*2.2.2 Volume delineation*

The imaging test used in the calculation algorithms for radiotherapy is CT; however, the definition of the gross tumor volume (GTV) in planning CT in liver tumors sometimes requires the use of supporting imaging tests that allow a correct visualization of the tumor, including different phases, sequences, or contrast acquisition times.

The efficacy of SBRT is totally dependent on the delimitation of the GTV, and an erroneous delimitation would mean on the one hand leaving tumor volume out of the irradiation field and on the other hand irradiating more healthy tissue than necessary, increasing the possibility of side effects.

The definition of the GTV in hepatocellular carcinoma requires the identification of abnormal areas in all phases of a multiphase CT and/or MRI. The definition of GTV typically represents a union of these findings. When vascular thrombosis is present, the definition of the lesion is more complex and is best visualized in the venous or late phases, requiring multiple images [28]. In liver metastases and pancreatic adenocarcinoma, PET/CT could be of added value in tumor delimitation to CT and/or MRI, although it is difficult to define the borderline uptake area (SUV).
