Tumour Imaging: ABCDS Approach

Musculoskeletal tumours span a vast spectrum from trivial "do-not-touch" lesions (non-ossifying fibroma, bone island) to lethal high-grade sarcomas. The single most important task for the examination candidate is not memorising every entity but applying a reproducible systematic framework to any radiograph, then deciding whether the lesion is non-aggressive (can be observed or treated by a generalist) or aggressive (must be referred to a specialist sarcoma centre before any intervention).

The radiograph remains the cornerstone of bone-tumour assessment: matrix mineralisation, periosteal reaction and zone of transition are best appreciated on plain film and frequently allow a confident diagnosis or a tight differential. Cross-sectional imaging then answers specific questions — MRI for marrow and soft-tissue extent and local staging, CT for cortical detail and pulmonary metastases, and FDG-PET/CT for whole-body staging and response.

A recurring exam theme is the danger of acting before staging. A poorly planned biopsy or an unwary excision of an unsuspected sarcoma can convert a limb-salvageable tumour into an amputation or compromise survival. The candidate who states early that any suspicious lesion is referred intact to a sarcoma multidisciplinary team, fully staged, and biopsied along a planned track demonstrates safe practice.

The topic repeatedly refers to "Enneking staging" — the examiner expects you to reproduce it. There are two systems, one for benign and one for malignant musculoskeletal tumours, both integrating imaging-defined compartmental anatomy.

For malignant lesions the system combines histological grade (G1 low, G2 high), anatomic site (T1 intracompartmental, T2 extracompartmental) and metastasis (M0 / M1):

Imaging does not just diagnose — it defines whether a tumour can be removed with an oncologically adequate margin, which is why MRI assessment of compartmental boundaries is mandatory. Enneking described four margins by the plane of dissection relative to the tumour and its reactive zone.

The decisive radiographic discriminator is the zone of transition combined with periosteal reaction and cortical status, integrated with patient age. The table below collapses the most common pattern-based decisions a candidate must make at the viva.

Primary bone sarcomas are rare worldwide (osteosarcoma incidence approximately 3-4 per million per year, peaking in adolescence, with a second peak in older adults often related to Paget disease or prior radiation). Ewing sarcoma is rarer still and shows marked geographic and ancestral variation, being notably less common in populations of African ancestry. Soft-tissue sarcomas are commoner than bone sarcomas (roughly 4-5 per 100,000 per year) but still account for under 1% of adult malignancies. Bone metastases vastly outnumber primary bone tumours overall and dominate the differential in patients over 40.