Inter-Observer Variability of Clinical Target Volume Delineation In Cone Beam CT-Guided Online Adaptive Accelerated Partial Breast Irradiation
Abstract
Purpose
Accelerated partial breast irradiation (APBI) combined with online adaptive radiotherapy (OART) offers precise treatment of early-stage breast cancer, but inter-observer variability (IOV) in target delineation must be accounted for to maintain treatment quality. This study quantified IOV in cone-beam CT-based adaptive APBI across multiple professional groups to inform optimal margin selection.
Methods
In a retrospective analysis, 14 early-stage breast cancer patients received CBCT-guided OART APBI (30 Gy/5 fractions). Fourteen observers from four groups, radiation oncologists (n=5), medical physicists (n=3), dosimetrists (n=3), and radiation therapists (n=3), independently edited GTV contours on CBCT images from fractions 1 and 5, self-timing the process. Consensus GTVs were formed by physician majority voting (3/5 threshold), with CTVs created by adding 1 cm margins. Individual observer CTVs and PTVs (1-5 mm isotropic margins) were compared to consensus CTVs via coverage (true positive rate), volume ratios, and geometric metrics (Dice similarity coefficient, 95th percentile Hausdorff distance). Unedited autocontours from deformable image registration provided baseline comparisons. Statistical analyses used Wilcoxon signed-rank and Kruskal-Wallis tests.
Results
Professional groups showed distinct margins for achieving ≥95% CTV coverage in 90% of cases: radiation oncologists needed 3 mm (37% volume expansion), physicists 4 mm (67%, due to high inter-observer discordance), dosimetrists 2 mm (45%), therapists 2 mm (46%), and unedited contours 1 mm (51%). Median contouring times varied significantly (p<0.0001): physicians 1.92 min, physicists 3.25 min, dosimetrists 6.36 min, therapists 5.48 min. Despite 3.30-fold speed differences, timing spread within observer groups were consistent (p=0.12).
Conclusion
Profession-specific margin requirements for CBCT-guided adaptive APBI: Physician editing cuts unnecessary normal tissue irradiation by 25% versus unedited autocontours, needing just 1 mm extra margin, justifying clinician input for tissue-sparing gains. High variability across professionals and patient-specific complexity call for competency-based credentialing and profession-specific margin evaluations.