Ans. c (Osteoid osteoma) (Ref. Maheshwari orthopedics 4th1215)Osteoid osteoma# It is the commonest true benign bone tumor.# Please note that otherwise the most common benign bone tumor is osteochondroma!# Osteochondroma is synomymous with exostosis.# Not a premalignant condition.# Tibia is most commonly affected bone.# Posterior elements of vertebrae is a common site.# Affects 5-25 years of age.# Classical clinical presentation is nagging pain during nighttime, relieved by aspirin.Radiological features:# Generally in diaphysis. Usually cortical.# Typically seen as a radiolucent nidus (usually < 1 cm) surrounded by reactive sclerosis.# May have central fleck of target calcification in the centre.# Spinal lesions: Most often affects lamina. 60% are in lumbar spine. Mature sclerotic lesion may present as sclerotic 'Ivory pedicle' or 'ivory lamina'.# Radionuclide bone scan may show "double density sign ". Q# CT remains the best modality to detect most sensitively the nidus, which is defining feature.# Rx:- Complete excison of nidus with sclerotic bone.- RFA.OSTEOBLASTOMA# <1% of all primary bone tumors# Mean age: 6-30 years (90%)# Lesion >1.5 cm; smaller lesions are classified as osteoid osteoma# Radiological features:Similar to osteoid osteoma:- Radiolucent nidus >2 cm in size.- Well demarcated.- Stippled / ringlike flecks of matrix calcification.# MC Location = spine (in posterior elements) followed by long bone diaphysis (e.g., femur).CHONDROBLASTOMA (Codman tumor)# 1% of primary bone neoplasms, and epiphyseal in origin.# Peak in 2nd decade.# Histologically,'chicken wire' calcification (pericellular deposition of calcification) is virtually PATHOGNOMONIC. # MC location = long bones (80%); 2/3 in lower extremity, 50% about knee.# Radiological features:- eccentric medullary, subarticular location with open growth plate (98% begin within epiphysis)# DD: (*) Giant cell tumor (usually larger + less well demarcated, not calcified, older age group with closed growth plate),