What are the next steps in managing a patient with a history of rectal cancer and recent PET/CT scan showing FDG uptake near the anorectal region?

Management of Suspected Recurrence in a Patient with History of Rectal Cancer

The next step in management should be a pelvic MRI to further evaluate the suspicious FDG uptake in the anorectal region seen on PET/CT. 1

Background and Current Findings

This patient has:

• History of rectal cancer
• Recent colonoscopy (2/13/2025) showing diverticulosis and internal hemorrhoids
• PET/CT (7/12/2025) showing FDG uptake in the anorectal region
• CT CAP (7/12/2025) negative for metastatic disease

Diagnostic Approach

Why Pelvic MRI is the Next Step

1. Superior Soft Tissue Resolution: MRI provides better characterization of rectal and perirectal tissues than CT or PET/CT alone 1

   • Can distinguish between post-treatment changes, fibrosis, and recurrent tumor
   • Provides detailed anatomic information about the anorectal region

2. Guideline-Based Recommendation:

   • NCCN guidelines recommend pelvic MRI for both initial staging and restaging of rectal cancer 1
   • ACR Appropriateness Criteria specifically recommends MRI for evaluation of suspected recurrence 1

3. Limitations of Current Imaging:

   • PET/CT has limited specificity in the post-treatment setting due to false positives from inflammation 1, 2
   • The current PET/CT finding is equivocal ("might correspond to subtle fullness versus partial distention")
   • CT alone has poor sensitivity for local recurrence in the pelvis 1

Diagnostic Algorithm

1. Pelvic MRI with contrast

   • High-resolution T2-weighted sequences
   • Diffusion-weighted imaging
   • Dynamic contrast enhancement

2. If MRI is inconclusive:

   • Consider endoscopic ultrasound (EUS) with biopsy of suspicious areas
   • Alternatively, consider PET/MRI which has shown improved accuracy (92% for local recurrence) compared to MRI alone (89%) 3

3. If MRI confirms suspicious lesion:

   • Tissue diagnosis via biopsy is required before definitive treatment

Interpretation Considerations

Potential False Positives on PET/CT

• FDG uptake in the presacral space can be due to benign inflammatory changes 2
• Physiologic uptake in displaced pelvic organs is a common cause of false-positive interpretations 4
• Post-treatment changes can show FDG avidity that mimics recurrence

Diagnostic Performance

• PET/CT alone has a sensitivity of 94% but specificity can be as low as 65% for pelvic recurrence 4, 5
• Combined PET/MRI has shown sensitivity of 94% and specificity of 94% for pelvic recurrence 5
• MRI has superior ability to evaluate the relationship of suspicious lesions to surrounding structures

Clinical Considerations

Timing of Evaluation

• The recent colonoscopy (2/13/2025) showed only benign findings
• The PET/CT finding is recent (7/12/2025) and requires prompt evaluation
• Early detection of recurrence significantly impacts treatment options and outcomes

Common Pitfalls to Avoid

• Relying solely on PET/CT findings: False positives are common in post-treatment settings 2
• Delaying evaluation: Early detection of recurrence improves surgical options and outcomes
• Proceeding directly to treatment without tissue diagnosis: Confirmation of recurrence is essential before initiating therapy

Summary

For a patient with history of rectal cancer and equivocal PET/CT findings in the anorectal region, pelvic MRI is the next appropriate step to better characterize the abnormality and distinguish between post-treatment changes and true recurrence. This approach aligns with current guidelines and offers the best diagnostic accuracy for local recurrence evaluation.