PCT CTLR EXPRESSED CONCERN REGARDING FALSE SECONDARY RADAR TARGET WHICH INDICATED TFC CONFLICTION.

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ON JUL/SAT/04; I WAS WORKING POTOMAC TRACON; CHESAPEAKE SECTOR; WOOLY POS. ACR X; A CRJ1; LOCATED 10 MI N OF BALTIMORE WAS INSTRUCTED TO CROSS BALTIMORE AT ONE ZERO THOUSAND FT. I LATER OBSERVED A NON-ARTS TAG SQUAWKING XXXX AT ONE ZERO THOUSAND FT; 10 MI W OF BALTIMORE; HDG EBOUND. IF THE TARGET WAS REAL; IT WOULD HAVE BEEN A DIRECT CONFLICT WITH ACR X XING BALTIMORE AT 10000 FT. I AMENDED THE ALT OF ACR X; AND TOLD HIM TO STAY AT HIS PRESENT ALT OF ONE TWO THOUSAND FT. A FELLOW CTLR CALLED ME ON THE LAND LINE TO ASK IF I ALSO SAW THAT TARGET SQUAWKING XXXX AT ONE ZERO THOUSAND FT. I CALLED AN ADJACENT SECTOR (ENSUE) TO SEE IF THEY HAD A 'RWY.' THEY DID NOT AND DID NOT OBSERVE THE 'GHOST' TARGET. THE 'GHOST' TARGET FINALLY DISAPPEARED. AFTER FULL ROUTING THE XPONDER CODE; IT APPEARS THAT TARGET (XXXX) WAS ABOUT 40 MI NW OF BALTIMORE. I FEEL THERE IS A PROB WITH THE RADAR COVERAGE AT POTOMAC TRACON. MGMNT WAS ALERTED TO THIS INSTANCE AND OTHERS IN THE PAST. THEY DENY THERE IS A RADAR PROB. HOW CAN CTLRS SAFELY SEPARATE ACFT IF 'GHOST' TARGETS APPEAR? WHAT IF WE TURN AN ACFT TO AVOID HITTING A 'GHOST' TARGET ONLY TO PUT THAT ACFT IN CONFLICT WITH A 'REAL' TARGET? THIS RADAR PROB NEEDS TO BE ADDRESSED IN THE INTEREST OF SAFETY.