You must get a detailed "plan" from your physician showing appointments and procedures extending over a period of at least 6 months. In some cases general appointments and maintenance - such as cleanings and checkups dont count towards the plan ... Only specialty or surgical. Obviously some cases have been allowed through but medical is the most difficult way to obtain the DTV.
On the application of says to enter within 90 days of applying ... But no one knows if they will cancel approvals after a certain period without entry.