Lipiodol: A potential direct surrogate for cone-beam computed tomography image guidance in radiotherapy of liver tumor

Purpose: To investigate the feasibility of using lipiodol as a direct surrogate for target localization using cone-beam CT (CBCT) image guidance in radiotherapy (RT) of patients with unresectable liver tumors after transarterial chemoembolization. Methods and Materials: Forty-six patients with an unresectable solitary liver tumor were enrolled for RT using active breathing control (ABC) and CBCT image guidance after transarterial chemoembolization. Each patient had pre- and posttreatment CBCT in the first 10 fractions of treatment. Lipiodol retention was evaluated using daily CBCT scans, and volume of lipiodol retention in the liver was calculated and compared between planning CT and post-RT CT. Influence of lipiodol on dosimetry was evaluated by measuring doses using an ion chamber with and without the presence of lipiodol. Margin analysis was performed on the basis of both inter- and intrafractional target localization errors. Results: Twenty-eight patients successfully completed the study. The shape and size of lipiodol retention did not vary substantially during the course of treatment. The mean Dice similarity coefficient for the lipiodol volume in pretreatment CT and that in posttreatment CT was 0.836 (range, 0.817-0.885). The maximum change (ratio of the lipiodol volume in pretreatment CT to that in posttreatment CT) was 1.045. The mean dose changes with the presence of <10 mL lipiodol were -1.44% and 0.13% for 6 MV and 15 MV, respectively. With ABC and online CBCT image guidance, clinical target volume-planning target volume margins were determined to be 2.5 mm in the mediolateral direction, 2.9 mm in the anteroposterior direction, and 4.0 mm in the craniocaudal direction. Conclusions: Lipiodol could be used as a direct surrogate for CBCT image guidance to improve the localization accuracy for RT of liver tumors. Combination of ABC and CBCT image guidance with lipiodol can potentially reduce the clinical target volume-planning target volume margin.