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In situ cannot spread internally. It is confined to the top most layer of skin which has no access to blood or lymph channels. You may have a genetic thing going on that is creating these dysplastic moles and in situ melanomas, but I would not worry about interal spread. Definitley not something insurance would ever approve a scan for. PET scans are the highest in radiation exposure of all types of scans, so exposing your body to that much unnessesary radiation is not recommended. I am sure Janner will have some good points for you here as well. 

I doubt you will get insurance to cover a PET scan.  Two primaries are just that – two independent occurences of melanoma.  Nothing traveled from one site to the other.  I had two primaries close together – can't remember the exact months apart (different calendar years) in 2000 and 2001.  PET scans only will see tumors 5mm and larger and with a new primary of under 1mm, you are not going to find a 5mm tumor somewhere else in your body.  In one statement you say <1mm and in another you say two in situ melanomas.  Which is it?  Because it does make a difference.  No one will give you a PET scan for two in situ melanomas.  And no oncologist will likely see you for two in situs.  If your second one has a depth, then an oncologist MIGHT see you. 

I have not seen an oncologist for my 3 primaries but my dermatologist is a cutaneous oncologist only dealing with cancer.  He's located at a cancer center and takes care of the early stage individuals.   Just for perspective, my 3rd and last primary was in 2001, my first in 1992.  Still alive with no metastasis and still stage 1.

If you have dysplastic nevus syndrome, then your risk of developing multliple primaries is higher.  Staying on top of your lesions is a big deal.  But early primaries are just that – early – and you must remember that they are staged independently.  One study years ago in Australia atually showed people with multiple primaries had better survival rates than those with just one.  So don't start doing a death watch :).  It's a shock but you just need to keep doing what you've been doing!!!

I had three melanomas removed within about six months – I would have actually had all of them at the same time, but one was removed quickly and I had to convince the derm to remove the other two because to me they had changed. All three were very thin – 1x in situ, 1x 0.3, 1x 0.2mm. Its what you call 'synchronous melanoma' – more than one primary at the same time. I have never thought about, nor been offered, certainly not pushed for, a PET scan. These are cutaneous primary melanomas, I'm just unlucky enough to have them all at once. This was three years ago, everything removed since has been normal or slightly dysplastic.