Answer: Almost complete response

Histology: This pancreas is remarkable for pools and pools of acellular mucin. The mucin not only involves the stroma of the pancreas, but is also present in the muscularis propia of the duodenum and in lymph nodes. Very rare atypical cells are present in the stroma.

Discussion: The differential diagnosis for stromal mucin in patients who haven’t been treated included mucin extruded from an intraductal papillary mucinous neoplasm (IPMN) and an invasive colloid carcinoma. The latter diagnosed by neoplastic epithelial cells floating in the stromal mucin, or neoplastic epithelial cells lining the pools of mucin in an abnormal location (such as in a nerve). In the present case, the patient received neoadjuvant therapy. The possibility that the stromal mucin represents mucin left behind by cancer that responded to treatment is supported by the abnormal location of the mucin (including in lymph nodes) and the rare residual atypical cells.