First case of primary phyllodes tumor of the pancreas: case report and findings of immunohistochemical and ultrastructural studies.

A 37-year-old Japanese man with a solid and cystic pancreatic mass was referred to our hospital. Computed tomography revealed a well-demarcated solid and cystic mass measuring approximately 3.0 cm in diameter in the pancreatic body. The patient underwent middle segment pancreatectomy, and the retrieved tumor specimen was found to be a well-demarcated solid and cystic lesion measuring 3.0 x 3.0 cm. On histological examination, the cyst walls were found to be lined with a monolayer of non-atypical tall columnar epithelial cells. The solid areas surrounded the cystic ones and showed storiform proliferation of spindle cells that contained round, oval, or elongated nuclei and were present among abundant collagen fibers. The solid areas sent phylloid projections into the cystic spaces and the main pancreatic duct. The spindle cells were found to be diffusely positive for alpha-smooth muscle actin, desmin, and h-caldesmon on immunohistochemical analysis. Electron microscopy revealed that these cells possessed well-developed myofilaments with dense bodies, pinocytic vesicles, and basal lumina. Neither metastasis nor local invasion was detected. After the operation (4 years), tumor recurrence has not occurred. The main differential diagnoses of spindle cell tumors are leiomyomas, leiomyosarcomas, inflammatory myofibroblastic tumors, solitary fibrous tumors, extra-gastrointestinal stromal tumors, and schwannomas. However, the histological findings in the present case differed from those of these tumors. The present lesion is the first reported case of a primary pancreatic phyllodes tumor.