Fiducial Markers are Necessary for Accurate Delivery of Liver SBRT

Abstract

We standardly use fiducial markers for target localization when administering liver stereotactic body radiation therapy (SBRT). However, there are few data demonstrating the impact of matching to fiducials on accuracy of liver SBRT delivery. Given the risks of percutaneous placement of fiducials, the aim of our study was to evaluate the benefit of fiducial-based SBRT and to potentially define a subset of patients that can be treated without fiducial markers. Nineteen metastatic (n = 15) and primary (n = 4) liver tumors were treated with SBRT. Doses ranged from 30-54 Gy in 3-5 fractions, and 75 individual fractions were delivered in total. Target localization at the time of treatment was performed using fiducial markers on cone beam computed tomography (CBCT). To assess the difference between fiducial-based and non-fiducial based set-up, each CBCT was then retrospectively realigned to best match the liver edge. The fiducial marker position was considered the gold standard. Shifts from the fiducial to liver edge alignment were recorded and an absolute Cartesian shift was calculated. To predict the dosimetric impact of these position errors, an artificial neural network was used to model the dose in the vicinity of the tumor. When aligning to the liver edge, the mean absolute Cartesian shift from fiducial alignment was 4.5 mm (0.73 – 13.3 mm). The lateral, longitudinal, and vertical shifts, were randomly distributed with mean values of 0.19 mm (-5.9 – 8.2 mm), -0.23 mm (-10.5 – 8.8 mm), and 0.43 mm (-9.0 – 8.4 mm), respectively. Thirty-three percent of fractions were found to have an absolute shift greater than 5 mm [our standard planning target volume (PTV) expansion] and 63% of patients had at least one absolute shift greater than 5 mm. In the neural network dose model, a 5 mm shift was associated with a dose reduction of 18.5% + 2.5%. The maximum shift seen was 13.3 mm which was associated with a projected target dose reduction of 45.0% + 5.6%. There were no tumor factors (including tumor size, distance from liver edge to the tumor, and distance from the tumor to the liver dome) that had a statistically significant correlation with worse alignment when matching to the liver edge without fiducials. Our findings demonstrate that treatment set-up without fiducial markers in liver SBRT leads to a mean positional error of 4.5 mm as compared to fiducial-based set-up. In one third of treatments, this variation was greater than our standard PTV expansion and would result in a substantial decrease in delivered dose. We were unable to define a subset of patients where fiducial markers may not be necessary. While these data suggest that with current technology, fiducial markers are necessary for the accurate delivery of liver SBRT; further work is needed to improve on-board imaging and localization techniques to allow for fiducial-less liver SBRT.