Four-dimensional dose evaluation for the liver stereotactic body radiotherapy with lipiodol retention after transcatheter arterial chemoembolization

To evaluate the effect of lipiodol on the four-dimensional accumulated dose of liver stereotactic body radiotherapy (SBRT) after transcatheter arterial chemoembolization (TACE). Ten patients who underwent liver SBRT with lipiodol retention after TACE were retrospectively analyzed. Free-breathing three-dimensional (3D) computed tomography (CT) and respiration-based four-dimensional (4D) CT scans were used. The tumor tissue and lipiodol region were delineated within the gross tumor volume (GTV). Volumetric modulated arc therapy plans were created using the X-ray Voxel Monte Carlo (XVMC), a fast MC algorithm. For 4D accumulated dose assessments, the dose was recalculated after copying the plan into each CT phase, and a deformable registration was used with the 3D CT as a reference. Additionally, dose variation due to tumor irradiation at the lipiodol location through motion was simulated by density override of planning target volume using tumor tissue density followed by dose recalculation. Dose to the lipiodol region increased after density override with tumor density. In eight out of ten patients, 4D doses covering 98% of the GTV and tumor tissue increased compared to 3D doses. Notably, for both 3D and 4D cases, doses to the tumor tissue exceeded those of the GTV. Attention to GTV coverage in 3D planning for liver SBRT without considering lipiodol was deemed sufficient. The similar 19.2 Gy-liver volume for 3D and 4D allowed for liver evaluation using the 3D plan as a quick substitute. In liver SBRT cases involving lipiodol after TACE, the XVMC planning system revealed higher 4D doses to tumor tissue compared to GTV, with 4D doses surpassing 3D. However, results were limited by XVMC accuracy, unable to reflect lipiodol's dose enhancement. For precise CT-based dose calculations involving lipiodol in the GTV, incorporating lipiodol's material information in the MC calculations is crucial.