Reactive Nodular Fibrous Pseudotumor Mimicking Metastatic Gastrointestinal Stromal Tumor to Perigastric Lymph Node: A Case Report and Review of the Literature.

Gastrointestinal stromal tumors (GIST) are the most common mesenchymal lesions of the gastrointestinal tract. Many are treated surgically with or without the use of adjuvant tyrosine kinase inhibitors. Metastases to lymph nodes are infrequent. In this article, we present a case of a perigastric nodule presenting 3 years postsurgical treatment for biopsy-proven GIST, clinically suspicious for a recurrent/metastatic GIST. The patient had also received adjuvant tyrosine kinase inhibitor therapy. Microscopic sections from the perigastric lesion showed a spindle cell nodule felt initially to represent a GIST with posttherapeutic changes. Together with morphology, immunohistochemical workup supported the myofibroblastic origin of the spindle cells, consistent with a reactive nodular fibrous pseudotumor, and definitively excluded metastatic GIST. This case highlights an important diagnostic pitfall and is the first known case of a GIST preceding a reactive nodular fibrous pseudotumor.