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Clinical Investigations |
1 Department of Nuclear Medicine, University of Texas M.D. Anderson Cancer Center, Houston, Texas
2 Department of Surgical Oncology, University of Texas M.D. Anderson Cancer Center, Houston, Texas
3 Department of Biostatistics, University of Texas M.D. Anderson Cancer Center, Houston, Texas
4 Department of Imaging Physics, University of Texas M.D. Anderson Cancer Center, Houston, Texas
| ABSTRACT |
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Key Words: parathyroid adenoma SPECT/CT hyperparathyroidism parathyroid scintigraphy radionuclide
| INTRODUCTION |
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Two recent case reports described the accurate location of ectopic parathyroid adenomas by fusing 99mTc-sestamibi SPECT and CT slices using fusion software (16,17). Kaczirek et al. have indicated the value of SPECT/CT acquisition using
-cameramounted anatomic x-ray tomography in 4 patients with ectopic parathyroid adenomas (18).
As SPECT/CT evolves, its applications and indications need to be evaluated clinically for more efficient and cost-effective use. To our knowledge, no studies have used a large number of patients to evaluate the clinical value of simultaneous SPECT and CT image acquisition using
-cameramounted anatomic x-ray tomography to locate parathyroid adenoma or hyperplasia. Our goal in this study was to evaluate the additive value of SPECT images to planar images and of SPECT/CT images to planar plus SPECT images. We hypothesized that fused SPECT/CT would provide additional information for diagnosing and locating parathyroid adenomas or hyperplasia.
| MATERIALS AND METHODS |
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Image Acquisition
The patients received 740925 MBq (2025 mCi) of 99mTc-sestamibi by intravenous injection. Anterior planar images of the neck and chest were obtained immediately after the injection and 60 min later, each for 10 min, using a high-resolution low-energy parallel-hole collimator and a large-field-of-view dual-detector
-camera with a mounted CT scanner (Hawkeye; General Electric Medical Systems). SPECT and CT images of the same area were obtained at 30 min after injection over a 360° arc, using 120 frames at 23 s per frame and 3° angles. The images were acquired into a 128 x 128 matrix and reconstructed using a 2-dimensional ordered-subset expectation maximization iterative technique (10 subsets and 2 iterations), both with and without CT-based attenuation correction and a Hanning 3-dimensional postfilter (cutoff frequency, 0.85 cycles/cm). The CT part was acquired at a slice step of 10 mm, a slice time of 14 s, a current of 2.5 mA, and a voltage of 140 kV. The SPECT acquisition took approximately 25 min, whereas the CT acquisition took approximately 10 min.
Image Interpretation
Two experienced nuclear medicine physicians interpreted the images. Each physician read the images independently while unaware of any clinical or radiologic information about the patients. The readers indicated whether the scan was negative or positive for locating the diseased parathyroid gland after reading the planar images alone, then the planar and SPECT images, and finally the planar and SPECT/CT images. Each reader had to respond to the following questions after SPECT and SPECT/CT interpretation: Did it change the diagnosis? Did it help in locating the gland? or Was it not helpful? Focal retention of tracer in abnormal parathyroid glands was visually compared between the early and the delayed images. Also evaluated were whether the thyroid gland had a normal or a multinodular appearance and whether the findings correlated with surgical and pathologic findings after surgical resection. We defined the identification rate as the percentage of diseased parathyroid glands, confirmed at surgery, to have been the glands identified at planar plus SPECT imaging, versus planar plus SPECT/CT imaging.
Biochemical Markers
The preoperative intact parathyroid hormone (PTH), free serum calcium (fCa), and total serum calcium (tCa) levels were also recorded to evaluate whether the scan findings and accuracy correlated with the level of biochemical markers. These levels were also recorded because their normalization indicated the adequacy and completeness of surgery.
Statistical Analysis
Interobserver agreement was evaluated using the McNemar test (P = 0.0 and
= 1.0 indicate excellent agreement). The correlation between the chemical markers of primary hyperparathyroidism and the scan findings was evaluated using a paired Student t test with a significance level of P < 0.05.
| RESULTS |
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-value was 0.39 with P = 0.00 for SPECT interpretation and 0.00 with P = 0.91 for SPECT/CT interpretation, indicating fair interobserver agreement for SPECT evaluation but good agreement for SPECT/CT evaluation.
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Thirty of the total removed glands were adenomas, 4 were hyperplastic, and 1 was a thyroid nodule. The delayed tracer retention pattern did not differ between the hyperplastic glands and the adenomas. Delayed tracer retention was noted in 3 of the hyperplastic glands. On the other hand, 17 of the adenomas showed retention, 12 showed washout, and 1 patient did not undergo delayed imaging. The scintigraphically unidentified second involved gland in 3 patients was hyperplastic in 1 patient and adenomatous in the other 2 patients. Thus, the pattern of tracer retention or washout cannot predict the pathology of the involved gland.
