SPECT/CT is a powerful diagnostic tool that has improved interpretation of classic radioiodine scintigraphy, and current management protocols and guidelines in thyroid cancer should be reassessed in light of this new technology, according to an article published in the May issue of the Journal of Nuclear Medicine.
“The synergistic combination of functional and anatomic information provided by SPECT/CT has been found to have many advantages over traditional planar imaging in different clinical settings,” wrote Anca M. Avram, MD, of University of Michigan Medical Center in Ann Arbor. “Optimal coregistration of tomographic volumes of data obtained by gamma cameras with inline CT, with the patient in the same bed position, allows precise anatomic localization of radioactivity foci. Additional benefits include CT-based attenuation correction and morphologic information from unenhanced CT with reduced milliampere-seconds and kilovoltage settings.”
Postoperative management of thyroid cancer patients has included radioiodine administration for years, explained Avram, with staging and risk stratification used to determine which patients should be referred for radioiodine therapy. Patients who would benefit from 131-I ablation are those with distant metastases, patients with a primary tumor larger than 4 cm and those with smaller tumors who have documented node metastases or another high-risk feature.
“Advantages of SPECT/CT applied either selectively or routinely to planar scintigraphy studies include accurate anatomic localization and characterization of radioiodine foci as benign (such as thyroid remnant tissue or physiologic activity in normal structures) or malignant in cervical nodal or distant metastases,” wrote Avram. She added that SPECT/CT can clarify interpretation of planar images when unusual radioiodine biodistributions are detected, thus reducing the number of false-positive diagnoses. SPECT/CT also has the advantage of CT-based attenuation correction, which can reveal more foci of pathologic activity compared to planar studies.
There are some disadvantages to SPECT/CT, however, which Avram outlined in the article. The hybrid modality’s imaging time is longer, potentially leading to more patient discomfort or claustrophobia on the SPECT/CT gantry. The additional radiation exposure from the CT component is a concern. Micrometastatic lesions also cannot be detected with SPECT/CT.
In the management of thyroid cancer, postoperative diagnostic radioiodine imaging helps to identify patients with unsuspected regional and distant metastases, according to Avram. It defines the target of 131-I therapy and is necessary for dosimetry calculations. Avram wrote she “considers that the patients’ risk stratification and staging should not be based solely on clinical and histopathologic criteria, but should include specific thyroid cancer imaging to evaluate for the presence of regional and distant metastases.”
Avram concluded by noting that the use of radioiodine SPECT/CT has led to a change in clinical management in a significant number of thyroid cancer patients—11 percent to 58 percent of patients in the various studies cited in the article. Management decisions changed based on information obtained with preablation scintigraphy included:
- Decisions on whether or not to proceed with 131-I therapy;
- Alterations to the prescribed 131-I activity for thyroid cancer patients;
- Decisions to refer patients for surgical debulking before 131-I therapy; and
- Selecting patient for external-beam radiation therapy.