Comparison of Fiducial Marker Implantation and Soft Tissue Registration Using Cone Beam CT in Image-Guided Radiation Therapy of the Prostate

Abstract

With the advent of Cone Beam CT (CBCT) in Image Guided Radiation Therapy (IGRT) of prostate cancer, daily registration can now be achieved with aligning patient position based on their soft tissue (ST) as opposed to surgically-inserted radio-opaque fiducial markers (FM). There are a limited number of studies directly comparing the differences in registration based on CBCT FM versus CBCT ST, despite differing standards of practice with the use of FM in IGRT of prostate cancers. There is disagreement in the literature whether CBCT ST offers an equivalent means of patient registration. For this study, we retrospectively collected the radiation treatment plan and daily CBCT imaging for 25 patients treated for prostate cancer at our center from 2014-2015. All patients had 3 FM implanted in the apex, mid and base of prostate under ultrasound guidance, pre-simulation. Three radiation therapists (each >2 years of experience in the use of CBCT ST and FM registration) performed registration of CBCT scans, uniformly spread over the treatment, to the reference CT based on both FM and ST using a stand-alone XVI (X-ray Volume Imaging) system by Elekta™ (Crawley, UK). Shifts in the RL, SI, and AP direction were calculated for each of the 5 fractions for each patient. Average shifts across patients were calculated and the results between the FM- and ST-based matching were compared using a Bland and Altman analysis. In a sample set of 5 patients, average shifts in the RL, SI, and AP directions were 2.5, 2.5, and 4.2 mm with CBCT FM, and 2.6, 1.4, and 4.0 mm with CBCT ST. A Bland and Altman comparison between CBCT FM versus CBCT ST resulted in a 95% confidence intervals of -2.4 to 1.6mm for RL, -1.9 to 4.4mm for SI, and -2.1 to 3.5mm for AP. The percentage of shifts in agreement between ±3mm was 98% for RL, 88% for SI, and 94% for AP, while the percentage of shifts in agreement between ±5mm was 98% for RL, 98% for SI, and 100% for AP. Our experience with CBCT ST has shown that it is comparable to CBCT FM alignment, in a center with radiation therapists experienced in aligning based on CBCT ST.