Gastrointestinal stromal tumor

GIST was introduced as a diagnostic term in 1983. Nonetheless, different GISTs have different risk assessments of their tendency to recur or to metastasize, dependent on their site of origin, size, and number of mitotic figures. A multi-omics study introduced a new molecular classification of GIST and identifies YLPM1, a potential tumor suppressor gene. The study categorised KIT/PDGFRA-mutated GISTs into 4 molecular subtypes. The C2 subtype, enriched with CD8+ T cells, shows potential responsiveness to immunotherapy. The C3 subtype, with frequent CDKN2A aberrations, could benefit from a combination of KIT and CDK4/6 inhibitors, showing strong synergistic effects. Additionally, C1 and C4 subtypes align with established risk classifications, supporting existing therapeutic strategies and prognostic outcomes. This new molecular classification provides new insights that guide personalized treatments. Due to the change in definition, clinical pathways of care before the year 2000 are largely uninformative in the current era. ==Signs and symptoms==

GISTs may present with trouble swallowing, gastrointestinal bleeding, or metastases (mainly in the liver). Intestinal obstruction is rare, due to the tumor's outward pattern of growth. Often, there is a history of vague abdominal pain or discomfort, and the tumor has become rather large by time the diagnosis is made. == Pathophysiology ==

GISTs are tumors of connective tissue, i.e. sarcomas; unlike most gastrointestinal tumors, they are nonepithelial. About 70% occur in the stomach, 20% in the small intestine and less than 10% in the esophagus. Small tumors are generally not aggressive, especially when cell division rate is slow. GIST tumors commonly metastasize to the liver (in 28% of cases) and/or to the greater omentum, lesser omentum, or mesentery (in 30% of cases). Less common areas of metastasis include the lungs, subcutaneous tissue, lymph nodes or bones. GISTs are thought to arise from interstitial cells of Cajal (ICC), that are normally part of the autonomic nervous system of the intestine. PDGFRA mutations Most GIST cells with wildtype (i.e. not mutated) c-KIT instead have a mutation in another gene, PDGFR-α (platelet-derived growth factor receptor alpha), which is a related tyrosine kinase. Mutations in c-KIT and PDGFrA are mutually exclusive [http://www.liferaftgroup.org/LondnPPT/Fletcher/Fletcher_files/frame.htm. Wild-type tumors Lesser numbers of GISTs appear to be associated with neither c-KIT nor PDGFR-α abnormalities. About 10-15% of gastrointestinal stromal tumors (GISTs) carry wild-type sequences in all hot spots of KIT and platelet-derived growth factor receptor alpha (PDGFRA) (wt-GISTs). These tumors are currently defined by having no mutations in exons 9, 11, 13, and 17 of the KIT gene and exons 12, 14, and 18 of the PDGFRA gene. ==Diagnosis==

