Pediatric Malignancy PET-MRI

Understanding Pediatric Malignancy PET-MRI Imaging

Pediatric malignancy PET-MRI represents a significant advancement in childhood cancer imaging, offering the combined benefits of metabolic assessment and superior soft tissue visualization without the radiation exposure associated with CT. This is particularly important in children, who are more sensitive to radiation effects and have many years of life ahead during which radiation-induced complications could develop.

Cancer in children differs fundamentally from adult cancers. Pediatric cancers are typically more aggressive, faster-growing, and more responsive to therapy. They also tend to occur in different locations—primarily in developing tissues such as bone marrow, nervous system, and connective tissues—rather than in epithelial surfaces like the lung, breast, or colon that are common sites of adult cancer.

The most common childhood cancers include leukemia (not typically imaged with PET-MRI), brain tumors, lymphoma, neuroblastoma, Wilms tumor, and bone and soft tissue sarcomas. PET-MRI plays a particularly important role in solid tumors where accurate staging and response assessment are crucial for treatment planning.

Why PET-MRI Is Preferred for Children

The choice of imaging modality in children requires special considerations:

Reduced radiation exposure is perhaps the most important advantage of PET-MRI over PET-CT. Children are significantly more radiosensitive than adults, with estimates suggesting they are 2-3 times more vulnerable to radiation-induced cancer. Additionally, they have a longer lifespan during which radiation-induced malignancies could develop. By replacing CT with MRI, PET-MRI eliminates this radiation source.

Superior soft tissue contrast of MRI is especially valuable in children because many pediatric cancers—such as sarcomas, neuroblastoma, and Wilms tumor—arise in soft tissues. MRI can show tumor extent, relationship to growth plates, and involvement of neurovascular structures with far greater detail than CT.

No need for sedation in many cases can be an advantage, as the MRI and PET portions can often be performed in a single session rather than requiring separate appointments. However, younger children often still require sedation to remain still for the required imaging time.

PET-MRI for Specific Pediatric Malignancies

Lymphoma (Hodgkin and Non-Hodgkin)

Lymphoma is one of the most common childhood cancers, accounting for approximately 15% of pediatric cancers. PET-MRI is valuable for:

• Initial staging: Detecting all sites of disease involvement
• Treatment response: Interim PET to assess response to chemotherapy
• End-of-therapy assessment: Distinguishing residual mass from active disease
• Surveillance: Detecting recurrence in high-risk patients

In children, the use of PET-MRI rather than PET-CT reduces cumulative radiation exposure, particularly important because lymphoma patients often undergo multiple imaging studies during their treatment and follow-up. Many children with lymphoma can expect to be cured, making long-term radiation avoidance particularly important.

Sarcomas (Ewing Sarcoma, Rhabdomyosarcoma, Osteosarcoma)

Sarcomas are cancers that arise from connective tissues—bone, muscle, fat, or other supporting tissues. They account for about 10-15% of childhood cancers. PET-MRI offers several advantages:

• Local staging: MRI shows tumor extent within bone or soft tissue, relationship to growth plates, and neurovascular bundle involvement
• Metastasis detection: PET identifies metastatic sites throughout the body
• Treatment response: Distinguishing residual tumor from treatment-related changes
• Surgical planning: Precise mapping of tumor extent for limb-sparing surgery

For sarcoma patients, accurate local assessment is critical for determining whether limb-sparing surgery is possible or whether amputation is required. MRI provides the detailed anatomical information needed for this decision, while PET shows the metabolic activity of the tumor.

Neuroblastoma

Neuroblastoma is a cancer that develops from nerve tissue and typically occurs in young children, with most cases diagnosed before age 5. PET-MRI is valuable for:

• Primary tumor assessment: MRI shows relationship to adjacent organs and blood vessels
• Metastasis detection: PET identifies metastatic sites in bone, bone marrow, and liver
• MIBG correlation: While MIBG scan remains the primary functional imaging for neuroblastoma, FDG PET-MRI can detect MIBG-negative disease
• Treatment response: Assessing response after chemotherapy, radiation, or immunotherapy

Neuroblastoma is unique among pediatric cancers because it can spontaneously regress in infants (especially stage 4S disease). PET-MRI helps distinguish regressing from progressing disease, avoiding unnecessary treatment in some infants.

Wilms Tumor and Other Renal Tumors

Wilms tumor (nephroblastoma) is the most common kidney cancer in children, typically diagnosed between ages 2-5. PET-MRI provides:

• Bilateral assessment: MRI can evaluate both kidneys simultaneously
• Local extent: Determining if tumor has extended beyond the kidney
• Metastasis detection: Identifying metastatic disease in lungs, liver, or lymph nodes
• Response assessment: Evaluating tumor shrinkage after chemotherapy

For Wilms tumor patients, treatment typically involves chemotherapy followed by surgery. PET-MRI can assess the response to chemotherapy and help plan the surgical approach.

Special Considerations in Children

Sedation and Motion Management

One of the challenges of pediatric PET-MRI is keeping children still for the required imaging time, which typically ranges from 30-60 minutes depending on the protocol.

• Children under 6 typically require sedation or general anesthesia
• Children 6-12 may be able to cooperate with preparation, video goggles, and child life specialists
• Adolescents 13+ can usually undergo the exam without sedation

The need for sedation has decreased as MRI sequences have become faster and as child life specialists have developed better techniques for helping children cope with the imaging environment. Some centers now use "scanese" (sedation-free) protocols for children as young as 5-6 years.

