This has been the case for inoperable tumors until the advent of imatinib mesylate, which is a molecular targeted therapy. Imatinib mesylate is the first effective systemic treatment for advanced GISTs, and yields a benefit of 50%-80%[15]. Although, the efficacy and safety of imatinib mesylate have been examined in large studies, few have focused on the pattern of tumor sellekchem changes after treatment. Knowledge on these patterns of response to treatment are important to adequately manage patients and to interpret clinical trials employing new tyrosine kinase inhibitors known as sunitinib[16]. The percentage overall response with imatinib treatment in this study was nearly equal to that in a previous study, which showed an overall response rate of about 50%, with 5% of those with a CR demonstrating a clinical response by CT scan[8].

This study demonstrated different patterns of CT changes in responders and non-responders during treatment with imatinib mesylate. Apart from the RECIST criteria, cystic change (GC, NC) was used to evaluate the response to treatment. If the lesions showed cystic change, even though it was a new lesion or a cystic change in a previous lesion, it was considered to be disease improvement. Several studies have supported the finding that cystic change is a feature of tumors which have responded to treatment, due to tumor necrosis and cystic or myxoid degeneration[10,17�C19]. Many authors have suggested new cystic lesions are characteristic of a response in small solid hepatic lesions, which cannot be seen in the initial image due to iso-density compared to liver parenchyma[20�C22].

FDG-PET in these patients has confirmed no glucose radiotracer uptake in the cystic lesions, whereas the tumor size remains unaltered[8,23,24]. This suggested there were no metabolically active tumor cells. The non-responder lesions showed three patterns of disease progression during treatment: GP, FP and NS. These patterns represented an increased solid component in terms of generalized or focal change. The FP pattern manifested as an increase in the solid component, such as increased wall thickness or a nodule within a mass[25,26]. Several studies have demonstrated that a solid nodule appearing inside a residual cystic mass indicating early retrogression after partial response to imatinib mesylate that corresponded to depicting new foci of increased FDG uptake in PET scan[11,23].

Few non-responders showed early FP or NS before developing GP, and finally, death. This may urge further treatment, such as surgery of the focal mass, or the new tyrosine kinase inhibitor sunitinib[16,27�C30]. Additionally, one of non-responders showed relapse of FP after cystic change of hepatic lesions. The authors speculated this event may have been a regrowth from AV-951 a residual tumor cell. However, the patient had prolonged stable disease before developing GP. Interestingly, there was one non-responder, who had two active sites of disease and showed a mixed response.