Microcystic adenoma of the pancreas: clinical, radiologic, and cytologic features

Cytology literature on pancreatic microcystic adenoma is sparse. It is important to separate microcystic adenoma from adenocarcinoma and mucinous cystic neoplasms on aspiration cytology, because patients with microcystic adenoma can be treated conservatively unless they are symptomatic. Potential pitfalls with endoscopic ultrasound (EUS) sampling of these lesions is discussed.

From January 1991 through June 2003, 10 patients with microcystic adenoma of the pancreas were diagnosed on fine-needle aspiration cytology. An additional patient, who was diagnosed with a mucinous cystic neoplasm by EUS sampling, was rediagnosed with microcystic adenoma on the excised specimen. Aspirate smears, cell blocks, core biopsies, subsequent excision (if any), and special stains were reviewed. Imaging studies and clinical data were available from the majority of patients.

The patients included 5 females and 6 males who ranged in age from 45 years to 84 years. Radiology studies showed tumors, which were heterogeneous with areas of fluid density and septations, located in the head, body, or tail of the pancreas. The masses ranged in size from 1 cm to 17 cm. The radiographic impression was highly suggestive of microcystic adenoma in six patients; detailed radiologic information was not available from three patients. On follow-up, six patients were alive and well at the last follow-up available, two patients died of unrelated sepsis, and three patients were lost to follow-up. Three of 11 patients underwent a Whipple resection. Cytology results: The cytologic features identified included the following: 1) Bland tumor cells were seen in sheets or small groups with a lack of nuclear abnormalities and moderate-to-scant cytoplasm with occasional clearing or vacuolation. Naked nuclei were present occasionally. Tumor cells were distinguishable from acinar cells based on larger cell size and granular cytoplasm in which prominent nucleoli were seen. 2) Relatively acellular, fibrovascular stroma was seen, usually located between tumor cells. 3) Calcifications were seen in four of eleven tumors. One tumor sampled by EUS revealed fragments of glandular-type epithelium with minimal atypia and was diagnosed erroneously as a mucinous cystic neoplasm. Cell blocks or core biopsies from most tumors showed fragments of dense stroma and cystic spaces lined by flattened epithelial cells. Subsequent Whipple resection in three patients showed histologic features of microcystic adenoma. Special stains performed in select tumors were positive for cytokeratin, carbohydrate antigen 19.9, and periodic acid-Schiff stain. Calretinin staining was negative in the tumor cells.

A cytologic diagnosis of microcystic adenoma is possible based on the criteria described above. Cell block and/or core biopsy, special stains, and radiologic information are key in making a definitive diagnosis. Patients with microcystic adenoma are spared a major surgical procedure unless they are symptomatic. With the EUS-guided modality of pancreatic sampling, caution should be exercised in misinterpreting benign glandular epithelium derived from the stomach or small bowel as a mucinous cystic neoplasm.