PET-CT in Orthopaedic Oncology

PET-CT in osteosarcoma staging has several roles: (1) Detection of skip lesions - separate tumour foci in the same bone, which affect surgical planning and prognosis. PET may detect skip lesions not seen on MRI. (2) Distant metastases - lung metastases (though CT chest is also needed for small nodules), bone metastases in other sites. (3) Baseline SUVmax - establishes pre-treatment metabolic activity for comparison after neoadjuvant chemotherapy. (4) Response prediction - higher pre-treatment SUV paradoxically associated with better response to chemotherapy. I would look for: primary tumour FDG uptake and extent, any separate foci in the same bone (skip lesions), other bone involvement, nodal disease, and lung lesions.

The primary tumour shows high FDG uptake (SUVmax 12), consistent with high-grade sarcoma. The inguinal lymph node with SUVmax 4 is concerning for nodal metastasis, though lymph node metastases are uncommon in most soft tissue sarcomas (less than 5% overall). However, certain histological subtypes have higher nodal metastasis rates: synovial sarcoma, epithelioid sarcoma, clear cell sarcoma, rhabdomyosarcoma, and angiosarcoma. The FDG-avid node should be biopsied or closely monitored. If confirmed as metastatic, this upstages the disease and affects prognosis and treatment planning. Alternative explanations for the node include reactive lymphadenopathy (infection, inflammation), so tissue confirmation is important before assuming metastatic disease.

FDG-PET can help differentiate benign from malignant cartilage lesions based on metabolic activity. Generally: Enchondromas have low FDG uptake (SUVmax typically less than 2). Low-grade (G1) chondrosarcomas have low to moderate uptake (often SUVmax 2-4). Higher-grade (G2-3) chondrosarcomas show higher uptake (SUVmax greater than 4-5). This patient's SUVmax of 3.5 is in an intermediate zone and concerning for at least low-grade chondrosarcoma. However, there is overlap between enchondroma and G1 chondrosarcoma, making PET alone insufficient for definitive diagnosis. Clinical context matters: symptoms (pain without fracture), location (axial vs appendicular), size, and imaging features (cortical destruction, soft tissue mass). This lesion warrants close follow-up or biopsy, as the SUV suggests it may not be a simple enchondroma.