In a pediatric patient with a suspected rhabdomyosarcoma, which imaging modalities and laboratory tests should be ordered for initial evaluation and staging?

Imaging and Laboratory Evaluation for Suspected Rhabdomyosarcoma

For a pediatric patient with suspected rhabdomyosarcoma, obtain MRI of the primary site with and without IV contrast, chest CT, and whole-body imaging (FDG-PET/CT or whole-body MRI), along with baseline laboratory tests including CBC, LDH, and alkaline phosphatase. 1, 2, 3

Initial Imaging Studies

Primary Site Evaluation

• MRI with and without IV contrast is the imaging modality of choice for evaluating the primary tumor site, providing superior soft tissue characterization and assessment of anatomic extent 3, 1
• MRI should include the entire anatomical compartment and adjacent structures to define local invasion 3, 1
• Diffusion-weighted imaging (DWI) sequences should be included when available, as they improve tumor characterization and can help differentiate viable tumor from necrosis 3
• CT may be used when MRI is contraindicated or unavailable, though it provides inferior soft tissue detail 3

Metastatic Disease Screening

• Chest CT (with or without contrast) is mandatory to detect pulmonary metastases, which are common sites of spread in rhabdomyosarcoma 1, 2
• Whole-body imaging is essential for complete staging, using either FDG-PET/CT or whole-body MRI 1, 4
• FDG-PET/CT provides both metabolic and anatomic information, with high sensitivity for detecting distant metastases including bone marrow involvement 4
• Whole-body MRI is increasingly employed and offers the advantage of avoiding radiation exposure in pediatric patients 5, 1

Critical Imaging Consideration for Alveolar Rhabdomyosarcoma

• When performing whole-body imaging, ensure complete coverage of hands and feet, as 14% of alveolar rhabdomyosarcoma patients may have silent distal extremity lesions that are easily missed 5
• This is particularly important because missing these lesions significantly impacts treatment decisions and prognosis 5
• In one registry study, 53% of whole-body examinations had partially or completely excluded the hands and feet, representing a major diagnostic pitfall 5

Laboratory Tests

Baseline Laboratory Studies

• Complete blood count (CBC) should be obtained, as anemia and leukocytosis may be present 6
• Lactate dehydrogenase (LDH) has prognostic value and should be measured at baseline 6
• Alkaline phosphatase (ALP) should be measured, particularly if bone involvement is suspected 6
• Renal function tests (urea, creatinine, glomerular filtration rate) are essential before initiating chemotherapy 6

Tissue Diagnosis Requirements

• Core needle biopsy or open biopsy is required for definitive diagnosis 2, 3
• Tissue should be sent for immunohistochemistry and molecular/cytogenetic studies 1
• Pathology review by a specialist sarcoma pathologist is recommended 1

Staging Imaging Protocol Summary

The complete staging workup should include:

• MRI of primary site with and without IV contrast 3, 1
• Chest CT to detect pulmonary metastases 1, 2
• Whole-body imaging (FDG-PET/CT or whole-body MRI) with complete coverage including hands and feet 5, 1
• Bone marrow evaluation can be accomplished through whole-body imaging rather than invasive bone marrow biopsy 4

Common Pitfalls to Avoid

• Do not perform biopsy before completing staging, as the biopsy tract must be planned to lie within the eventual resection bed 6
• Do not exclude distal extremities from whole-body imaging, particularly in alveolar rhabdomyosarcoma where silent lesions occur in 14% of cases 5
• Do not rely on plain radiographs alone, as they provide insufficient information for soft tissue sarcomas 2, 3
• Do not omit chest imaging, as pulmonary metastases significantly impact prognosis and treatment planning 1, 2
• Ensure all imaging is reviewed by radiologists with musculoskeletal oncology expertise before biopsy planning 6