Evaluation of the geometric accuracy of anatomic landmarks as surrogates for intrapulmonary tumors in image-guided radiotherapy

Objectives: The purpose of this study was to evaluate the geometric accuracy of thoracic anatomic landmarksas target surrogates of intrapulmonary tumors for manual rigid registration during image-guided radiotherapy(IGRT). Methods: Kilovolt cone-beam computed tomography (CBCT) images acquired during IGRT for 29 lungcancer patients with 33 tumors, including 16 central and 17 peripheral lesions, were analyzed. We selected the“vertebrae,” “carina,” and “large bronchi” as the candidate surrogates for central targets, and the “vertebrae,”“carina,” and “ribs” as the candidate surrogates for peripheral lesions. Three to six pairs of small identifiablemarkers were noted in the tumors for the planning CT and Day 1 CBCT. The accuracy of the candidate surrogateswas evaluated by comparing the distances of the corresponding markers after manual rigid matching basedon the “tumor” and a particular surrogate. Differences between the surrogates were assessed using 1-wayanalysis of variance and post hoc least-significant-difference tests. Results: For central targets, the residualerrors increased in the following ascending order: “tumor,” “bronchi,” “carina,” and “vertebrae;” there was asignificant difference between “tumor” and “vertebrae” (p = 0.010). For peripheral diseases, the residual errorsincreased in the following ascending order: “tumor,” “rib,” “vertebrae,” and “carina;” There was a significantdifference between “tumor” and “carina” (p = 0.005). Conclusions: The “bronchi” and “carina” are the optimalsurrogates for central lung targets, while “rib” and “vertebrae” are the optimal surrogates for peripheral lungtargets for manual matching of online and planned tumors.