This is the moment you have been afraid of . The moment you freak out and start asking everyone you know
"what am i gonna do ?"
"do you have a topic for me ?!"
"how did you do it ?"
"what the hell is a thesis ?!!"
Well..after asking and panicking for a while you find out that there is no short answer for that no quick fixes it is one of these few annoying things in life you have to figure out by your self.
All i am going to do here is to help you organize you thoughts give you a few hints to get you started
So first things first how to start
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Brain storming 
This is easy just focus
Start by labeling the major topics in your work ,the most common cases you encounter ,the recent advances you here about - and accessible to you - in a rough draft in your mind,just broad ideas .
Exclude the ones that are killed in research before and the ones that are currently your colleagues are working on ..duh
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Seek the Internet wisdom
Thank god for the internet .. in old times you had to
travel for knowledge
After choosing some key words go to famous medical search engine e.g. pub-med and start searching your key phrases you'll get hundreds of papers on every subject .just read the titles and abstracts see what other doctors are interested in in their research.
What aspects are still vague and some times they clearly say we need more work to prove certain points,how they designed their work regarding the time,No. of cases,techniques.it won't take much time but it will help to see the big picture and know how it is done.
Give a special attention to recent papers on any topic ,and always look for what they are trying to prove
That will give you an idea how to choose the capital question for you thesis .It should be unique and specific and up to date.
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Refining 
Don't get overwhelmed
Now write about 10 ideas then review them by asking your self these question;
Is this research manageable ?
Could i find the cases in time, do i have the equipment ,is it expensive?
Is it useful to my career ?
Is it the next big thing or that's an obsolete technique ,working with theses cases will improve my skills ?
What about the supervisors?
Do they have some knowledge about this subject , are they interested in this specialty ,what line of work do they prefer?
After scraping off some ideas from your draft you should end up with 3-4 topics now is the time to make your move
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Ask your supervisor 
Some times things get rough!
You should consult with your Junior supervisor 1st he could add some tweaking to them before meeting with the senior ones.
Keep in mind that some times your supervisors give you ideas for research some times this saves the effort but in most cases these ideas are not feasible and if this is the case you have to present your's and convince them that you have done your home work,you know what you are doing and you have a clear plan.
At the end there is no perfect topic try to find something you love and you will enjoy working with.
I hope this was helpful best of luck.
A.M.Tawfeek
