Significance of the Splenic Vein and Its Branches in Pancreatoduodenectomy with Resection of the Portal Vein SystemSignificance of the Splenic Vein and Its Branches in Pancreatoduodenectomy with Resection of the Portal Vein System 門脈系合併膵頭十二指腸切除術における脾静脈およびその分枝の重要性