Sarcopedia

Unique biphasic epithelial-fibrous tumour of bone Tibial predilection is virtually diagnostic Cytokeratin positivity confirms epithelial component Ostefibrous dysplasia-like variant may precede or coexist with classic form

Fluid-fluid levels on MRI are characteristic but not pathognomonic USP6 FISH useful to confirm primary ABC vs secondary Must exclude telangiectatic osteosarcoma - shares imaging features Sclerotherapy and embolisation effective alternatives to surgery

Diagnosis of exclusion - must rule out Malignant Fibrous histiocytoma and other Fibrous lesions Histologically identical to soft tissue counterpart Close Follow-up recommended due to recurrence risk

Benign notochordal remnant Observation appropriate

Cytological atypia mimics malignancy but behaviour is Benign Lack of medullary continuity distinguishes from osteochondroma USP6 rearrangement links it to nodular fasciitis (reactive vs neoplastic debate) Recurrence in >50% after simple excision

Chondroblastic variant may resemble chondrosarcoma - osteoid must be identified Often shows poorer chemotherapy response than other OS subtypes Distinguishing from dedifferentiated chondrosarcoma is critical (different treatment and prognosis) IDH1/2 mutation testing helps exclude chondrosarcoma

One of the few truly epiphyseal Bone tumours (along with GCT and clear cell chondrosarcoma) H3F3B K36M IHC is Highly sensitive and specific for diagnosis Extensive perilesional oedema on MRI is characteristic and can mislead towards aggressive diagnosis Benign pulmonary implants documented - do not require treatment

EWSR1-GRM1 fusion gene is characteristic and diagnostically useful No IDH mutations - helpful to distinguish from Low-grade chondrosarcoma Histological misdiagnosis as chondrosarcoma not uncommon due to cellularity Curettage with adjuvants preferred to reduce recurrence

IDH mutation status important for prognosis and emerging therapy Wide excision critical Chemotherapy generally not effective

Brachyury (T protein) IHC is Highly sensitive and specific for chordoma Proton beam radiotherapy is treatment of choice for skull base chordoma Sacral resection requires detailed preoperative planning for sphincter preservation Dedifferentiated variant has sarcomatous component and is Highly aggressive

Abrupt bimorphic histology is characteristic - unlike secondary High-grade sarcoma IDH mutation confirms chondroid origin of Low-grade component Treated as High-grade sarcoma despite cartilaginous component Prognosis dramatically worse than conventional chondrosarcoma

Distinguishing enchondroma from Low-grade chondrosarcoma is one of the most challenging problems in musculoskeletal pathology Pain without fracture and endosteal scalloping >2/3 are red flags for chondrosarcoma IDH mutation does not distinguish enchondroma from chondrosarcoma Maffucci Syndrome has near 100% lifetime risk of chondrosarcoma

Predominantly lytic lesion - may be confused with fibrosarcoma of Bone or MFH Identification of osteoid (even focal) is required for OS diagnosis Generally more chemosensitive than chondroblastic OS Must be distinguished from High-grade surface osteosarcoma

Benign self-limited lesion Observation standard of care

GNAS1 mutation testing is diagnostic and distinguishes from Low-grade OS or other fibro-osseous lesions Malignant transformation risk is very Low but increases with radiation exposure (avoid radiotherapy) Bisphosphonates are the mainstay of non-surgical management Regular screening for endocrine abnormalities in McCune-Albright

H3F3A mutations define giant cell tumour Typically epiphyseal around knee High recurrence risk requires close Follow-up Denosumab effective for inoperable disease

Benign incidental finding Observation appropriate for asymptomatic lesions

Rare primary Bone malignancy Usually DLBCL histology Assess for systemic involvement before treatment

Most common conventional OS subtype - 'classic' osteosarcoma Dense ivory sclerosis on X-ray is Highly characteristic Histological response to neoadjuvant chemotherapy is the single most important prognostic factor High-dose methotrexate is uniquely active in OS - not used in other sarcomas

Most common bone tumour HME carries malignancy risk Monitor cartilage cap size on imaging

Benign congenital Fibrous lesion Anterior tibial bowing is characteristic Resolves with skeletal maturity

Nocturnal pain relieved by aspirin/NSAIDs is virtually pathognomonic CT is essential to identify the nidus - MRI alone is unreliable due to surrounding oedema CT-guided RFA is the treatment of choice - safe, effective, minimal recovery time Spinal lesions cause painful scoliosis which resolves after treatment

Multiple osteomas should prompt consideration of Gardner Syndrome - refer for APC testing and colonoscopy Ivory appearance on CT is diagnostic - no further investigation required for typical asymptomatic lesion Frontal sinus is the most common location Treatment only required when symptomatic

Neoadjuvant chemotherapy essential Biopsy must not compromise surgical margins Limb-salvage surgery preferred when feasible

New pain in Paget disease should prompt urgent imaging to exclude sarcomatous transformation 5-year survival <10% - much worse than conventional osteosarcoma Bisphosphonate treatment of Paget disease may reduce (but not eliminate) risk of transformation Multidisciplinary planning essential - surgery most important modality

MDM2 and CDK4 amplification are diagnostically useful - also present in DDLPS (different anatomical context) Posterior distal femur is the classic location Low-grade lesion does NOT require chemotherapy Dedifferentiated component (High-grade) must be identified - changes management completely

Rare subtype 1-2% of OS Adolecence and young adults (10-30) Arrises from periosteum

Most common primary Bone malignancy in older adults Workup requires comprehensive staging Reversible disease with continuous treatment

Solitary lesion but High risk of progression Treat as High-risk disease Close monitoring essential

Benign self-limited lesion Observation appropriate initially Multiple effective injection treatments

Small cell variant but aggressive like conventional OS Osteoid production distinguishes from Ewing Treated identically to conventional OS

Fluid-fluid levels on MRI mimic ABC - aggressive periosteal reaction and soft tissue mass are red flags for OS Never biopsy a suspected ABC before ruling out telangiectatic OS - confirm with MRI soft tissue component Treated identically to conventional osteosarcoma with MAP chemotherapy Osteoid must be identified in septal sarcomatous cells for correct diagnosis