A patient presents with pain in the thigh, relieved by aspirin. X-ray shows a radiolucent mass surrounded by sclerosis. Diagnosis is:

Ref: Robbins textbook of Pathology, 8th edition. & Apley's System of Orthopaedics and Fractures, 8th ed.Explanation:Osteoid osteomaThese benign bone tumors are less than 2 cm in greatest dimensionOccur in 2nd and 3rd decadesMai : Female ratio is 2 : 1.They can arise in any bone but have a predilection for the appendicular skeleton and posterior elements of the spine.In 50% of cases the femur or tibia is involvedThey commonly arise in the cortex and less frequently within the medullary cavity.Osteoid osteomas produce severe nocturnal pain that is relieved by aspirinThe pain is caused by excess prostaglandin E2 (PGE2) production by the proliferating osteoblasts .Osteoid osteoma is frequently treated by radioablation.