Assessment of cone beam <scp>CT</scp> registration for prostate radiation therapy: Fiducial marker and soft tissue methods

Abstract

Abstract Introduction This investigation aimed to assess the consistency and accuracy of radiation therapists ( RTs ) performing cone beam computed tomography ( CBCT ) alignment to fiducial markers ( FMs ) ( CBCT FM ) and the soft tissue prostate ( CBCT ST ). Methods Six patients receiving prostate radiation therapy underwent daily CBCTs . Manual alignment of CBCT FM and CBCT ST was performed by three RTs . Inter‐observer agreement was assessed using a modified B land– A ltman analysis for each alignment method. Clinically acceptable 95% limits of agreement with the mean ( LoA mean ) were defined as ±2.0 mm for CBCT FM and ±3.0 mm for CBCT ST . Differences between CBCT ST alignment and the observer‐averaged CBCT FM ( AvCBCT FM ) alignment were analysed. Clinically acceptable 95% LoA were defined as ±3.0 mm for the comparison of CBCT ST and AvCBCT FM . Results CBCT FM and CBCT ST alignments were performed for 185 images. The CBCT FM 95% LoA mean were within ±2.0 mm in all planes. CBCT ST 95% LoA mean were within ±3.0 mm in all planes. Comparison of CBCT ST with AvCBCT FM resulted in 95% LoA of −4.9 to 2.6, −1.6 to 2.5 and −4.7 to 1.9 mm in the superior–inferior, left–right and anterior–posterior planes, respectively. Conclusions Significant differences were found between soft tissue alignment and the predicted FM position. FMs are useful in reducing inter‐observer variability compared with soft tissue alignment. Consideration needs to be given to margin design when using soft tissue matching due to increased inter‐observer variability. This study highlights some of the complexities of soft tissue guidance for prostate radiation therapy.