© Pol J Radiol, 2011; 76(3): 38-48 Werewka-Maczuga A et al – Characteristics of computed tomography… Figure 8. Axial CT scans of the abdomen. Two examples of GIST tumors with uncertain point of origin due to their large sizes. Picture on the left presents a rare primary retroperitoneal tumor. Figure 9. Axial CT scans of the abdomen. Two typical small bowel and duodenal GIST tumors. In these cases, radiological reports suggested primary lymphomas. Lack of enlarged lymph nodes in GISTs makes the differential diagnosis easier. Figure 10. Axial CT scans of the abdomen. Two cases of small peritoneal metastases, which could be easily missed by the radiologist • rare incidence of this tumor, which inclines the reporting radiologist to diagnose lesions other than GIST. Figures 4–9 presenting examples of lesions that posed diag- nostics difficulties. Dissemination of the primary process is often found at the moment of diagnosis. It is most commonly seen in the liver or peritoneum [6]. Sporadically, one may encounter pri- mary metastatic tumors in other organs, including lymph nodes – in very advanced stages. While looking for meta- static tumors, one may also encounter problems, especially in case of small nodules and a mild spread to the perito- neum, as shown in Figure 10. In these cases, it is particu- larly important to fill the lumen of the GI tract with non- contrasting fluid or contrast agent [7,8]. The figures above also prove that ultrasound scan may not be appropriate for periodic follow-up examinations in patients after resection of the primary tumor because the chances for visualizing such lesions are slim. Regrettably, this is a common practice of some clinicians despite firm guidelines on periodic follow-up using computed tomogra- phy [9]. A radiologist searching for sarcoma spread to the liver expects metastatic nodules that become well enhanced after injection of a contrast medium, best visible in the arterial or venous phase of a dynamic study. However, very diverse pictures may be encountered, which was illustrated in Figures 11–15. Occurrence of isodense GIST metastases has been described in the literature [10–12]. The degree of their enhancement after application of the contrast agent is similar to that of the surrounding hepatic parenchyma, which may result in overlooking all or some of the lesions, especially in a sin- gle-phase study. In case of tumors that are poorly distin- guishable from the surrounding tissue there may be prob- lems with taking measurements. Monitoring of response to treatment poses a separate, fundamental diagnostic 41