Quantifying Undetected Dosimetric Deviations In Multi-Fraction Interstitial HDR Brachytherapy without Adaptive Replanning: A Retrospective Analysis
Abstract
Purpose
Catheter displacement in multi-fraction HDR-brachytherapy is a recognized yet poorly quantified concern. This study evaluates the magnitude of these deviations and their impact on biologically-equivalent-dose (EQD2) metrics.
Methods
Eleven interstitial-HDR-brachytherapy cervical cancer patients (median 10 catheters/case, range=7-20) treated with multi-fraction regimens were retrospectively analyzed. Original Fx1 catheter positions were digitally reconstructed on Fx2 anatomy. Three-dimensional catheter deviations were calculated, including overall magnitude, directional components (ΔX, ΔY, ΔZ), and angular distribution relative to patient anatomy. Structures were contoured on the Fx2-CT images. Dose distributions were recalculated using identical dwell times. Target structures and OARs were analyzed using D90, D100 (targets) and D2cc (OARs), with all doses converted to EQD2 (α/β=10 targets, α/β=3 OARs).
Results
Mean catheter displacement across all cases was 1.69±0.26 mm (range: 1.32-2.14 mm), with maximum deviations of 4.87±2.35 mm (range:2.64-11.03 mm). Directional analysis revealed systematic shifts: ΔX(lateral)=0.03±0.33 mm, ΔY(anterior-posterior)=0.10±0.56 mm, ΔZ(superior-inferior)=-0.74±0.22 mm. Angular distribution analysis demonstrated the largest deviations tend to occur in the posterior sector of the implant geometry. HRCTV D90 decreased by 0.18±0.77 Gy(EQD2) on average, with similar trends for the GTV and IRCTV. OAR D2cc increased by 2.63±1.22 Gy(EQD2) for the worst-affected OAR per case. Correlation analysis revealed moderate association between catheter displacement magnitude and target D90 deterioration (Pearson R=-0.399, p=0.287). Structure-specific analysis identified the rectum as most vulnerable to dose escalation, with mean D2cc increase of 2.18±1.38 Gy(EQD2). Conversely, the sigmoid experience a clinically meaningful dose reduction, with mean D2cc decrease of 1.21±1.87 Gy(EQD2).
Conclusion
Catheter displacement without adaptive replanning leads to substantial, often undetected deviations. When expressed in EQD2, target underdosage and OAR overdosage may reach clinically significant thresholds. Directional patterns suggest anatomical factors may contribute to systematic displacement. These findings underscore the critical need for adaptive replanning workflows in brachytherapy where inpatient workflow is necessary.