Gastrointestinal Stromal Tumors (GIST)
GISTs form the largest category of primary, nonepithelial ,neoplasms of gastrointestinal tract(GIT).They can be defined as neoplasms derived from pluripotent mesenchymal cells of GIT capable of either partial or terminal differentiation along a variety of cell lines.
Group 1: Form the largest category, composed of tumours showing differentiation towards smooth muscle cells Arise from muscularis propria, muscularis mucosae or vessel related smooth muscle cells; Immunohistochemistry :Positive for actin,desmin and vimentin; Electron microscopy: Pinocytic vesicles and cytoplasmic microfilaments
Amin’s Classification: Size No of mitoses Benign < 5 cm’s < 5/50 HPF Borderline > 5 cm’s < 5/50 HPF Malignant Any > 5/50 HPF
Nuclear atypia, necrosis, haemorrhage etc are not reliable indicators of malignancy. It is widely accepted that the no of mitoses holds the key to the prognosis. However, There is difference of opinion on the exact no of mitoses necessary to determine whether a tumour is benign (Leiomyoma, Leiomyoblastoma) or malignant (Malignant Leiomyoblastoma or Leiomyosarcoma).e.g.: > 10 mitoses / 10 HPF in the case of a leiomyosarcoma and > 4 mitoses / 50 HPF in the case of a malignant leiomyoblastoma according to some.
STUMP: Stromal Tumours of Uncertain Malignant Potential, is the term used by pathologists to describe a neoplasm when they are unable to distinguish a benign from a malignant one.
Group 2: Form the second largest category, composed of tumours which show differentiation towards neural elements. Regarded as malignant. Also known as GANS (Gastrointestinal Autonomic Nervous System Tumours), Plexosarcomas, myenteric plexus tumours, etc. Immunohistochemistry:Positive for NSE (Neuron Specific Enolase), S 100 protein, neurofilaments, vimentin. Electron microscopy: Long cytoplasmic processes resembling axons,joined by primitive cell junctions,scattered neurotubules and dense core secretory type granules.
Group 3: Tumours showing dual differentiation, regarded as malignant
Group 4: Tumours lacking differentiation, regarded as malignant.
However,it must be mentioned that these tumours are capable of differentiation along vascular (CD 34 + , F.VIII + ) or fibroblastic ( Vimentin + ) lines as well.
Pathology: Gross: Large, soft, rubbery, well circumscribed tumours lacking definite capsule.May be cystic due to necrosis. Areas of haemorrhage and focal ulceration may be present. Microscopy: Elongated cells with varying amounts of eosinophilic cytoplasm and spindle shaped nuclei.