Contrast-enhanced CT is a powerful technique for GIST diagnosis,
allowing for characterization of tumors.
[5] Indeed,
CT scan represents the most suitable method for: determining size,
shape and extention of GISTs; evaluating lesion uniformity (expressed in Hounsfield Units) and enhancing patterns; and staging (presence of liver,
peritoneal or other metastases).
CT appearances of GISTs are various,
depending on their localization and size.
Small GISTs are well-demarcated spherical and homogeneus soft tissue masses.
The lesion arises on the inner aspect of the muscle wall or muscularis mucosae and tends to displace the submucosa and mucosa rather than the firmer muscle wall.
[6] In some cases (GISTs over 3 cm of diameter) may be attached to the intestinal wall by a thin pedicle.
Frequently these lesions have a distinctive vascularization.
In arterial phase (30-35 sec) small GISTs are characterized by intense and early contrast-enhanced.
In venous (70 sec) and late (180 sec) phase they keep a light contrast-enhanced compared to the surrounding parenchymas due to their fibrotic component. (Fig.10)
Instead,
large GISTs appear on CT like huge and dishomogeneous masses with extensive hemorrhages,
necrosis linked to a large cystic spaces or cavities with air-fluid levels (Fig.11).
Calcifications were present in a minority of cases (Fig.12).
In these cases,
the masses exhibit a peripheral enhancement after intravenous contrast administration in arterial and venous phases,
due to the presence of vital neoplastic tissue and to the prominent arteries supplying the superficial layers of such tumors (Fig.13).
The central regions of large GISTs appear dishomogeneus also in venous and late phase,
owing to the extensive areas of hemorrhages and necrosis.
In some cases,
the central areas presented a soft contrast enhancement,
related to the presence of cord tissues connecting to the neoplastic mass.
(Fig.14)
Liver metastasis from malignant GISTs are large necrotic in nature with peripheral enhancement.
(Fig.15)
Coronal and sagittal CT reconstructions are useful to establish the organ of origin of large GISTs and to evaluate the involvement of surrounding structures and the metastasis presence.
The CT assessment of GIST morphological changes after chemotherapy represents another critical point.
Usually,
the lesion size decreases in responding tumors; however cases of tumor size increase are resulting from intratumoral hemorrhage,
necrosis or myxoid degeneration (Fig.16).
Moreover,
the mass density changes after chemotherapy,
becoming homogeneous and hypodense,
according to mitosis activity and intratumoral vessel reduction[7].
(Fig.17)
For all the above reasons,
the follow-up after chemotherapy is essential to evaluate dimensional and density changes of the tumor mass,
by means of RECIST criteria integrated with CHOI criteria.
[7] (Tab.
3) [8][9]