A Novel Patient-Specific Gating Method to Maximize Duty Cycle In Pancreatic Tumors
Abstract
Purpose
The Elekta Comprehensive Motion Management (CMM) system enables real-time monitoring of tumor motion with high temporal resolution and accuracy. Using histogram analysis of tumor positional oscillations, we propose a patient-specific gating strategy to maximize duty cycle and compare it with standard exhale and 10-phase mid-position gating.
Methods
Fifteen pancreatic cancer patients treated on an MR-Linac with CMM were analyzed. Tumor positions were extracted from log files. Baseline drift was removed using a low-pass filter, and data were recentered to the midpoint of respiratory peaks and valleys. All fractions were combined to generate a tumor position spectrum over the treatment course. Position histograms were constructed, and margins from 1 to 8 mm were evaluated. For each margin, the center was placed within the histogram to define a gating region (typically PTV), and the cumulative histogram within this region represented the duty cycle. Duty cycles were calculated by sweeping the margin center across the histogram. Three gating positions were compared: (1) the optimal position with maximum duty cycle, (2) the 10-phase mid-position, and (3) the exhale position (40%–60% phase).
Results
Tumor motion amplitude ranged from 5.5 to 13.7 mm (mean 9.2±2.2 mm). At small margins, the optimal position was near exhale but shifted with increasing margin, passing the 10-phase mid-position and approaching the midpoint of the respiratory waveform. Exhale gating closely matched the optimal position at small margins but diverged at larger margins, whereas the 10-phase mid-position showed the opposite trend. The duty cycle versus margin curves intersected mainly between 2 and 4 mm, where the patient-specific optimal position showed the greatest advantage.
Conclusion
Histogram analysis allows determination of patient-specific optimal gating positions based on breathing patterns and gating/PTV margins and is generally applicable across gating systems. Exhale and 10-phase mid-position gating can serve as practical surrogates depending on margin selected.