When searching for a multiple word concept in PubMed, you will often get significantly different numbers of results if you do not use quotation marks around the terms as compared to using quotation marks.  Here is an example:

lymph node dissection – 40,235

“lymph node dissection” – 10,124

When you search for a concept without quotation marks, you will retrieve records which contain synonyms for the terms you entered as well as vocabulary (MeSH) terms for the concept.  For lymph node dissection, records with the term lymph node excision will be returned, as well as those appended with the MeSH term, lymph node excision.

Fewer records will be retrieved with “lymph node dissection” since all results must contain this exact phrase.