Yes, it is possible to get a seat for a postgraduate (PG) program at AIIMS Delhi, in radiology. But is highly competitive and typically requires candidates to meet specific eligibility criteria and pass an entrance examination. AIIMS Delhi generally admits candidates from across India, including those who have obtained their MBBS degrees from state medical colleges like PGIMS Rohtak or other institutions.

• Eligibility: To be eligible for AIIMS PG courses, candidates are generally required to have a recognized MBBS degree from a university recognized by the Medical Council of India (MCI) or the National Medical Commission (NMC).
• Entrance Examination: Admission to AIIMS PG courses, including radiology, is primarily based on performance in the AIIMS PG Entrance Examination (AIIMS PGEE). AIIMS conducts an entrance exam every 6 months for entry into its MD, MS courses. You can regularly visit the Collegedunia website or exam portal of AIIMS to check for all the details regarding exam pattern, eligibility, seat distribution, registration for the exam, and other details.
• Reservation: Although internal applicants (those who pursued their MBBS at AIIMS) typically receive 50% of the PG seats at AIIMS, outsider candidates do have a chance.
• Counselling Process: Successful candidates in the AIIMS PGEE are called for counselling, where you can select your desired specialty and institute based on their rank and availability of seats.

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.