Purpose: Data on organ mobility is required for optimizing radiotherapy. Renal mobility was studied in four-dimensional computed tomography (4DCT) scans acquired during uncoached respiration.
Methods and materials: The 4DCT scans of 54 patients, in whom at least the upper pole of both kidneys were visualized in all 10 respiratory phases, were analyzed. Scans were performed on a 16-slice CT scanner (slice index and reconstruction, 2.5 mm) during quiet, uncoached respiration. Mobility of the renal apex was evaluated in all patients by use of the z-position on CT slices. Reproducibility of mobility was studied in 8 patients who underwent 1 or 2 repeat 4DCT scans.
Results: Mobility was predominantly craniocaudal, with a mean of 9.8 mm for the left kidney and 9.0 mm for the right kidney. Large interpatient variations were observed that ranged from 2.5 to 30 mm (left) and 2.5 to 20 mm (right), and mobility of 1 kidney did not predict for mobility of the contralateral organ. Reproducibility of renal mobility and position at end-expiration was poor, with positional variations in repeat scans appearing to correlate with changes in the amplitude of respiratory waveform and total lung volume.
Conclusions: Large interpatient variations in renal movement occur during uncoached respiration, which indicates that respiratory coaching is useful for 4DCT imaging and treatment delivery.