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Does the traffic light protocol capture dose deviations? A longitudinal CBCT-based dose-monitoring study in adaptive
Dylan Callens1,2,3, Emma Van Riet1,2,3, Jan Verstraete2,3
1Laboratory of Experimental Radiotherapy, KU Leuven, Leuven, Belgium.
Introduction:
Traffic light protocols (TLP) standardize IGRT by reporting anatomical alterations presumed to proxy dosimetric change, yet this assumption is rarely verified. We therefore characterize the daily delivered dose and its longitudinal evolution during adaptive radiotherapy for LA-NSCLC and quantify how TLP reporting relates to dosimetric alteration.
Materials And Methods:
Twenty-two patients with LA-NSCLC enrolled in a prospective trial (604 fractions, mid-treatment adaptation) underwent multi-task automated daily CBCT-based dose monitoring. For each fraction, the delivered dose and the accumulated dose were compared with the reference plan utilizing the existing dose-volume constraints. Longitudinal dose evolution and TLP-code associations were analyzed with piecewise linear mixed-effects models, with significance based on BH-FDR-adjusted q-values (q < 0.10).
Results:
Daily delivered dose respected planning constraints (median differences within ±3%) but varied between patients. At the population level, only lung V20Gy showed a deviation rate from start (β = 0.082%-points/day; q = 0.044). The deviation rate after plan-adaptation was not significant. Population deviation rates were largely dominated by inter-patient variability. Three DVH-parameters were significantly associated with TLP-codes. Code orange for tumour alterations was associated with PTVp V95% reduction as well as spinal canal D0.03cc increase of respectively -1.73%-points (q = 0.011) and 2.15%-points (q = 0.011). Code orange for spinal-canal alterations was also associated with PTVp V95%-reduction (β = -2.03%-points, q = 0.002). Code orange for lungs showed no significant association with any dose constraint after adjustment for days on treatment.
Conclusion:
TLP-code orange reporting is a partial proxy for PTV-coverage loss but an unreliable proxy for organ-at-risk dose. Dose deviation rates during treatment are patient specific. This highlights the need for continuous daily dose monitoring to inform ART-decisions during treatment.
