Do I need to pre approve with Medicare a dermatology visit and Basal cell cancer removal?

[plsprius]

...Also, even if you sign that you will be responsible for payment, if Medicare does not pay, as long as the Doctor participates with Medicare, you cannot be billed. You can only be billed if that procedure is listed as never being payable by Medicare-I hope this helps...

Can you explain this further (maybe with a reference)? Certainly, this seems like good news for those on original Medicare.