image of GIST in fundus of stomach, seen on retroflexion. CT scanning is often undertaken (see the radiology section). Negative immunohistochemistry staining for β-catenin in cell nuclei is a consistent finding in uterine leiomyomas, and helps in distinguishing such tumors from β-catenin positive spindle cell tumors. The definitive diagnosis is made with a biopsy, which can be obtained endoscopically, percutaneously with CT or ultrasound guidance or at the time of surgery. A biopsy sample will be investigated under the microscope by a pathologist physician. The pathologist examines the histopathology to identify the characteristics of GISTs (spindle cells in 70-80%, epitheloid aspect in 20-30%). Smaller tumors can usually be confined to the muscularis propria layer of the intestinal wall. Large ones grow, mainly outward, from the bowel wall until the point where they outstrip their blood supply and necrose (die) on the inside, forming a cavity that may eventually come to communicate with the bowel lumen. When GIST is suspected—as opposed to other causes for similar tumors—the pathologist can use immunohistochemistry (specific antibodies that stain the molecule CD117 [also known as c-KIT] —see below). 95% of all GISTs are CD117-positive (other possible markers include CD34, Discovered On GIST-1 (DOG1), desmin, and vimentin). Other cells that show CD117 positivity are mast cells. If the CD117 stain is negative and suspicion remains that the tumor is a GIST, the newer antibody DOG1 can be used. Also, sequencing of KIT and PDGFRA can be used to prove the diagnosis. Imaging The purpose of radiologic imaging is to locate the lesion, evaluate for signs of invasion and detect metastasis. Features of GIST vary depending on tumor size and organ of origin. The diameter can range from a few millimeters to more than 30 cm. Larger tumors usually cause symptoms in contrast to those found incidentally which tend to be smaller and have better prognosis. Small GISTs Since GISTs arise from the bowel layer called muscularis propria (which is deeper to the mucosa and submucosa from a luminal perspective), small GIST imaging usually suggest a submucosal process or a mass within the bowel wall. In barium swallow studies, these GISTs most commonly present with smooth borders forming right or obtuse angles with the nearby bowel wall, as seen with any other intramural mass. The mucosal surface is usually intact except for areas of ulceration, which are generally present in 50% of GISTs. Ulcerations fill with barium causing a bull's eye or target lesion appearance. In contrast-enhanced CT, small GISTs are seen as smooth, sharply defined intramural masses with homogeneous attenuation. Large GISTs As the tumor grows it may project outside the bowel (exophytic growth) and/or inside the bowel (intraluminal growth), but they most commonly grow exophytically such that the bulk of the tumor projects into the abdominal cavity. If the tumor outstrips its blood supply, it can necrose internally, creating a central fluid-filled cavity with bleeding and cavitations that can eventually ulcerate and communicate into the lumen of the bowel. In that case, barium swallow may show an air, air-fluid levels or oral contrast media accumulation within these areas. Also, overtly malignant behavior (in distinction to malignant potential of lesser degree) is less commonly seen in gastric tumors, with a ratio of behaviorally benign to overtly malignant of 3-5:1. Even if radiographic malignant features are present, these findings may also represent other tumors and definitive diagnosis must be made immunochemically. ==Management==

For localized, resectable adult GISTs, if anatomically and physiologically feasible, surgery is the primary treatment of choice. Laparoscopic surgery, a minimally invasive abdominal surgery using telescopes and specialized instruments, has been shown to be effective for removal of these tumors without needing large incisions. An exception to this is where the anatomical position of the tumour means that surgery is technically difficult or complex. For example, rectal GIST often requires radical surgery to achieve complete resection, involving abdominoperineal resection and permanent stoma. In these situations, neoadjuvant imatinib can significantly decrease tumor size and mitotic activity and permit less radical sphincter-preserving surgery. Recently, in PDGFRA-mutated GIST, avapritinib has been approved by FDA. Now real-world data are coming for avapritinib as well Regorafenib (Stivarga) was FDA-approved in 2013 for advanced GISTs that cannot be surgically removed and that no longer respond to imatinib (Gleevec) and sunitinib (Sutent). ==Epidemiology==

GISTs occur in 10-20 per one million people. The true incidence might be higher, as novel laboratory methods are much more sensitive in diagnosing GISTs. The estimated incidence of GIST in the United States is approximately 5000 cases annually. This makes GIST the most common form of sarcoma, which constitutes more than 70 types of cancer. The majority of GISTs present at ages 50–70 years. Across most of the age spectrum, the incidence of GIST is similar in men and women. Adult GISTs are rare before age 40. Pediatric GISTs are considered to be biologically distinct. Unlike GISTs at other ages, pediatric GISTs are more common in girls and young women. They appear to lack oncogenic activating tyrosine kinase mutations in both KIT and PDGFRA. Pediatric GISTs are treated differently from adult GISTs. Although the generally accepted definition of pediatric GIST is a tumor that is diagnosed at the age of 18 years or younger, "pediatric-type" GISTs can be seen in adults, which affects risk assessment, the role of lymph node resection, and choice of therapy. == Citations ==