PET in Pediatric Oncology

Quick Answer

Pediatric oncologic imaging pairs FDG-PET/CT (Hodgkin and non-Hodgkin lymphoma, sarcomas, Langerhans-cell histiocytosis) with MIBG (neuroblastoma, the paradigm childhood theranostic target). The high-yield differences from adults are physiologic pitfalls that are stronger in children — prominent thymus, brown fat, physeal (growth-plate) uptake, and reactive marrow — and weight-based dosing with strict ALARA dose minimisation. Response criteria carry over (Deauville for lymphoma), while neuroblastoma has its own semiquantitative MIBG scores (Curie, SIOPEN) that guide risk and therapy.

Disease-specific roles

What each modality does:

• Hodgkin & non-Hodgkin lymphoma: FDG-PET/CT for staging and interim/end-of-treatment response by the Deauville 5-point score; PET-adapted therapy (escalate/de-escalate) is central to pediatric lymphoma trials.
• Sarcomas (Ewing, osteosarcoma, rhabdomyosarcoma): FDG for staging, grading/prognosis, and response; complements MRI (primary) and is sensitive for nodal/distant and marrow disease.
• Neuroblastoma: MIBG is the workhorse — diagnosis, staging, and selecting ¹³¹I-MIBG therapy (the target must be MIBG-avid). ~10% are MIBG-non-avid → FDG is the fallback. Semiquantitative Curie and SIOPEN scores quantify skeletal/soft-tissue burden.
• Langerhans-cell histiocytosis: FDG-PET is sensitive for active lesions and monitoring.

Pediatric-specific pitfalls

Physiologic FDG pitfalls are more pronounced in children: a prominent/rebounding thymus (inverted-V anterior mediastinal uptake, especially after chemotherapy) mimics residual lymphoma; brown adipose tissue is common in lean children (keep warm); growth-plate (physeal) uptake is normal and symmetric at the metaphyses; and reactive/G-CSF-stimulated marrow is diffuse and symmetric. Recognising these prevents overcalling disease.

Thymic rebound after chemotherapy is the classic pediatric trap — homogeneous, arrowhead-shaped anterior mediastinal uptake that should not be called relapse. Physeal uptake must not be mistaken for skeletal metastasis; metastatic uptake is focal and off the growth plate.

Dosimetry and safety

Children are more radiosensitive and have a longer horizon for stochastic risk, so imaging follows ALARA: weight- (or body-surface-area) based activity per pediatric dosage cards (e.g. EANM pediatric dosage card / North American consensus guidelines), the lowest activity that yields a diagnostic study, attention to cumulative dose across serial scans, and thyroid blockade for iodinated agents (MIBG). See the pediatric dose estimator.

Common Pitfalls / Cautions

• Thymic rebound, brown fat, and physeal uptake are the dominant false positives — know them cold.
• MIBG-non-avid neuroblastoma (~10%) needs FDG; do not assume MIBG covers all cases.
• Minimise and track cumulative radiation across serial pediatric studies.

Board Pearls

• MIBG-avidity is fairly specific for neurogenic tumors (neuroblastoma) — other pediatric small-round-blue-cell tumors (Ewing, rhabdomyosarcoma, lymphoma) are FDG-avid but MIBG-negative, so tracer choice follows tumor type.
• DOTATATE (SSTR) PET is an emerging alternative in neuroblastoma and opens a ¹⁷⁷Lu-DOTATATE theranostic route, particularly for MIBG-non-avid tumors.
• Diffuse marrow FDG uptake from G-CSF or anemia can mask marrow disease — schedule PET before or well after growth-factor support and correlate with marrow biopsy.
• Preparation is pediatric-specific — adequate fasting/glucose control, warming to suppress brown fat, and frequently sedation/anesthesia in young children to avoid motion.

Differential Diagnosis

• Reactive or infectious lymphadenopathy — common FDG-avid nodes in children mimic lymphoma; weigh symmetry, size, and clinical context.
• Benign FDG-avid bone lesions — non-ossifying fibroma, fibrous dysplasia, Langerhans-cell histiocytosis, and healing fractures can mimic sarcoma metastasis.
• Post-treatment inflammation — surgical beds, radiation change, and marrow rebound simulate residual or recurrent tumor.
• Infection / granulomatous disease versus metastasis, especially in immunosuppressed children.
• For a neurogenic adrenal/abdominal mass, neuroblastoma (MIBG-avid) versus other small-round-cell tumors and benign adrenal lesions — MIBG confirms neurogenic origin.

Reporting Checklist

• State the tracer, weight/BSA-based activity (ALARA), and preparation (fasting/glucose, warming, sedation, thyroid blockade for MIBG).
• Apply the correct scoring system — Deauville for lymphoma response, Curie/SIOPEN for neuroblastoma burden.
• Explicitly classify equivocal foci as physiologic (thymus, physis, brown fat, marrow) versus disease.
• Compare with priors and note cumulative radiation dose across serial studies.
• Correlate with anatomic imaging (MRI for primary/CNS) and, where relevant, marrow sampling; for neuroblastoma, state MIBG-avidity to guide ¹³¹I-MIBG therapy.

Self-Check

Key References

• EANM pediatric dosage card and North American consensus pediatric dosing guidelines; ALARA/Image Gently principles.
• Deauville-based PET-adapted pediatric lymphoma trials; MIBG Curie/SIOPEN scoring and ¹³¹I-MIBG therapy selection in neuroblastoma.