Pathogenesis: vascular compromise with obliterative endarteritis + periarteritis → hypovascularity + hypoxia → damage to osteoblasts + osteoclasts → hypocellularity + fibrosis → decreased matrix production (growing bone + periosteal new bone most sensitive)

Dose effects:

depend on age of patient, absorbed dose, size of radiation field, beam energy, fractionation

1. FOCAL MARROW DEPLETION
   Pathophysiology: marrow edema, vascular congestion, suppressed hematopoiesis; replacement of marrow elements by fibrosis + fat (complete by 3 months)
   • homogeneous high-intensity signal within radiation port on T1WI
   • occasionally bandlike appearance characterized by peripheral zone of low SI (red marrow) and a central zone of high SI (fatty marrow) on T1WI
2. BONE GROWTH DISTURBANCE
   • Appendicular skeleton
     • joint space widening ← cartilage hypertrophy (after 8–10 months)
     • growth plate widening in 1–2 months, often returning to normal by 6 months
     • permanent alteration in bone length / size ← premature fusion of physis
     • metaphyseal bowing
     • sclerotic metaphyseal bands
     • metaphyseal irregularity + fraying resembling rickets
     • longitudinal striations
     • overtubulation (= abnormal narrowing of the diaphyseal shaft)
     
     Cx: slippage of femoral / humeral epiphysis ± ischemic necrosis (after doses of >25 Gy)
   • Axial skeleton (dose of <15 Gy)
     • “bone-within-bone” appearance after 9–12 months
     • irreversible scalloping + irregularity of vertebral endplate with decreased height of vertebra (= failure of vertical growth)
     • scoliosis concave toward side of irradiation ← asymmetric vertebral growth + muscular fibrosis
     • hypoplasia of ilium + ribs
     • acetabular dysplasia, coxa vara / valga
3. RADIATION OSTEITIS = OSTEORADIONECROSIS = RADIATION NECROSIS
   = bone mottling due to osteopenia + coarse trabeculation and focally increased bone density ← attempts of osseous repair with deposition of new bone on ischemic trabeculae
   Dose:>6,000 cGy in adults; >2,000 cGy in children
   Time of onset: 1–3 years following radiation therapy
   Location: mandible, ribs, clavicle, humerus, spine, pelvis, femur
   • focal lytic area with abnormal bone matrix:
   • radiolucency confined to radiation field with narrow zone of transition
   • periostitis
   • increased fragility with sclerosis (= pathologic insufficiency fracture)
   • ± cortical thinning from chronic infection
   
   MR:
   • increased intensity of spinal bone marrow on T1WI + T2WI corresponding to radiation port (fatty infiltration)
   
   NUC:
   • bone scan with decreased uptake in radiation field
   
   Cx: increased susceptibility of irradiated bone to infection
   DDx: recurrent malignancy, radiation-induced sarcoma (soft-tissue mass), infection
4. BENIGN NEOPLASM
   Most likely in patients <2 years of age at treatment; with doses of 1600–6425 rads
   Latent period: 1.5–5–14 years
   1. Osteochondroma = exostosis (exclusively in children under 2 years of age during treatment)
   2. Osteoblastoma
5. MALIGNANT NEOPLASM
   = RADIATION-INDUCED SARCOMA
   Latency period: 3–55 (average of 11–14) years
   Minimum dose: 1,660–3,000 rad
   Criteria:
   1. malignancy occurring within irradiated field
   2. latency period of >5 years
   3. histologic proof of sarcoma
   4. microscopic evidence of altered histology of the original lesion
   
   Histo:
   1. Osteosarcoma (90%) = 4–11% of all osteogenic sarcomas
   2. Fibrosarcoma >chondrosarcoma >malignant fibrous histiocytoma
   • pain, soft-tissue mass, rapid progression of lesion