-Surface bone lesions are bone lesions arising peripheral to the medulla.
-The surface of bone consists of cortex and periosteum.
-The periosteum has an outer fibrous part and an inner part consisting of mesenchymal cells (which can differentiate into osteoblasts or osteoclasts).
-A primary surface lesion of bone may arise from any of the mesenchymal elements that normally reside there,
or it may be the product of the pleuripotential cells of the parosteal tissues. Thus,
a surface lesion may contain osteoid,
cartilage,
fibrous tissue,
fat,
or blood vessels.
-The most common types are those producing cartilage or bone.
-Surface bone lesions consist of neoplastic and non-neoplastic type lesions.
-Periosteal reaction or periostitis is the reaction of cortical bone to insult.
-Aggressive periosteal reaction is interrupted and may be lamellated (onion skin),
spiculated (sunburst/hair on end) or disorganised. Focal triangular periosteal reaction at the edge of a lesion (Codman's triangle) is classically associated with malignant lesions but can be seen in infection and sub periosteal haematoma.
-Non-aggressive periosteal reaction is solid or unilamellated,
well defined and continuous.
-Perioisteal reaction can occur in neoplastic and non neoplastic bone lesions.
-There are mimics of surface bone lesions such as eccentric medullary based lesions and soft tissue lesions forming adjacent to bone.
-Non-neoplastic surface bone lesions include infection and post traumatic lesions such as stress fractures,
myostis ossificans/heterotopic ossification,
fluorosis and bone avulsions.
-Neoplastic surface bone lesions include parosteal osteosarcoma,
periosteal osteosarcoma,
and high grade surface osteosarcoma.
-Imaging features can be used to differentiate aggressive from non-aggressive lesions and to a certain extent differentiate between subcategories of neoplastic lesions.
-Aggressive periosteal reaction,
large soft tissue mass,
wide zone of transition/irregular borders and enhancement indicate aggressive lesions.
-A full compliment of imaging with xray,
CT (assess osseous component/matrix) and MRI (to assess enhancement and matrix but also to locally stage) +/- nuclear medicine or ultrasound is required to aid in the differentiation of surface bone lesions.
-Serial imaging is helpful in differentiating between myositis ossificans and periosteal osteosarcoma.
-Biopsy should not be performed without significant consideration and full imaging workup as some no-touch lesions such as myostis ossificans/periostitis can mimic sarcoma on imaging and pathology.
In addition,
biopsy of aggressive lesions could possibly create seeding from the biopsy tract into the soft tissues.
-All biopsies should be performed in consultation with the surgical team to ensure an appropriate path be chosen to prevent seeding of uninvolved compartments and avoid neurovascular structures.
-Radiology report of surface bone lesions should comment on size,
location,
matrix,
degree of medullary involvement,
soft tissue component (ie muscle compartment involved) and neurovascular structures involved.
An imaging based differential diagnosis should be offered.
-When considering a differential diagnosis it is important to take into account clinical factors such as age,
duration and any history of trauma.
As a caveat it should be noted that often in cases of sarcoma a history of trauma is given,
thus trauma does not exclude aggressive neoplastic lesions.
Summary of DDx for surface bone lesions:
Neoplastic
Bone Forming: Osteoma,
osteochondroma,
osteoid osteoma,
paracortical osteoblastoma and surface osteosarcoma (parosteal,
periosteal and high grade)
Cartilage Forming: Periosteal chondroma,
Nora lesion/Bizarre Parosteal Osteochondromatous Proliferation (BPOP) and parosteal chondrosarcoma.
Fibrous: Fibrous cortical defect,
non ossifying fibroma and osteofibrous dysplasia.
Fat Forming: Parosteal lipoma.
Vascular: Paracortical hemangioma and paracortical angiosarcoma.
Metastasis: hypertrophic arthropathy.
Non-neoplastic
Osteomyelitis,
fracture healing,
subperiosteal haematoma or periostitis ossificans,
fluorosis,
rickets,
thyroid acropachy,
Caffey disease,
renal osteodystrophy and subperiosteal ganglion.
Mimics (not true surface lesions)
Myositis ossificans,
any eccentric bone lesion or adjacent soft tissue lesion invading bone surface.