How much experience do you have setting up calculation expressions?
Create a script with the send mail script. When you click specify, you bring up the standard email dialog and there's a button with an arrow on it to the right of the message box.
Click that button and enter an expression like:
"Hi Dr. " & Yourtablename:: DoctorLastName
If you don't check the "Perform with out dialog" box, the script should pop up the email dialog with the text you've specified as the start of the message. You can also specify expressions to load data into any of the other text boxes in the send mail dialog.
You may want to consider using a single field for the body of the email, and by using a button and script, you can insert the last name of the doctor, etc. and other desired information into the field before editing it.
Then I would use a script to send it, which at the same time, could add the contents of that field to a related table for future reference. This record of sent emails may be a better reference than depending on your email software to retain sent emails, and the previously sent emails could then be shown in a portal on your layout if that would be helpful.
If you are sending the same message to many doctors, you could do a search for the applicable ones, use at least two separate fields for the body of the email, create a loop in your script, and send multiple emails. Again you can retain copies of all email sent in a related table.