Nidus is seen in:

Osteoid Osteoma-- mc at femur diaphysis. mc benign true bone tumor,. Typical presentation - Pain that is worse at night and is relieved by aspirin When the lesion is in a veebra, scoliosis may occur. Both osteoblastic & osteoclastic cells are seen The lesion has central lysis with surrounding sclerosis. Growing centre is called as nidus (seed). CT is the best study to identify the nidus and confirms the diagnosis.