Gastrointestinal stromal tumors (GISTs) represent the
most common mesenchymal malignancy of the
gastrointestinal tract. The incidence of GISTs was
estimated to account for approximately 1% to 3% of all
malignant gastrointestinal tumors
1. GISTs were
initially a descriptive term developed in 1983 by Mazur
and Clark to define intra-abdominal tumors excluding
carcinomas
2. GISTs originate from stem cells that
differentiate toward the interstitial cells of Cajal
(ICCs). ICCs are the pacemaker cells of the
gastrointestinal tract and act to coordinate gut
peristalsis by linking the smooth muscle cells of the
bowel wall with the autonomic nervous system
3.
GISTs are characterized by the expression of CD34
and c-kit (CD117). Kit is the cell transmembrane
receptor with a tyrosin kinase activity. There are
frequent mutations of kit in GISTs. These mutations
results in constitutive activation of kit signaling which
leads to uncontrolled cell proliferation and resistance to
apoptosis
4.
Surgical resection is the only curative treatment
for non metastatic GIST. Recurrence of the disease is
common and metastatic GIST is fatal. The rates of
objective antitumor response to a variety of
chemotherapy agents for patients with GIST were
routinely reported as 0%, at best, less than 5% 1. The
median survival of metastatic disease has been reported
to range between 11 and 21 months 5.
Imatinib mesylate (Glivec; Novartis
Pharmaceuticals, Basel Switzerland) is small molecule
tyrosine kinase inhibitor that suppresses intracelluler
ABL kinase, chimeric BCR-ABL fusion oncoprotein of
chronic myeloid leukemia, platelet-derived growth
factor receptor and transmembrane receptor c-kit
products 4, 6. Clinical studies for metastatic and
unresectable GISTs have demonstrated partial response
rate 40%-69% in patients treated with imatinib
mesylate. But complete response is rare.
In the literature besides studies in which no
complete response was obtained in GISTs using
imatinib mesilate therapy there are also few cases
demonstrating complete response to therapy 8, 9. In
the study of Andtbacka et al a 7 total of 46 metastatic
or unresectable GISTs were evaluated and following
complete metabolic response proven with radiologic
and metabolic imaging was detected only in 1 patient.
However histopathological evidence lacks for this
particular case. Similarly in the report of Chiang et al
4, although complete metabolic response was
detected in 3 of 42 patients using radiologic methods
and F18-FDG PET, no histopathological verification
was reported. Goh et al 10 reviewed 37 unresectable
and recurrent GIST cases and reported pathologic
complete response in 4 patients. F18-FDG PET study
was reported as stable disease in 1 and as complete
metabolic response in remaining 3 patients. There are
also cases reported by Salazar et al 11, Melichar et al
5 and Suzuki et al 12 in whom pathologic complete
responses were reached.
Our GIST case treated with imatinib mesilate is
one of the rare complete response cases verified with
both CT, F18-FDG PET/CT study and pathological
examination.
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