Lung tumor motion measured using retrospective 4D CT and correlated with tumor location and attachment

Abstract

To measure lung tumor motion using retrospective 4D CT and correlate the motion with tumor position and attachment. Lung patients were scanned using retrospective 4D CT to account for respiratory motion of the tumor. For 4D CT, multiple images are acquired at each slice location and then sorted according to the phase of the respiratory cycle in which the image was acquired. In order to acquire the 4D CT, each patient was positioned on the CT couch with his/her arms in a wingboard and feet tied together. While acquiring a scout CT and a helical CT (not yet the 4D CT), the patient’s breathing cycle is observed using the “RPM computer” (Varian Medical Systems). The respiratory cycle is observed using a marker block taped to the patient’s abdomen about 5 cm below the xiphoid. The motion of the marker block is relayed to the RPM computer by an infrared camera. An audible breathing coach is programmed with the patient’s breathing cycle time and started. The patient is asked if the rhythm is comfortable and if not, slight adjustments are made to the time until a comfortable rhythm is achieved. The CT slice spacing is set to 2.5 mm so that motions as small as 2.5 mm in the superior-inferior (S-I) direction can be observed. The volume to be scanned is determined from the scout image. The number of images is limited to 1500. The user programs the scanner to remain at each slice location acquiring multiple images for at least one full breathing cycle of the patient. All of these images are synchronized with the breathing cycle and sorted according to the breathing phase at which they were acquired. A 3D image is created for each breathing phase. The location of the tumor was identified by the lung lobe in which the tumor resided. Any attachment observed superiorly, inferiorly, anteriorly, posteriorly, medially, or laterally was recorded even if the attachment was only via a distinguishable vessel. Tumor displacement was measured by locating each edge of the tumor on the phases representing its most superior and most inferior positions. If the Medial-Lateral (M-L) displacement and the Anterior-Posterior (A-P) displacements were less than 3 mm, the superior and the inferior edges of the tumor were recorded on the phases representing its most superior and most inferior positions. The average displacement of the inferior and superior edges was identified as the tumor displacement. Tumors with A-P or M-L displacements of more than 3 mm were contoured before identifying their displacements. To contour the tumors, the 3D images representing its most superior and most inferior positions were exported to the planning computer (Eclipse, Varian Medical Systems). The tumor was contoured on both 3D images. The planning computer was then used to identify the center of the tumor on each phase using a built in center of mass calculation. These centers at the superior and inferior positions were recorded and their S-I difference represents the S-I displacement of the tumor. Lung tumor motion has been measured in 10 patients and associated with tumor position and attachment. Tumors located in the superior and inferior lobes of the lungs have been observed. Superior, inferior, anterior, posterior, lateral, and medial attachments have all been observed. The tumors that moved more than 3 mm were all located in the inferior lobes. Of these tumors, those attached posteriorly were observed to move the most. Tumors with extensive attachment moved the least. Tumor S-I motions from 0 to 13.8 mm were observed.