if you're accessing the news item by using CurrentDocument data, you can simply use CurrentDocument.DocumentForeignKeyValue which will be the PageNewsId assuming the current document is of class cms.news.
The field DocumentForeignKeyValue on the CMS_Document record, stores the primary key of the page type record as a reference. It does this because the CMS_Document table stores the page and language data while the CMS_News page type table only stores the page types data. There are other references but this is a simple explanation of what is going on.