Can FDG‑PET/CT be used to monitor recurrence of rhabdomyosarcoma in an adolescent who has completed surgery, chemotherapy, and radiation?

FDG-PET/CT for Monitoring Rhabdomyosarcoma Recurrence in Adolescents

Yes, FDG-PET/CT can be used to monitor for recurrence of rhabdomyosarcoma in adolescents who have completed multimodal therapy, as it is highly accurate for detecting recurrent soft tissue sarcomas and provides superior sensitivity compared to conventional imaging alone.

Primary Recommendation

FDG-PET/CT is highly effective for detecting recurrent rhabdomyosarcoma and should be considered when there is clinical suspicion of recurrence, equivocal findings on conventional imaging (CT/MRI), or rising tumor markers 1. The American College of Radiology guidelines support FDG-PET/CT for detecting tumor recurrence, especially when residual abnormalities on physical examination or other imaging studies need differentiation between viable tumor versus post-treatment fibrosis or necrosis 1.

Diagnostic Performance in Rhabdomyosarcoma

• FDG-PET/CT demonstrates high sensitivity (94%) and specificity (92%) for detecting recurrent soft tissue sarcomas, outperforming contrast-enhanced CT alone (78% sensitivity, 67% specificity) 1
• In pediatric rhabdomyosarcoma specifically, PET/CT shows 77% sensitivity and 95% specificity when compared to final clinical determination of disease extent 2
• All primary rhabdomyosarcoma sites demonstrate positive FDG uptake at diagnosis, with standardized uptake values (SUVmax) ranging from 2.4 to 12.7 (mean 6.4) 2, 3
• PET/CT is particularly valuable for detecting unsuspected and unusual metastatic sites that may not be apparent on conventional imaging 4

Optimal Timing for Surveillance Imaging

Critical timing considerations to minimize false-positive results:

• Wait at least 3 months after radiation therapy before performing PET/CT to avoid false-positives from therapy-related FDG uptake 5
• Wait at least 1 month after surgery to reduce false-positive results from post-operative inflammation 6
• For chemotherapy alone, perform PET/CT at 6-8 weeks after completion of treatment 7
• For combined chemoradiotherapy, perform PET/CT at 8-12 weeks after completion 7

When PET/CT Provides Maximum Clinical Value

Use FDG-PET/CT as a problem-solving tool in these specific scenarios:

• When MRI or CT findings are equivocal for recurrence versus post-treatment changes 1
• When there is clinical suspicion of recurrence (symptoms, rising tumor markers) but conventional imaging is inconclusive 1
• To differentiate residual abnormalities from viable tumor versus fibrosis/necrosis after treatment completion 1
• For guiding biopsy to the most metabolically active region of suspected recurrent tumor 1

Complementary Imaging Strategy

MRI remains the primary surveillance modality for the local tumor site, with PET/CT serving as an adjunct 1:

• MRI with contrast is the mainstay for evaluating local recurrence at the primary soft tissue tumor site 1
• PET/CT and MRI show comparable performance for detecting local recurrence (MRI: 90% sensitivity, 98% specificity vs. PET/CT: 95% sensitivity, 96% specificity) 1
• PET/CT provides superior whole-body assessment for detecting distant metastases compared to conventional imaging 1

Important Caveats and Pitfalls

Be aware of these limitations when interpreting PET/CT:

• False-positive results can occur from inflammation, infection, or post-treatment changes if imaging is performed too early after therapy 6
• False-negative results may occur in small lesions below the resolution limit of PET scanners 2
• In one pediatric series, PET failed to capture disease at 10 sites that were visualized by CT, MRI, or bone scan, highlighting the need for complementary imaging 2
• Inflammatory and infectious processes are the primary cause of false-positive FDG-PET/CT results 6

Practical Algorithm for Surveillance

1. Primary surveillance: Use MRI with contrast for the local tumor site at regular intervals 1
2. Add PET/CT when:
   • MRI/CT findings are equivocal or concerning for recurrence 1
   • Clinical symptoms suggest recurrence but conventional imaging is negative 1
   • Tumor markers are rising without clear anatomic correlate 1
3. Timing: Ensure adequate interval from last treatment (3 months post-radiation, 1 month post-surgery) 6, 5
4. Interpretation: Compare metabolic activity to baseline PET/CT if available; biopsy confirmation recommended for FDG-avid lesions 6, 3

Prognostic Value

Baseline and follow-up PET/CT parameters have prognostic significance:

• Higher SUVmax values at diagnosis correlate with worse outcomes 8
• Complete metabolic response on follow-up PET/CT is associated with significantly better prognosis 3
• Metabolically active lymph node and distant metastases on PET/CT indicate significantly lower survival rates 8