Growth Plate Considerations

Children's bones contain growth plates (physes) that are metabolically active and can show physiologic FDG uptake. This is important when assessing bone tumors or metastases:

• Normal growth plates show symmetric, moderate FDG uptake
• Tumorous involvement typically shows more intense, asymmetric uptake with destructive changes on MRI
• Comparison to contralateral side helps distinguish normal from abnormal

Additionally, the relationship of tumors to growth plates is critical for surgical planning. Tumors near growth plates may affect future bone growth, influencing decisions about limb-sparing versus amputation approaches.

Radiation Sensitivity

Children are more vulnerable to radiation effects than adults for several reasons:

• More rapidly dividing cells are more susceptible to radiation damage
• Longer lifespan allows more time for radiation-induced cancers to develop
• Smaller body size means higher radiation dose per unit volume
• Multiple imaging studies during childhood create cumulative exposure

These factors make PET-MRI particularly attractive for pediatric oncology, as it eliminates the radiation dose from the CT component of PET-CT.

Clinical Applications and Impact on Management

Treatment Response Assessment

Assessing how tumors respond to therapy is critical in pediatric oncology, as it guides decisions about continuing, escalating, or de-escalating treatment. PET-MRI provides complementary information:

• Size reduction: MRI measures anatomical tumor shrinkage
• Metabolic response: PET shows whether tumor cells are still metabolically active
• Treatment-related changes: MRI can show necrosis, fibrosis, or hemorrhage within treated tumors
• Residual viable tumor: Areas of persistent FDG uptake indicate remaining cancer

In some pediatric protocols, early response assessment after 2-3 cycles of chemotherapy can identify patients who may benefit from treatment escalation. Conversely, excellent early responders may be candidates for reduced therapy, potentially decreasing long-term toxicities.

Long-Term Follow-Up

Because most children with cancer are now cured, long-term follow-up is essential to monitor for:

• Local recurrence: Tumor returning at the original site
• Distant metastases: New tumors in other organs
• Second malignancies: New primary cancers, sometimes related to previous treatment
• Treatment effects: Late effects of surgery, radiation, or chemotherapy

PET-MRI provides comprehensive surveillance without additional radiation exposure, which is important for children who have already received significant radiation as part of their cancer treatment.

Evidence-Based Outcomes

Quality of Life Considerations

Long-term survivors of childhood cancer face unique challenges:

• Second cancers: Radiation exposure increases risk of secondary malignancies decades later
• Growth abnormalities: Radiation to growth plates can cause limb length discrepancies
• Organ dysfunction: Radiation to abdominal organs can cause liver, kidney, or intestinal problems
• Cognitive effects: Brain radiation can affect learning and cognitive development

By using PET-MRI instead of PET-CT whenever possible, radiation exposure is minimized, potentially reducing these long-term complications.

Preparing for Your Child's Scan

Before the Appointment

Preparation depends on whether sedation will be used:

• Fasting: Typically 6 hours for FDG scans; 4 hours if sedation is planned
• Medications: Take usual medications unless instructed otherwise
• Comfort items: Bring favorite blanket, toy, or video player for the waiting area
• Clothing: Dress in comfortable metal-free clothing (sweatpants, t-shirt)
• History: Provide complete medical history including previous scans and treatments

Day of the Procedure

The PET-MRI examination typically takes 2-3 hours total:

1. Check-in and preparation
2. Tracer injection: Radiotracer administered intravenously
3. Uptake period: 60-90 minutes in a quiet room; child should remain calm and still
4. MRI acquisition: 30-60 minutes in the scanner
5. Recovery: If sedation was used, monitoring until fully awake

During the MRI

Child life specialists help children cope with the MRI experience through:

• Age-appropriate explanation of what will happen
• Practice sessions with mock scanners to desensitize children
• Video goggles or entertainment systems during the scan
• Parental presence in the scanner room when safe and appropriate
• Coaching and encouragement throughout the exam

Understanding Your Child's Results

Frequently Asked Questions

Is PET-MRI safe for children?

Yes, PET-MRI is very safe for children. The main risks are the small radiation dose from the PET tracer (similar to a CT scan) and potential allergic reaction to the tracer. The MRI component uses no radiation and is safe for children.

Can parents stay in the room during the scan?

Whether parents can stay depends on the facility's policies and whether sedation is being used. Many centers allow a parent to remain in the scanner room during setup and until imaging begins, then wait in the nearby area during image acquisition.

How often will my child need these scans?

The frequency depends on the type of cancer and treatment protocol. Common schedules include: staging at diagnosis, response assessment during treatment (often after 2-3 cycles), end-of-treatment evaluation, and then periodic surveillance for several years after treatment completion.

Will my child's growth be affected by the scan?

No, PET-MRI does not affect growth. The small radiation dose from the PET tracer is far below levels that would affect growth or development. The MRI component uses magnetic fields and radio waves, not radiation.

References

1. Children's Oncology Group. Long-Term Follow-Up Guidelines for Childhood Cancer Survivors. 2024.
2. Society of Nuclear Medicine and Molecular Imaging. SNMMI Practice Guidelines for Pediatric PET/MRI. 2023.
3. Pediatric Radiology. Image Gently Campaign: Radiation Protection in Pediatric Medical Imaging. 2023.
4. American Journal of Roentgenology. PET-MRI in Pediatric Oncology: Clinical Applications and Future Directions. 2022.

Medical Disclaimer: This information is educational only. Always discuss findings with your healthcare provider for personalized medical advice.