© Pol J Radiol, 2011; 76(3): 38-48 Werewka-Maczuga A et al – Characteristics of computed tomography… Figure 25. Axial CT scans of the abdomen – consecutive images during treatment with Imatinib within approximately one year. There is an apparent evolution of lesions in the following months of therapy evidenced by decrease in the sizes of tumors and lowering of their densities. The picture on the left shows progression despite continuous diminishing of the lesions. Note the new enhancing nodule within the biggest tumor (a “nodule within a nodule”) after injection of the contrast agent. Figure 26. Axial CT scans of the abdomen – consecutive images during treatment with Imatinib. Changing sizes and degrees of contrast enhancement of metastatic lesions in the following months of therapy. Sizes of the tumors were constantly decreasing despite progression of the disease evidenced by appearance of new solid tissue within the neoplastic ‘cyst’. decrease in size while other lesions remain unchanged or become enlarged, especially the ‘cystic’ lesions. Metastases may also diminish as the disease progresses and the only sign of progression may be an increase in tumor density or thickening of the wall in cases of ‘neoplastic cysts’, which was illustrated in Figures 23–26. One should also remember about the possibility of so-called slow disease progression. In some patients, enlargement of metastatic changes occurs very slowly and the patient evidently benefits from continuation of chemotherapy. The longest-treated person in Cracow facility has remained in good condition for the last six years despite a massive neo- plastic dissemination to the liver and constant enlargement of most foci since the very beginning of the therapy. In such cases, treatment should be carried on despite seeming indications for its discontinuation. There are also unique images of metastatic nodules within metastatic tumors, first observed in GIST chemotherapy. They are caused by new local mutations within the ‘old’ neoplastic tissue, which gives rise to new lines of cells resistant to the applied drug. In CT, such changes are visu- alized as new, tiny areas of solid tissue, usually undergoing strong enhancement following contrast injection, usually located near the wall on the inside (rarely on the outside) of the old ‘neoplastic cyst’ that was formed during che- motherapy. As the dimensions of the old tumor usually do not change and may even diminish, they are classified as SD or PR according to RECIST scale, while in reality the disease progresses. Knowledge about this new progres- sion pattern allows for early recognition of resistance to therapy with CT examinations. It should be noted that nodules may appear in every location within the old meta- static lesion. Live neoplastic cells always remain inside, as 47