Evaluation of the use of planning CTs with slow revolution time (4s/slice; 'slow CTs') in the planning procedure of radiotherapy for lung cancer patients in comparison to commonly used 'fast' planning CTs; impact on margin assessment of planning target volume (PTV) design.

Eighteen lung cancer patients (six upper, lower lobe and central tumors, respectively) have been scanned each by three series of slow CTs and one fast CT (spiral CT). Patients have been freely breathing. The largest transversal tumor diameters in slow and fast CTs have been measured. Tumor edge positions have been determined for six spatial directions in all slices of the three slow CT series.

Slow CTs show larger dimensions of the visible tumor than fast CTs. The median difference of the diameter for all tumors is 2mm (range 0-4mm). Slow CTs deliver constant depictions of lung tumors within a range of 1.6mm in all directions. This margin is considered to be sufficient to compensate for tumor movements by respiration and cardiovascular motions (internal margin). A margin of 7 mm added to the GTV of a single slow CT series to draw the PTV is proposed.

Slow planning CTs show larger, but highly constant depictions of lung tumors in comparison to conventional fast CT scanning, yielding an integral delineation of almost all positions of the moving tumors. Thus the use of slow planning CTs enables the drawing of tighter margins in external beam treatment planning of lung cancer.