Mean PTH, tCa, and fCa levels were not significantly different between the patients with positive scan findings and the patients with negative scan findings (PTH = 186.7 and 123.5 pg/mL, respectively, P = 0.12; tCa = 10.3 and 10.5 mg/dL, respectively, P = 0.61; fCa = 1.33 and 2.0 mmol/L, respectively, P = 0.44). All patients who underwent surgery demonstrated a decrease in their chemical markers, with a mean intact PTH of 74 (reference range, 1065 pg/mL), a mean tCa of 9.2 (reference range, 8.410.2 mg/dL), and a mean fCa of 1.22 (reference range, 1.131.32 mmol/L). Only 1 patient demonstrated normalization of his intact PTH level but persistently elevated tCa and fCa levels postoperatively. These findings indicated the adequacy and completeness of the operations in all patients.
| DISCUSSION |
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Of note was the difference in interobserver agreement about the usefulness of conventional SPECT in addition to planar scanning. Reader 1 found SPECT helpful in identifying diseased glands for 9 patients, whereas reader 2 found it helpful for 20 patients. The difference occurred because reader 1 considered focal retention of tracer on delayed planar images to be consistent with parathyroid adenoma whereas reader 2 depended frequently on SPECT images to identify pathologic processes in or posterior to the thyroid gland. However, both readers agreed that fusing CT images with SPECT images provided little additional information on parathyroid glands that were in the correct anatomic location.
We could not evaluate the sensitivity and specificity of SPECT/CT because 16 patients did not undergo surgery (10 because of negative scan findings). Thus, the actual number of false-negatives and true-negatives is unknown. Therefore, we used the term identification rate instead of the term sensitivity to evaluate the accuracy of our imaging method. Overall, we had good identification rates of 89% in all patients and 100% in patients with multinodular thyroid gland. These are higher than the previously reported decreased sensitivity of 71% for patients with multinodular goiter, compared with 91% for patients with normal thyroid glands (13). This difference might be due to the small number of patients with multinodular thyroid glands in our study population.
We did not observe a statistically significant relationship between the chemical markers of hyperparathyroidism and the identification rate of the involved parathyroid gland as previously suggested by Staudenherz et al. (13) and Parikshak et al. (19). Our findings agree with those of Clark et al., who previously reported a lack of correlation between biochemical markers or thyroid gland pathology and imaging sensitivity (20). Similarly, there was no particular difference in the pattern of tracer retention or washout between hyperplastic glands and adenomatous glands with comparable identification rates. Our finding agrees with that of Wakamatsu et al., who previously demonstrated comparable sensitivities of 36.7% and 39.7% for the identification of hyperplastic and adenomatous glands, respectively (2).
In our study, the involved parathyroid gland (superior or inferior) was misidentified in 8 patients. Misidentification can result in more extensive intraoperative dissection to find the involved gland, thus leading to a larger surgical scar and longer operative time and defeating the purpose of accurately locating the involved gland for minimally invasive surgery. However, in all cases the involved gland was on the same side as the scintigraphically identified abnormal gland. This is believed to be due to the growth pattern of superior parathyroid adenomas, which tend to enlarge inferiorly behind the thyroid gland, thus appearing to be an inferior thyroid adenoma. This has to be taken into consideration when interpreting parathyroid scans. Clark et al. have observed a similar problem and have suggested the addition of lateral images to the standard imaging protocol (10). Additionally, the use of an intraoperative
-probe is highly recommended for better locating the glands in such cases (21). Also of note is the difficulty we faced in identifying involvement of a second parathyroid gland in 3 patients. This difficulty supports the importance of combining preoperative imaging with intraoperative PTH level measurement to identify pathologic residual parathyroid gland after removal of the scintigraphically identified parathyroid adenoma or hyperplasia (22).
Although this study was retrospective, all patients were imaged consistently, increasing our confidence in the results. The lack of operative results for patients with negative scan findings hinders the complete evaluation of false-negative SPECT or SPECT/CT findings. However, considering the increasing trust of surgeons in parathyroid nuclear scans and the adoption of minimally invasive surgery, it would be difficult to conduct a prospective study in which patients with negative scan results still undergo surgery.
Finally, we recommend that SPECT/CT be reserved for ectopic parathyroid adenomas and that planar and conventional SPECT be used for normally located parathyroid adenomas or hyperplasia.
| CONCLUSION |
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| FOOTNOTES |
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For correspondence or reprints contact: Isis W. Gayed, MD, Department of Nuclear Medicine, Unit 83, University of Texas M.D. Anderson Cancer Center, 1515 Holcombe Blvd., Houston, TX 77030.
E-mail: igayed{at}di.mdacc.tmc.edu
| REFERENCES |
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