Assessment with cone-beam computed tomography of intrafractional motion and interfractional position changes of resectable and borderline resectable pancreatic tumours with implanted fiducial marker.

OBJECTIVE: The volume of targets to which a high radiation dose can be delivered is limited for pancreatic radiotherapy. We assessed changes in movements of pancreatic tumours between simulation and treatment and determined compensatory margins.
METHODS: For 23 patients, differences in implanted fiducial marker motion magnitude (MMM) and mean marker position (MMP) between four-dimensional CT and cone-beam CT were measured. Subsequently, residual uncertainty was simulated after no action level (NAL) and extended no action level (eNAL) protocols were adopted.
RESULTS: With no correction, respective 95th percentile of MMM were 4.5 mm, 6.2 mm and 16.0 mm and systematic (random) errors of MMP were 2.8 mm (3.3 mm), 3.2 mm (2.0 mm) and 5.9 mm (4.0 mm) in the left-right (L-R), anteroposterior (A-P) and superoinferior (S-I) directions, so that large margins were required (L-R, 10.5 mm; A-P, 11.7 mm; and S-I, 24.8 mm). NAL reduced systematic errors of MMP, but resultant margins remained large (L-R, 8.0 mm; A-P, 9.6 mm; and S-I, 18.1 mm). eNAL compensated for time trends and obtained minimal margins (L-R, 6.7 mm; A-P, 6.7 mm; and S-I, 15.2 mm).
CONCLUSION: Motion magnitude and position of pancreatic tumours during simulation are frequently not representative of that during treatment. eNAL compensated for systematic interfractional position change and would be a practical approach for improving targeting accuracy. Advances in knowledge: Considerably large margins, especially in the S-I direction, were required to compensate for intrafractional motion and interfractional position changes of the pancreatic tumour. An application of eNAL was an effective strategy to diminish these margins.