Review Article Figure 24. Axial CT scan of the abdomen. The same tumor as in Figure 16 (the bottom row) after approximately three years of treatment with Imatinib. The size of the tumor has further decreased despite actual progression of the neoplastic process as evidenced by the appearance of contrast enhancement within the tumor. 46 © Pol J Radiol, 2011; 76(3): 38-48 Figure 22. Axial CT scans of the abdomen before and after contrast injection. In the scans taken after treatment with Imatinib (the lower row), there is an increase in tumor density due to hemorrhaging. It is easier to differentiate between a true progression and hemorrhagic changes causing an increase in density of the tumor on images acquired before contrast injection. In such cases, there is no increase in contrast enhancement either. Figure 23. Axial CT scans of the abdomen. Diminishing of the size of peritoneal metastatic tumor during treatment with Imatinib despite an actual progression of the disease as evidenced by the thickening of the walls of a ‘neoplastic cyst’ and appearance of enhanced paramural nodules. documentation are unavailable for comparison, these ‘cysts’ may be reported by a radiologist as simple cysts. During therapy of GIST, a phenomenon of pseudoprogres- sion of a different kind may occur, which can also lead to discontinuation of an effective drug. Some of smaller met- astatic lesions in the liver may be at first isodense to the surrounding liver parenchyma and exhibit similar degree of enhancement after application of the contrast medium, which makes them invisible in the initial CT examination. Following treatment, ‘new’ lesions may appear, which is a result of density lowering in previously present but unrec- ognized foci. Such situation was illustrated in Figure 21. During chemotherapy, one may also encounter a ‘worri- some’ increase in the density of a lesion following treat- ment, which in fact is caused by hemorrhages within the tumors. Such situation was presented in Figure 22. There are also surprises regarding changes in tumor size during therapy. One may encounter metastases that