Q: Can I use an iMac as a PC monitor with a Thunderbolt cable?

Sorry to be the bearer of bad news, the answer is no. Please read Use your iMac as a display with Target Display Mode - Apple Support Apple is very specific about which computers will work. There is one solution available however it is based on Wi-Fi and the performance is poor, IMHO not even worth considering.

Your welcome.

If you have iMac late 2009 Indeed you can

Now I'm using my PC And plugging my imac 27 inch 2009 Late at the same time

I'm using mindisplayport to displayport and it worked like a magic with my gtx 970 graphics card:)

I saw a guy in youtube using thunderbolt cable from his imac to his pc and it worked

there is a chance if you have a 2009 imac

Msh3lany wrote:

 

If you have iMac late 2009 Indeed you can

 

Now I'm using my PC And plugging my imac 27 inch 2009 Late at the same time

 

I'm using mindisplayport to displayport and it worked like a magic with my gtx 970 graphics card:)

 

I saw a guy in youtube using thunderbolt cable from his imac to his pc and it worked

 

there is a chance if you have a 2009 imac

You did not read the OP's post, he clearly asked about a iMac with Retina 5K, these were introduced 5 years after the late 2009 27" iMacs!