© Pol J Radiol, 2011; 76(3): 38-48 Werewka-Maczuga A et al – Characteristics of computed tomography… Figure 15. Axial CT scans of the abdomen – consecutive phases of dynamic CT examination. Untypically, metastases are best seen before intravenous injection of contrast medium; the degree of their enhancement is similar to that of liver parenchyma. Table 1. RECIST classification. Table 2. Classification according to Choi. Complete regression Partial regression Stable disease Progressive disease Complete regression Partial regression Stable disease Progression CR PR SD PD CR PR SD PD Response to treatment according to RECIST scale Regression of all known foci of the disease and lack of new lesions At least 30% reduction in the sums of dimensions that could be measured, i.e. larger than 1 cm Dimensions of the lesions do not fulfill criteria of progression or regression Enlargement of lesions by at least 20%, appearance of new lesions Response to treatment according to Choi Disappearance of all known foci of the disease and lack of new lesions Reduction in the sizes of lesions by at least 10% or reduction in post-contrast enhancement by 15% or more Dimensions and contrasting of the lesions do not fulfill the criteria for progression or regression Increase in the size of the lesion by at least 10% without reduction in contrasting, appearance of a new lesion or enhancing nodule within the old lesion problem because of an untypical picture of a tumor dur- ing therapy and danger of discontinuation of the treatment following erroneous interpretation of radiographic images. According to the RECIST 1.1 scale, commonly used in clini- cal studies for assessment of response to therapy, the only criterion of response is the reduction in tumor size, as shown in Table 1. Alerady in 2004, three years after introduction of Imatinib to treatment of GIST, prof. Choi proposed extending the “response to treatment” scale by adding criteria of evalu- ation of tumor density changes following treatment, as assessed after injection of a contrast medium [13]. Classification according to Choi was presented in Table 2. A decrease in density of lesions in CT images in response to effective treatment (confirmed with PET study) is cur- rently a well-documented phenomenon in GIST chemo- therapy [12,14,15]. At first, sizes of the lesions may remain unchanged despite good therapeutic response, and the only sign of treatment efficacy is a reduction of density in CT images. In such cases, the RECIST classification defines the disease as stable, while in fact it is regressing. It has also been found that lowering of density of the lesions has a greater prognostic value in terms of therapeutic effica- cy than reduction in their sizes. Figure 16 pictures typical cases of good response to Imatinib treatment. The most important phenomenon that may occur during GIST chemotherapy is, so called, pseudoprogression of the lesions. It signifies an increase in tumor size despite a very good response to therapy (confirmed by PET study). In such cases, enlargement is always accompanied by a decrease in density in CT [11,16,17]. This phenomenon is relatively rare, but it may have fatal consequences for the patient if therapy response monitoring is performed using only RECIST scale, which takes into consideration only the sizes of lesions. Application of Choi’s classification, which includes evalua- tion of changes in tumor density following treatment, elimi- nates the threat of discontinuation of an effective drug. Awareness of the pseudoprogression phenomenon in GIST therapy, as well as in targeted molecular therapies of other 43