BACKGROUND: The lack of anatomical details in standard ¹³¹Iodine whole body scanning (¹³¹I WBS) interferes with the proper localization of metastatic differentiated thyroid carcinoma (DTC) lesions. In addition, nearby or overlapping variable physiological distribution of ¹³¹I may affect the specificity of ¹³¹I uptake, giving indeterminate results. The aim of this study was to demonstrate the clinical usefulness of simultaneous co-registration of ^99mTc MDP bone scanning as an anatomical landmark with ¹³¹I scanning in the evaluation of metastatic DTC.
MATERIAL AND METHODS: Twenty-five patients (16 females and 9 males, mean age ± SD = 52 ± 13 years) with metastatic DTC (17 papillary, 8 follicular), were included. Whole body scanning using a 256 x 1024 matrix and an 8 cm/min scan rate were obtained 48 hours after oral administration of 185-370 MBq 131I and 2 hours after IV administration of 185-370 MBq ^99mTc MDP using a dual head gamma camera equipped with high energy parallel hole collimators. Occasionally, additional simultaneous co-registration of localised detailed images was also performed using a 256 x 256 matrix size. The two planar images were fused with optional fusion of SPECT images.
The data from standard ¹³¹I scanning and fused ¹³¹I/ ^99mTc-MDP scanning were separately assessed by two nuclear medicine physicians. Fusion images were considered to improve image interpretation in comparison with standard ¹³¹I scanning when they provided better localization of lesions.
RESULTS: All lesions in the present study were validated by radiological images and clinical follow up for at least 12 months. Forty-eight metastatic lesions were confirmed as follows: 2 in the skull, 10 in the neck, 20 in the thorax, 12 in the pelvic-abdominal region and 4 in the extremities. Standard ¹³¹I WBS showed 54 extra-thyroidal foci with 8 false positive lesions of which 2 were located in the scalp and 6 in the pelvic-abdominal region extra-skeleton (i.e. sensitivity 100%, specificity 86%). Out of the 48 validated lesions, 16 were indeterminately localized: 10 in the thorax (3 mediastinal nodal lesions, 5 vertebral lesions and 2 ribs) and 6 in the pelvic-abdominal region (2 upper sacral, 2 sacroiliac region and 2 ischial bone). Fusion images confirmed the precise localization of the pathological uptake in the validated 48 lesions (sensitivity 100%, specificity 100%). There were 2 (4%) indeterminate lesions in fused planar imaging that were clearly localized via fused SPECT images.
CONCLUSIONS: Fusion images using simultaneous co-registration of ¹³¹I and ^99mTc MDP scanning is a simple and feasible technique that improves the anatomically limited interpretation of scintigraphy using ¹³¹I alone in patients with metastatic differentiated thyroid carcinoma. The diagnostic advantage of this technique seems to be more apparent in the thoracic and pelvic- abdominal regions in contrast to the neck and extremities.