The Use of Dual-Energy CT to Eliminate the Need of Pre-Contrast CT and Registration Error for Contrast CT-Based Radiation Treatment Planning

Abstract

The delineation of targets for CT based radiation treatment (RT) planning frequently utilizes contrast enhancement via Iodine bolus. However, dose calculations should not be performed on a CT with a contrast agent because the patient won’t receive bolus during treatment. Hence it is typical to acquire CT twice during RT simulation: once before the injection of bolus and once after. The registration between these two CT sets can introduce error, particularly for abdominal and thoracic tumor sites, due to the motion between the two acquisitions. In this work, we investigate the feasibility of using virtual non-contrast (VNC) images derived from dual-energy CT (DECT) to eliminate the pre-contrast CT and the registration error. CT datasets, including DECT and conventional 120 kVp pre- and post-contrast CTs, acquired for 10 pancreatic cancer patients were used. The DECT sets were acquired simultaneously using a dual source CT simulator. For each case, a VNC was derived from the DECT and was registered to the pre-contrast CT. The gross tumor volume (GTV) and organs at risk (OAR) were delineated on the contrast CT and then populated to the pre-contrast CT. An IMRT plan of 50.4 Gy to the GTV in 28 fractions was calculated on the pre-contrast CT. The dose distribution was reconstructed on the VNC image. Dose volume parameters (DVP) of the original plan on the pre-contrast CT were compared to reconstructed plan on the VNC. The motion related differences between the pre- and post-contrast CTs were assessed in the target region. The CT number in the bone of the VNC sets dropped an average of 65 HU in comparison to the pre-contrast CTs. On average, the distance between the centroids of the duodenum differs by an average of 6.7 ± 4.0 mm and as much as 13.3 mm as measured on the pre- and post-contrast CTs. The dose distributions on the pre-contrast CT and VNC are almost identical. The GTV mean dose and GTV maximum dose differ by 0.1% and 0.2% respectively between the two plans for the cases analyzed. For organs at risk the maximum point dose difference is 0.3% and 0.5% for the duodenum and cord respectively; the liver mean dose differs by 0.1%; the kidney V15Gy differs by 1.0%. The VNC images derived from DECT can be used to replace the conventional pre-contrast CT for RT planning, eliminating the need for a pre-contrast CT scan and subsequently eliminating the error in the registration of pre- and post-contrast CT. The use of VNC improves clinical workflow and reduces imaging dose to the patient.