there is a simple dictionary feature available in RapidMiner which at leasts supports word replacements including those using regular expressions. The used operator is called "DictionaryStemmer". A dictionary in RapidMiner is a file containing matching rules, i.e. each line contains matching rules for a given entity. A rule is either a term or a regular expression, for example:
car_manufacturer: bmw chrysler ford toyota
I don't know any medical dictionaries but that does not mean too much
A general dictionary is WordNet but actually we did not notice too much success with using a dictionary for text classification / clustering so we usually avoid the additional work.