Why can't my dr. find the source of the melanoma? i've had numerous pet and cat scans.

Source?: I'm not sure what you mean. Melanoma is usually diagnosed on the skin...And the source is the skin. From your question, it sounds like it spread and must have been detected in an organ or bone or brain. Ct scans and pet scans are helful at following the disease but not perfect and can miss many small tumors.

Depends: Typically a thorough dermatology eval is recommended- sometimes the primary may remain occult.

Unknown primary: It is possible that your body has completely fought off the primary skin lesion and now it is gone. Patient with unknown primaries while are not common do occur.