Yes it is possible. There are a lot of mediocre students who evolve with time and end up cracking IIT or AIIMS.

• AIIMS conducts an entrance exam every 6 months for entry into its MD, MS courses. 
• You can regularly visit the exam portal of AIIMS to check for all the details regarding exam pattern, eligibility, seat distribution, registration for the exam, and other details. 
• They also release exam notifications in newspapers.
• AIIMS generally reserves 50% of its PG seats for its internal candidates (those who have pursued their MBBS from AIIMS), which doesn't mean external candidates don't stand a chance.

Since MD radiology at AIIMS, New Delhi is your target, you will require a rank in top 3-5 to secure a place in MD radiology. It might be the toughest exam but it is not impossible. Try dedicating more time preparing for this, which might give you positive results.