Large hard lump can feel had for 5+yr slight left on tlbone. ct scan showed nada, but 5 drs can feel. what wont show ct? pylondial ruled out.

? pilonidal cyst: Usually a pilonidal cyst is easily felt on examination. If infected your physician can i ;d. No ct scan evaluation is indicated.

Hard=bone: It sounds like a bony protuberance since you say it is hard. If it doesn't hurt and doesn't look like a significant abnormality on cat scan then i would forget about it. Since it's stable for five years it is probably not going to be a problem.

? Cebaceous cyst?: Quite likely if just below the skin with a negative ct and now 5 years out. Quite likely not a malignant growth. The area is quite uncomfortable to sit on with that lesion there, so sometimes it's hard to decide the right thing to do, so you avoid messing with it surgically as long as you can.