Main Session
Sep
29
PQA 06 - Genitourinary Cancer, Gynecological Cancer, and Health Care Access and Engagement
Presenter(s)
Zuzana Cermakova, MD, PhD - University Hospital Ostrava, Ostrava, Ostrava
Z. Z. Cermakova1,2, L. Knybel2,3, J. Cvek2,3, and T. Blazek3; 1University Hospital Ostrava, Ostrava, Ostrava, Czech Republic, 2University of Ostrava, Faculty of Medicine, Ostrava, Czech Republic, 3University Hospital Ostrava, Ostrava, Czech Republic
Purpose/Objective(s):
Late genitourinary (GU) toxicity remains a concern after prostate SBRT. Bladder dose is associated with urinary morbidity, and MRI-guided margin reduction reduced GU toxicity in MIRAGE . In our fiducial-guided practice using 3 mm PTV margins, bladder V20Gy < 17cc constraint is challenging to meet. Margin reduction may improve bladder sparing but requires confirmation of reproducible geometric accuracy, quantified by fiducial-based rigid body error (RBE). We prospectively evaluated inter- and intrafraction geometric variability to assess the feasibility of reduced PTV margins.Materials/Methods:
This prospective single-center analysis included 20 consecutive patients treated with fiducial-guided prostate SBRT (5×7,25 Gy, every other day). A total of 100 fractions were analyzed. RBE is defined as the difference in relative fiducial positions between in-room imaging and the navigation dataset. RBE was recorded at the start, midpoint, and end of each fraction. The intrafraction variability was defined as the standard deviation of the RBE during 1 treatment fraction.). The interfraction RBE variability was defined as the standard deviation of the mean daily RBE over the whole treatment course. Feasibility of PTV margins 2,0 vs 1,5 mm was evaluated using an RBE cutoff of = 2,0 mm, distinguishing multi-fiducial from single-fiducial deviations. The system has predefined RBE threshold of 1,5 mm what ensures that the targeting error due to rigid body deformation is below 1 mm.Results:
A total of 300 intrafractional RBE measurements were analyzed across 100 fractions. The overall mean RBE was 1.88 mm (SD 0.69). Mean RBE increased by fraction number: 1.31 mm (SD 0.65), 1.60 mm (SD 0.52), 2.16 mm (SD 0.56), 2.26 mm (SD 0.60), and 2.08 mm (SD 0.61) for fractions 1–5, respectively. Intrafraction variability was 0,25 ± 0,19 mm. Interfraction variability, was 0.52 ± 0,19 mm, indicating moderate heterogeneity among individuals. Using a predefined RBE threshold of =2,0 mm as the criterion for a 2,0 mm PTV margin, 48/100 fractions met this condition, while 52/100 exceeded it; however, in the range of 1,5–2,0 mm was always exhibited by only one of four fiducials, supporting the feasibility of a 1,5 mm margin.Conclusion:
Interfraction rigid-body geometric error increased across fractions. Using a =2,0 mm geometric threshold, reducing PTV margins to 1,5 mm would have been feasible in 48% of fractions with higher RBE values being exhibited by only a single fiducial. Reduced margins is achievable in a substantial subset of treatments, but uniform implementation may require verification of geometric stability. These findings support investigation of adaptive PTV margin strategies, particularly in subsequent fractions.