Beyond the snapshot: continuous optical surface monitoring reveals non-linear temporal dynamics and postural stability window in free-breathing breast radiotherapy

Purpose This study aimed to evaluate the clinical consistency between AlignRT surface monitoring and fan-beam computed tomography (FBCT)–based single-timepoint volumetric verification in free breathing (FB) breast radiotherapy, and to quantitatively characterize the nonlinear temporal dynamics of intrafractional motion. Materials and methods A retrospective analysis was conducted on 485 FB volumetric modulated arc therapy (VMAT) fractions from 38 patients treated following breast conserving surgery. Intrafractional motion was monitored concurrently using FBCT (endpoint positional snapshots) and AlignRT (continuous six degree of freedom tracking). Raw surface signals were decomposed into baseline drift and respiratory motion components using a 0.05 Hz zero phase low pass filter. Linear mixed effects models (LMMs) and segmented regression analyses were employed to evaluate clinical predictors and characterize temporal stability patterns. Results Both AlignRT and FBCT demonstrated excellent agreement along the lateral axis (median difference, 0.021 mm). In contrast, the vertical axis exhibited opposing directional shifts, in which FBCT indicated a slight anterior displacement (0.06 mm), whereas AlignRT detected a posterior drift (−0.17 mm), likely reflecting differences between internal bony expansion and external soft tissue deformation. LMM analysis revealed that higher respiratory frequency was significantly associated with reduced motion amplitudes. Segmented regression identified a multiphasic temporal trajectory, including an observed “stability window” between 100.7 s and 138.8 s, after which postural degradation significantly increased. Conclusions AlignRT provides an indispensable continuous-tracking supplement to single-timepoint FBCT. A shallow, rapid breathing pattern and treatment delivery within the ~100-140s stability window may help optimize posture maintenance. Continuous surface monitoring may therefore play a critical role in maintaining dosimetric accuracy during extended treatment delivery.