Author: Joseph A. Miccio, Nicholas J. Potter, Zain Siddiqui 👨🔬
Affiliation: Penn State Health Milton S. Hershey Medical Center 🌍
Purpose: Fiducial markers are the gold standard for prostate alignment during CT-based image-guided radiotherapy (IGRT). Radiopaque hydrogel rectal spacers reduce rectal toxicity by displacing the rectum and allow for visualization during CT simulation and cone-beam CT (CBCT). This study evaluates whether spacer-based alignment is comparable to fiducial-based alignment during prostate IGRT. We hypothesize similar target positioning accuracy and excellent inter-rater reliability (IRR) between alignment methods.
Methods: An ongoing prospective phase II trial (NCT05650021) is evaluating alignment accuracy using fiducial markers and radiopaque hydrogel spacers in prostate cancer IGRT. Patients had both fiducial markers and hydrogel spacers inserted transperineally under ultrasound guidance. Daily CBCT imaging was performed, and offline alignments were independently completed by using fiducial and spacer-based methods. Linear and rotational shifts were compared. Good agreement between fiducial and spacer-based alignment was defined as an absolute vector shift <3mm. IRR was assessed using average intraclass correlation coefficients (ICC) with a two-way random-effects model and absolute agreement.
Results: As of 01/01/2025, 13 patients had been evaluated for their first fraction of IGRT. The average vector shift between fiducial-based and spacer-based alignment was 3.02 mm (Range: 0.51–11.48 mm). Good agreement was observed in 69.23% of patients, though 30.77% showed clinically significant differences. Fiducial-based translational shifts (x, y, z) had near-perfect IRR (ICC > 0.99), while rotational shifts yaw and roll showed excellent IRR (ICC 0.8–0.9). Pitch had poor IRR (ICC = 0.28). Spacer-based alignment demonstrated near-perfect IRR for translational shifts (x, y, z) and excellent IRR for rotational shifts (ICC > 0.8).
Conclusion: Spacer-based alignment for prostate IGRT has the potential to be comparable to fiducial-based alignment, with most patients demonstrating good agreement. However, clinically relevant differences in select patients suggest the two alignment methods may not be interchangeable. IRR was generally excellent though rotational pitch showed better agreement with spacer-based alignment.