Clinical utility of the new American Joint Committee on Cancer staging system for gastrointestinal stromal tumors: current overall survival after primary tumor resection.

BACKGROUND: The objectives of the current study were to assess the reliability of the new revision of the American Joint Committee on Cancer (AJCC) staging system for gastrointestinal stromal tumors (GISTs) based on the National Comprehensive Cancer Network-Armed Forces Institute of Pathology risk classification and to analyze the factors that influence after resection for primary GISTs in 2 AJCC groups: patients with GISTs originating from the stomach and omentum (G-GISTs) and patients with other primary GISTs located mainly in the small bowel (nongastric GISTs [NG-GISTs]).
METHODS: The authors prospectively analyzed a group of 640 patients with primary, CD117-positive GISTs who underwent surgery with curative intention (R0/R1 resection), including 340 G-GISTs (55.5%) and 300 NG-GISTs (44.5%). Factors were explored that had an effect on disease-free survival time (DFS), which was calculated from the date of radical operation to the date of recurrence or last follow-up. The median follow-up was 39 months.
RESULTS: Compared with NG-GISTs, G-GISTs were characterized by a significantly lower median size (5.3 cm and 8.5 cm, respectively; P < .0001) and lower mitotic activity (median, 3 in 50 high-power fields [HPF] vs 5 in 50 HPF; P < .0001), and they were diagnosed in older patients (median age, 62 years vs 57 years; P = .002). The most commonly detected mutations in G-GIST were those located in KIT exon 11 (60.5%) and platelet-derived growth factor receptor alpha (PDGFRA) exon 18 (19%) versus KIT exons 11 and 9 in NG-GISTs (72% and 17.4%, respectively). The prognosis of patients who had G-GISTs was significantly better compared that of patients who had NG-GISTs, with 5-year DFS rates of 69% (median, 83 months) versus 43% (median, 33 months), respectively (P < .00001). The most significant prognostic factors that correlated with shorter DFS in both G-GISTs and NG-GISTs were primary tumor size >5 cm and >10 cm (P < .0001) and mitotic index >5 in 50 HPF and >10 in 50 HPF (P < .0001). The 5-year DFS rates in G-GISTs according to AJCC stage categories were as follows: 96% for stage IA tumors, 92% for stage IB tumors, 51% for II tumors, 22% for stage IIIA tumors, and 22% for stage IIIB tumors (P < .0001). The 5-year DFS rates in NG-GISTs according to AJCC categories were as follows: 92% for stage I tumors, 66% for stage II tumors, 28% for IIIA tumors, and 16% for IIIB tumors (P < .0001). The high prognostic significance of the AJCC classification also was confirmed for overall survival data, including the impact of therapy with tyrosine kinase inhibitors.
CONCLUSIONS: The reliability of AJCC risk classification after resection of primary GIST was confirmed for DFS and overall survival. Patients with primary G-GISTs had a better prognosis than patients with NG-GISTs. In both groups, primary tumor size and mitotic activity were the most important prognostic factors in terms of DFS.