Osteoradionecrosis (ORN) refers to a severe delayed radiation-induced injury and is characterised by bone tissue necrosis and failure in healing. There is some overlap with the term radiation osteitis.
Microvascular damage is thought to result in altered blood supply to the bone. This limits the ability of the tissues to repair normal wear and tear which, in turn, eventually results in the tissue breakdown seen on imaging.
Usually large radiation doses are required for osteoradionecrosis to occur (e.g. < 3000-5000 cGy) 2.
While it can involve any bone within the irradiated field, there are specific sites in which osteoradionecrosis is more commonly seen. These include:
- mandible: mandibular osteoradionecrosis: this site is particularly prone due to its superficial location and high doses of radiation required to radically treat naso-oro-pharyngeal tumours
- chest wall-shoulder-humerus-scapula
- bony pelvis
While there are general features, radiographic features can somewhat vary with the site of involvement.
With mandibular osteoradionecrosis, there can be ill-defined cortical destruction without sequestration. In osteoradionecrosis of the ribs, clavicle, scapula, and humerus, radiography may demonstrate 2 :
- osteopaenia: typically occurs after ~1 year after irradiation 4
- disorganization and coarsening of trabecular architecture
- cortical irregularity
- heterogenous bone density
With mandibular osteoradionecrosis, CT may additionally show cortical interruptions and loss of spongiosa trabeculation 6. In other sites CT may show the presence of subtle fractures, alterations in bone architecture and dystrophic soft-tissue calcification.
On MRI, they can be development of new heterogeneous signal within the marrow of an irradiated area (intermediate or low T1 signal, intermediate or high T2 signal). Osteoradionecrosis with or without osteomyelitis can be extremely difficult to differentiate from recurrent tumour. Adjacent muscles may appear oedematous and show intense enhancement, which can be difficult to differentiate from recurrent tumor if bone changes are not visible on CT 10.
In osteoradionecrosis of the spine, haematopoietic cellular elements of the spinal marrow can also be replaced with fat, which then has 2
- T1: high signal intensity
- T2: intermediate signal intensity
Treatment and prognosis
It either stabilizes or gradually worsens, which then becomes notoriously difficult to manage.
To give a meaningful differentiation, location and imaging modality needs to be taken into account. General differential consideration include:
- original tumour recurrence
- radiation-induced secondary tumour e.g. sarcoma
- complicating infection: osteomyelitis (can also be an association) 10
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