The use of respiratory-correlated four-dimensional CT where kidney motion has the potential to impact upon the radiotherapy planning process

Purpose Four-dimensional CT (4D-CT) image data can be sorted into bins of each 10% of the respiratory cycle. From these data, an averaged scan (AS) and a maximum intensity projection (MIP) are derived. While sorting based on the respiratory cycle applies to lung tumours, the use of 4D-CT for the kidney has not been analysed. Methods Both kidneys from five patients were contoured from every phase of the 4D-CT, MIP and AS (in total 116 kidneys were contoured). For each patient, the kidney volumes on the AS and calculated mathematical average (CMA) from individual phases were compared. The kidney volumes on the MIP and Boolean expansion of all phases were compared. The maximum excursions of the superior and inferior poles of the kidneys were measured in the craniocaudal and anterior-posterior (AP) directions. Mid-kidney excursion was measured in the AP and medial-lateral directions. Results Mean difference between CMA and AS was -0.91 cm³ (range -8.04 to 5.07 cm³). Mean difference between Boolean and MIP was 18.92 cm³ (range -2.24 to 44.34 cm ³). Mean maximum displacement of mid-kidney was 0.28 cm anteriorly (range 0 to 1.0 cm), 0.22 cm posteriorly (range 0.1 to 0.4 cm), 0.28 cm medially (range 0.1 to 0.5 cm) and 0.27 cm laterally (range 0.1 to 0.5 cm). Mean maximum excursion of superior pole was 0.31 cm anteriorly (range 0.2 to 0.5 cm), 0.29 cm posteriorly (range 0.2 to 0.5 cm) and 1.23 cm craniocaudally (range of 0.6 to 2.7 cm). Mean maximum excursion of inferior pole was 0.52 cm anteriorly (range 0.2 to 0.9 cm), 0.23 cm posteriorly (range 0.1 to 0.4 cm) and 0.67 cm craniocaudally (range of 0.3 to 1.8 cm). Conclusion The current study using 4D-CT has shown that the derived MIP is a good surrogate for the evaluation of three-dimensional kidney displacement, and that the volume of the AS is not largely different from the average volume for individual phases. The 4D-CT is useful for the creation of individual patient internal target volume or planning organ at risk volume when kidney motion impacts on radiotherapy planning.