Purpose: The no action level (NAL) protocol reduces systematic displacements relative to the planning CT scan by using the mean displacement of the first few treatment fractions as a setup correction in all subsequent fractions. This approach may become nonoptimal in case of time trends or transitions in the systematic displacement of a patient. Here, the extended NAL (eNAL) protocol is introduced to cope with this problem.
Methods and materials: The initial setup correction of eNAL is the same as in NAL. However, in eNAL, additional weekly follow-up measurements are performed. The setup correction is updated after each follow-up measurement based on linear regression of the available measured displacements to track and correct systematic time-dependent changes. We investigated the performance of eNAL with Monte Carlo simulations for populations without systematic displacement changes over time, with large gradual changes (time trends), and with large sudden changes (transitions). Weekly follow-up measurements were simulated for 35 treatment fractions. We compared the outcome of eNAL with NAL and optimized shrinking action level (SAL) protocol with weekly measurements.
Results: Without time-dependent changes, eNAL, SAL, and NAL performed comparably, but SAL required the largest imaging workload. For time trends and transitions, eNAL performed superiorly to the other protocols and reduced systematic displacements to the same magnitude as in case of no time-dependent changes (SD approximately 1 mm).
Conclusion: Extended NAL can reduce systematic displacements to a minor level irrespective of the precise nature of the systematic time-dependent changes that may occur in a population.