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New prognostic parameters for very-low-risk gastrointestinal stromal tumors.

BACKGROUND
According to the National Institutes of Health consensus criteria, gastrointestinal stromal tumors (GISTs) smaller than 2 cm in diameter with less than 5 mitotic figures per 50 high-power fields are considered very-low-risk GISTs, but these two indices alone cannot reliably predict a benign outcome during long-term follow-ups. Therefore, identification of additional parameters for predicting the clinical behavior of GISTs is necessary.


METHODS
Eighty-eight patients with tumors that meet the very-low-risk GIST criteria were retrospectively investigated and morphological parameters of tumors associated with the biological behavior of very-low-risk GISTs were evaluated in the present study. The Kaplan-Meier method was used to calculate disease-free survival rates.


RESULTS
Eighty-one patients were followed up for one to 16.3 years. Five cases of relapses were identified in the patients. Distinctive infiltrative growth patterns such as muscularis propria, muscularis mucosa, or nerve infiltration were identified by microscopy in 4 patients with the relapse, including three patients who experienced multiple recurrences. The infiltrative growth features became more obvious in multiple recurrent tumors compared to the single recurrent tumor, while only one developed relapse in 76 patients without infiltration (P < 0.0001).


CONCLUSION
Microscopic infiltrative growth patterns of the tumor may have clinical significance in predicting the prognosis of very-low-risk GISTs.