I'm not sure I fully understand what you are aiming for here, but my first question would be, are you expecting to deal with more than one treatment id in the report? If the answer to this is yes then you probably should be looking at a single datasource or, prefereably, the way you have already implemented it with sub-reports.
If you are only looking to report on one treatment id then the question is, how is the treatment id selected, is this going to be a parameter that the end user selects or enters when running the report.
Give me a bit more info & I'll try to help out.