Review Article © Pol J Radiol, 2011; 76(3): 38-48 Figure 16. Axial CT scans of the abdomen. Two examples of response to therapy in advanced GISTs. Both tumor sizes and densities have strongly decreased (to the level of fluid density). Liver metastases have a pseudocystic appearance. Figure 17. Axial CT scans of the abdomen. Metastatic tumor in the liver before (images in the upper row) and after two months of treatment with Imatinib (the lower row). Tumor size has slightly increased, but its density is significantly lower. Such increase in size during treatment is due to tumor tissue edema and intra-neoplastic hemorrhages. neoplasms is still not widespread while this problem occurs more and more often as new drugs appear on the market. Figures 17 and 18 presents case of a slight increase in lesion size after treatment accompanied by lowering of its contrasting, which classifies it as SD according to RECIST scale and as PR according to Choi classification. Apart from an increase in tumor size during effective che- motherapy, other issues may arise, making a proper radio- logical interpretation difficult: • transformation of metastatic foci into lesions imitating simple cysts, • appearance of images with new metastatic foci during effective chemotherapy, 44