The factors of fiducial marker motions and individual margin assessment in postoperative breast cancer radiotherapy

BACKGROUND: As a surrogate for the breast tumor bed, individual fiducial markers frequently move during radiotherapy. This study aimed to classify the motions and calculate the individualized target margin. METHODS: The mammary basal diameters (D) and heights (H) were measured to represent breast sizes for 15 patients after breast-conserving surgery. The clinical target volume (CTV) was divided into 4 quadrants by a coordinate system with the center of mass of the tumor bed as the origin. The lateral, anteroposterior, and craniocaudal motions of markers were calculated (M(LR), M(AP), M(SI)) based on the difference of the setup errors between the spine matching and the fiducial markers matching. The distances between markers and the innermost, foremost, and uppermost borders of CTV (DS(LR), DS(AP), DS(SI)) were recorded. RESULTS: In the first quadrant, M(AP) was strongly correlated with D×H (r>0.80) when D×H <99.89 cm(2). Both M(LR) and M(AP) were positively linearly related to DS(LR), DS(AP), DS(SI) (r>0.85, R(2)>0.75). M(SI) was also positively linearly correlated with DS(AP) and DS(LR) (r>0.90, R(2)>0.80). In the fourth quadrant with D×H <90.71 cm(2), only M(LR) and DS(LR) showed a linear positive correlation (r>0.90, R(2)>0.75), whereas the others showed linear negative correlations (r>−0.90, R(2)>0.80). The planning target volume (PTV) margin varied significantly between the first and fourth quadrant (P<0.05), and the largest margin was 12.4 mm in the craniocaudal direction of the first quadrant with D×H ≥99.89 cm(2). CONCLUSIONS: Fiducial motion is susceptible to breast size and fiducial position, and the individualized PTV margins should take the above factors into account.