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The differential diagnosis of gastrointestinal stromal tumours (GIST) is complex. GIST may be differentiated from other GI tumours such as leiomyosarcomas, leiomyomas, schwannomas, malignant melanoma, and fibromatosis. Available imaging modalities to evaluate GIST include:

• CT
• Endoscopic ultrasound
• Magnetic resonance imaging (MRI)
• Fluorine-18-fluoro-deoxyglucose (FDG) positron emission tomography (PET)

Contrast-enhanced CT scan is the imaging modality of choice to visualise suspected abdominal masses⁶, and is used in staging and surgical planning. CT scanning provides for visualisation of the full extension of large exophytic GIST masses (tumour grows between the bowel loops) and sometimes within the bowel lumen, and is able to detect local invasion as well as distant metastases. CT scanning also helps to guide tissue biopsy. A small tumour found incidentally during endoscopy can be evaluated using endoscopic ultrasound or CT.

MRI is the procedure of choice for imaging rectal GISTs because it provides better soft-tissue contrast resolution and direct multiplanar imaging, and a better delineation of tissues.

PET may help to distinguish tissue functionalities-such as the difference between recurrent tumour and scar tissue-or in highlighting early functional changes and responses to treatment. Evaluation of FDG uptake using PET scanning is recommended when an early detection of response to Glivec treatment is required, eg, to consider surgery after Glivec cytoreduction of rectal tumours⁶. PET scan may also be useful in judging cases suspected to be metastatic.

GISTs are fragile and may bleed easily. If a suspected GIST is considered to be resectable after primary workup, then biopsy before surgery is not recommended because the risk of tumour spill and cellular dissemination is high. However, if a preoperative biopsy is scheduled, an experienced multidisciplinary team is preferred⁶.

The diagnosis of GIST relies on standard histologic examination and immunohistochemical analysis of several markers, including KIT. Equivocal cases should be submitted to a central review by an expert in sarcoma pathology, experienced in the diagnosis of GIST⁶.

Cell Type

Definitive diagnosis of GIST requires tissue biopsy of resected material and evaluation by an experienced pathologist. Small tissue samples are often inconclusive, as the histologic appearance of the tumour can vary throughout a single mass and can change with therapy²⁷. Fixed tissue samples are evaluated in the surgical pathology laboratory, by light microscopic examination after thin sectioning and staining with a general morphological stain such as haematoxylin and eosin.

Histologically, GISTs can be classified into 3 broad categories: spindle-cell type (70%), epithelioid type (20%), and mixed spindle and epithelioid cell type (mixed morphology) (10%)⁶. GISTs have a remarkably uniform appearance, with only a small minority having a notable cytologic pleomorphism (<2% to 3%)¹.

   Spindle Cell        Epithelioid Cell   Mixed Morphology

Figure 1. GIST histopathological cell types. Images reproduced from Fletcher CD, et al. Hum Pathol. 2002;33:459-465.

GISTs with spindle-cell type morphology have relatively uniform eosinophilic cells arranged in short fascicles or whorls. In relation to smooth muscle neoplasms, GISTs have a paler eosinophilic cytoplasm with more ovoid/shorter nuclei¹. GISTs of epithelioid type morphology have rounded cells with variably eosinophilic or clear cytoplasm¹.

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