When to Order PET Scan for a Pelvic Mass
FDG-PET/CT should be ordered for a pelvic mass primarily in two specific scenarios: (1) when a patient has a known history of malignancy and presents with a new adnexal mass to identify other sites of disease, or (2) when a pelvic mass is highly suspicious for malignancy and has already been characterized by ultrasound and CT/MRI, to aid in staging and detection of distant metastases. 1
Primary Indications for PET/CT
Known History of Malignancy
- FDG-PET/CT plays a role specifically in women with a known history of malignancy who present for evaluation of an adnexal mass to identify other sites of disease. 1
- This is the clearest guideline-supported indication from the ACR Appropriateness Criteria 1
Highly Suspicious Malignant Mass - For Staging Only
- When ultrasound has already confirmed a highly suspicious malignant adnexal mass, CT abdomen and pelvis with IV contrast is the primary modality of choice for staging 1
- PET/CT is not the first-line staging modality according to ACR guidelines, though research suggests it has high diagnostic value 2
- In one prospective study of 97 patients with Risk of Malignancy Index >150, PET/CT demonstrated 100% sensitivity and 92.5% specificity for diagnosing malignant pelvic tumors 2
When NOT to Order PET/CT
Initial Characterization of Pelvic Mass
- There is no indication for FDG-PET/CT in the initial evaluation of asymptomatic clinically suspected adnexal masses 1
- Transvaginal ultrasound with color Doppler is the essential first-line imaging modality for all suspected adnexal masses 3
- For indeterminate masses, MRI pelvis with and without IV contrast is the next appropriate step, not PET/CT 1, 3
Follow-up of Benign or Indeterminate Masses
- There is no indication for FDG-PET/CT in the follow-up of benign adnexal masses in premenopausal women 1
- There is no indication for FDG-PET/CT in the follow-up of indeterminate adnexal masses 1
Algorithmic Approach to Imaging Selection
Step 1: Initial Evaluation
- Perform transvaginal ultrasound with color Doppler for all suspected pelvic masses 1, 3
- Assess for simple cyst (benign in 100% of premenopausal women), specific benign features (endometrioma, dermoid, hydrosalpinx), or suspicious malignant features 1
Step 2: If Mass is Indeterminate on Ultrasound
- Order MRI pelvis with and without IV contrast for problem-solving 1
- Do NOT order PET/CT at this stage 1
Step 3: If Mass is Highly Suspicious for Malignancy
- Order CT abdomen and pelvis with IV contrast as the primary staging modality 1
- Consider PET/CT only if patient has known history of other malignancy to identify additional disease sites 1
Step 4: Recurrent Disease Surveillance
- PET/MRI or PET/CT can be used for restaging in patients with suspected recurrence of gynecologic malignancies, with PET/MRI showing 98% sensitivity and 94% diagnostic accuracy 4
Critical Pitfalls to Avoid
Physiologic Uptake Mimics
- Normal physiologic FDG uptake occurs in bowel loops, blood vessels, ureters, and urinary bladder 5
- In premenopausal patients, endometrial activity changes cyclically and ovarian uptake may be functional 5
- These normal variants can lead to false-positive interpretations 5
Ordering Sequence Errors
- Never order PET/CT before completing ultrasound and appropriate cross-sectional imaging (CT or MRI) 1
- PET/CT is not a characterization tool for initial pelvic mass evaluation 1
- The ACR guidelines explicitly state CT is the modality of choice for staging highly suspicious masses, not PET/CT as first-line 1
Pregnancy Considerations
- FDG-PET/CT may only be considered in pregnant patients with known history of malignancy, and only if pregnancy is terminated 1
Special Populations
Postmenopausal Women
- Even in postmenopausal women with indeterminate masses, the recommended imaging progression is ultrasound → MRI, not PET/CT 1
- Simple cysts occur in 17-24% of postmenopausal women and are benign 1