DOI: 10.1097/MNM.0b013e32835afb67
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PMID: 23111381
Issn Print: 0143-3636
Publication Date: 2013/02/01
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Excerpt
We read with considerable interest the article by Soussan et al. 1 recently published in this journal. The authors of the article had conducted a retrospective study to determine whether early acquisition of 18F-DOPA PET/CT could improve the detection of medullary thyroid carcinoma (MTC) lesions. Fifteen dual-phase 18F-DOPA PET/CT acquisitions performed on 14 patients with MTC, which were positive on at least one of the two phases, were analyzed. The median time of early and delayed acquisitions was 15 and 94 min, respectively. More lesions or more intense 18F-DOPA uptake was found in 14 true-positive examinations in the early phase compared with the delayed phase. The authors concluded that early 18F-DOPA PET/CT acquisition is more appropriate for the detection of MTC lesions compared with acquisition at 60 min or later 1.
We appreciate the innovative and original contribution of the authors. Nevertheless, some limitations of the study could hinder the drawing of final conclusions on this topic.
As recognized by the authors themselves, the main limitation is the lack of a standardized 18F-DOPA PET/CT acquisition protocol. The early acquisition time varied from 3 to 30 min, whereas the delayed acquisition time varied from 70 to 150 min. Further, in nine cases (60%) the delayed acquisition time was longer than 60–90 min, which is the protocol usually followed for 18F-DOPA PET/CT 2,3.
To provide further data on the value of early 18F-DOPA PET/CT, we report the findings of a prospective study conducted using a standardized dual-phase 18F-DOPA PET/CT protocol. Fifteen patients (six men/nine women; mean age 58.7 years; range: 41–85) with suspected MTC recurrence after thyroidectomy based on increased calcitonin levels were prospectively enrolled from November 2010 to October 2011 in our center. PET/CT images were acquired 15 min (early phase) and 60 min (delayed phase) after intravenous injection of 4 MBq/kg of 18F-DOPA. No carbidopa pretreatment was performed, but a fasting state before 18F-DOPA injection was required. Images were acquired using a Gemini PET/CT System (Philips Medical System, Best, the Netherlands). Early images included the neck and the thorax in all cases and, if clinically indicated, the abdomen as well (eight cases); whole-body PET/CT was performed in the delayed phase. Two experienced nuclear medicine physicians independently evaluated early and delayed images and, blinded to clinical data, reached a consensus. Both visual and semiquantitative analyses (using the SUVmax) were performed. Any focal accumulation of 18F-DOPA outside the normal distribution or higher than the surrounding physiological uptake was considered an abnormal finding. Studies were classified as positive if at least one abnormal focus was found. Washout rate (%) was calculated as 100×(delayed SUVmax−early SUVmax)/early SUVmax. As increased calcitonin levels are related to residual and/or recurrent disease, any 18F-DOPA PET/CT-negative result was interpreted as false negative 3. The number and sites of abnormal foci detected by both phases were also recorded. The results of 18F-DOPA PET/CT on both phases were compared on a per-patient and per-lesion basis. Histology, other imaging studies, or follow-up served as the reference standard. We used McNemar’s test to compare the results of early and delayed 18F-DOPA PET/CT on a per-patient and per-lesion basis. A t-test was used to compare mean SUVmax values in MTC lesions between early and delayed PET/CT acquisition. A P-value less than or equal to 0.05 was considered statistically significant.
The main findings of our prospective study are reported in Table 1. Of 15 MTC patients evaluated, four were negative and 10 were positive on both 18F-DOPA PET/CT phases; one patient (patient 10) was positive only in the early phase.
