My 12 yr old has a lump below knee, medially. Xray shows a lucent lesion with ill defined margin and adjacent irregular cortical bone in tibia. Ideas?

Ped. Orthopedics. Time to see a well-qualified pediatric orthopedist.
Many possibilities. Some causes of painful “lumps” may include: bony overgrowths, soft tissue masses, ganglions, fibromas, and cysts. Have it evaluated by a professional to get the right diagnosis.