Invisible Tattoos

About two years ago, I had radiation treatments to treat my cancer.

As I was lying on the table during a pre-radiation procedure, they told me that they needed to give me tattoos to guide the laser machine when it delivers the radiation to the targets. I had no knowledge of this before we started the procedure. I didn’t have my phone with me, I wasn’t in any position to call a rav at the moment, and a decision needed to be made then and there while I was still lying in that position, so I told them to go ahead. I got several tattoos (just tiny blue dots).

I’m not interested in hearing if you think I did the right thing or the wrong thing in telling them to go ahead. That’s not the point of this post. If you think I was wrong, so noted.

I’m going to be doing radiation treatments again in the near future. I will not need new tattoos, so it’s not an issue. However, the techs told me that they can now do invisible tattoos. These are tattoos that use ink that only shows up under an ultraviolet lamp.

While it’s certainly preferable (to me, anyway) from an aesthetic view, I’m wondering if that would be more acceptable from a halachic point, or if it would still be forbidden.

The Wolf

(Yes, of course, I know not to base a final opinion solely on the words of posters in the Coffee Room. No need to mention it. I just want to hear what people think, both halachically and personally.)