Fiducial tracking fidelity in robotic prostate SBRT: a comparison of a 3-fraction boost following pelvic nodal irradiation and definitive 5-fraction treatment.

[PURPOSE] Pelvic nodal irradiation is often used for high-risk prostate adenocarcinoma. A commonly used alternative to low dose rate (LDR) brachytherapy, a 3-fraction SBRT boost with fiducial tracking may allow for better coverage of extracapsular extension and macroscopic seminal vesicle invasion. This study evaluates the practical impact of prior pelvic nodal irradiation on fiducial tracking during a subsequent 3-fraction robotic stereotactic body radiation therapy (SBRT) boost for high-risk prostate cancer and compares these outcomes to a cohort of patients undergoing definitive 5-fraction SBRT.

[METHODS] In this institutional analysis, we prospectively collected fiducial tracking data for patients receiving a 3-fraction boost to the prostate and seminal vesicles after conventional nodal radiation. We also identified patients treated with 5-fraction SBRT with a low risk of nodal involvement. Monte Carlo estimates of the Fisher's Exact Test assessed fiducial tracking loss. Continuous variables within the 5- and 3-fraction cohorts were compared using the Mann-Whitney Test. Changes in fiducial tracking and their association with pre-treatment factors were analyzed through the Kruskal-Wallis test and Monte Carlo for tracking patterns, and Spearman Correlation Coefficient and Mann-Whitney Test for deviations in tracking over 5 fractions.

[RESULTS] A total of 405 patients were treated from April 2021 to September 2023 with: (1) 5-fraction SBRT (n = 309, 76%), and (2) 3-fraction boost after nodal irradiation (n = 96, 24%). There was no significant fiducial tracking loss over the three-fraction boost treatment regimen that proceeded nodal treatment (p = 0.63). However, there was a significant (p < 0.001) loss of fiducial tracking fidelity as demonstrated by progressive loss of one tracked fiducial over 5-fractions. There was significantly more volatility observed in the 5-fraction versus 3-fraction boost treatment (median volatility 2.4 vs. 0.0, p < 0.001). There were no significant associations between fiducial tracking, independently for 3- or 5-fractions, using either analysis method or volatility for ADT, time from fiducial placement to SBRT, CTV, and QOD vs. daily SBRT.

[CONCLUSIONS] Pelvic nodal treatment does not affect the quantity/quality of fiducial tracking in 3-fraction treatments. However, 5-fraction treatments showed a progressive loss and increased volatility in fiducial tracking over time. No pre-treatment factors significantly influenced fiducial tracking changes in either cohort, though ADT use trended towards increased volatility in the 5-fraction group. With a minimum of 4 fiducials placed for treatment, the loss/volatility of a single fiducial had no clinical impact on the tracking system.