Residual image registration error by fiducial markers in accelerated partial breast irradiation using C?arm linac: a phantom study

摘要

External beam accelerated partial breast irradiation (APBI) is an alternative treatment for patients with early-stage breastcancer. The efficacy of image-guided radiotherapy (IGRT) using fiducial markers, such as gold markers or surgical clips, hasbeen demonstrated. However, the effects of respiratory motion during a single fraction have not been reported. This studyaimed to evaluate the residual image registration error of fiducial marker-based IGRT by respiratory motion and propose asuitable treatment strategy. We developed an acrylic phantom embedded with surgical clips to verify the registration errorunder moving conditions. The frequency of the phase difference in the respiratory cycle due to sequential acquisition wasverified in a preliminary study. Fiducial marker-based IGRT was then performed in ten scenarios. The residual registrationerror (RRE) was calculated on the basis of the differences in the coordinates of clips between the true position if not movedand the last position. The frequencies of the phase differences in 0.0–0.99, 1.0–1.99, 2.0–2.99, 3.0–3.99, and 4.0–5.0 mmwere 23%, 24%, 22%, 20%, and 11%, respectively. When assuming a clinical case, the mean RREs for all directions werewithin 1.0 mm, even if respiratory motion of 5 mm existed in two axes. For APBI with fiducial marker-based IGRT, theintroduction of an image registration strategy that employs stepwise couch correction using at least three orthogonal imagesshould be considered.