (1000-Max.) was changed to be used to identify the magnetic disk within the port, and a contour matching the reported physical dimensions of the port was created. A suitable density was then assigned to the contoured port structures (stainless steel with mass density range 6.2-8 g/cm3). A representative plan was created consisting of opposed tangential fields prescribed to 50Gy in 25 fractions. Finally, dose was computed using each of the three port contours with the AXB algorithm within the TPS. DVHs and two dose points near the skin and on the chest wall were calculated for each port contour and compared. The contour generated using # 3 was considered most accurate, and served as a baseline for comparison. Results Maximum dose differences of > 10% in the PTV and chest wall and > 20% in skin were recorded when comparing the three port contouring methods. Contouring the port using only the bone window/level setting (# 1) produced the greatest difference from # 3. The # 2 is closer to # 3 (approximately 3%-5%) which is acceptance model in clinics. The dose differences in PTV, skin and chest wall were increased with energy increased. Conclusion The AXB algorithm, iMAR, and extended HU table were shown to provide more accurate dose calculation in the presence of metal in a previous study. This study has shown that the method used to contour the metal port is a critical aspect of this calculation. TPS calculations based on the ideal model of the metallic port 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 radiation complications.