OtherClinical Investigations

An Intrapatient Comparison of 99mTc-EDDA/HYNIC-TOC with 111In-DTPA-Octreotide for Diagnosis of Somatostatin Receptor-Expressing Tumors

Michael Gabriel, Clemens Decristoforo, Eveline Donnemiller, Hanno Ulmer, Christine Watfah Rychlinski, Stephen J. Mather and Roy Moncayo
Journal of Nuclear Medicine May 2003, 44 (5) 708-716;

Abstract

The aim of this study was to compare the imaging abilities of the recently developed somatostatin analog, 99mTc-hydrazinonicotinyl-Tyr3-octreotide (99mTc-HYNIC-TOC [99mTc-TOC]), with 111In-diethylenediaminepentaacetic acid-d-Phe1-octreotide (111In-OCT [Octreoscan]) in patients undergoing routine somatostatin receptor (SSTR) scintigraphy. Methods: Forty-one patients (20 men, 21 women; age range, 29–75 y; mean age, 56.7 y) with either histologically proven or biologically and clinically suspected endocrine tumors were enrolled in the study. Four groups were distinguished: (a) patients being evaluated for the detection and localization of neuroendocrine tumors (n = 6), (b) tumor staging (n = 19), (c) patients being investigated to determine the SSTR status of tumor lesions (n = 11), and (d) patient follow-up studies (n = 5). Each patient received a mean activity of 150 MBq 111In-OCT and 350–400 MBq 99mTc-TOC. Scintigraphy with 99mTc-TOC was performed 4 h after injection and scintigraphy with 111In-OCT was performed 4 and 24 h after injection. SPECT studies of areas of interest were performed 4 h after injection for both tracers as well as at 24 h after injection for 111In-OCT. The time interval between the studies using each tracer ranged from 2 to 22 d (mean interval, 9.3 d). Results: 111In-OCT and 99mTc-TOC showed an equivalent scan result in 32 patients (78%), 9 cases showed discrepancies (22%), false-negative results with 111In-OCT were seen in 6 cases (14.6%), whereas 99mTc-TOC was false-positive in 2 cases (4.9%). 111In-OCT was true-negative in both cases. The false-positive findings of the 99mTc-TOC studies were caused by nonspecific uptake in the bowel. In 1 case, 99mTc-TOC correctly identified a metastasis in the lumbar spine but both scan results were false-positive because of an inflammatory process. In 21 patients with SSTR-expressing tumors, the semiquantitative region-of-interest analysis showed that 99mTc-TOC achieved higher tumor-to-normal tissue ratios than 111In-OCT. Conclusion: This study revealed a higher sensitivity of 99mTc-TOC as compared with 111In-OCT as an imaging agent for the localization of SSTR-expressing tumors. To avoid false-positive findings with 99mTc-OCT due to nonspecific tracer accumulation, additional scanning at 1–2 h after injection should be done.

Many human tumors are known to express somatostatin receptors (SSTRs) with varying intensity (1–8). Scintigraphy with the radiolabeled somatostatin analog 111In-diethylenediaminepentaacetic acid-d-Phe1-octreotide (111In-OCT [Octreoscan; Mallinckrodt Medical, Petten, The Netherlands]) has gained acceptance as a diagnostic procedure for demonstrating neuroendocrine and other SSTR-positive tumors (9–13). However, this radiopharmaceutical has several drawbacks that are related to the use of 111In because this radiolabel has limited availability, high costs, and a medium γ-energy leading to suboptimal image resolution and relatively high radiation burden for the patient. We have recently described the development of a 99mTc-labeled somatostatin analog, based on Tyr3-octreotide (TOC) and hydrazinonicotinic acid (HYNIC) as complexing ligand for 99mTc (14). HYNIC-TOC can be labeled with 99mTc using ethylenediamine N,N′-diacetic acid (EDDA) as coligand, resulting in a hydrophilic and stable complex, 99mTc-EDDA/HYNIC-TOC (99mTc-TOC). In the initial study, the imaging characteristics of 99mTc-TOC were compared with those of 111In-labeled octreotide analogs in 10 tumor patients (15). The pharmacokinetic properties of 99mTc-TOC were found to be better than those of 111In-OCT. Higher target-to-nontarget ratios and higher absolute tumor uptake values were observed for 99mTc-TOC and the optimal acquisition time for 99mTc-TOC imaging was identified as 4 h after injection. The findings of this preliminary study indicated that 99mTc-TOC is a promising candidate to replace 111In-OCT in diagnostic nuclear medicine. Besides these advantages, generator availability and improved image quality of 99mTc over 111In make it a promising substance.

Clinical protocols for SSTR scintigraphy with 111In-OCT include imaging at 24 h after injection with additional imaging at 4 or 48 h after injection (13), whereas a 99mTc-labeled analog cannot be used for scanning at such late time points. On the other hand, a single 1-d protocol with early imaging at 4 h after injection would improve patient compliance and simplify this important diagnostic procedure. The aim of our study was to evaluate the 2 radiopharmaceuticals in a larger series of patients routinely undergoing SSTR scintigraphy using a single-acquisition, 1-d protocol for 99mTc-TOC as compared with the standard dual-time acquisition, 2-d protocol (4 and 24 h after injection) for 111In-OCT. Using a prospective cross-over study design, diagnostic efficacy and functional characteristics were compared in a study of sufficient size to be able to statistically compare the 2 radiopharmaceuticals.

MATERIALS AND METHODS

Patients

The clinical study was approved by the local ethical committee and all patients gave their informed consent before inclusion. Comparative scintigraphy with 99mTc-TOC and 111In-OCT was performed on 41 patients (20 men, 21 women; age range, 29–75 y; mean age ± SD, 56.7 ± 10.8 y), details of which are given in Table 1. For analysis, the patients who were enrolled in this study were divided into 4 study groups (13). The first group (Detection; n = 6) consisted of patients who underwent SSTR scintigraphy for the initial detection and localization of suspected neuroendocrine tumors and potential metastases. Patients with histologically proven endocrine malignancies were enrolled for staging in the second group (Staging; n = 19). In the third group (SSTR status; n = 11), the SSTR status of tumor lesions was evaluated for planning long-acting octreotide therapy. In the fourth group (Follow-up; n = 5), follow-up was performed on patients after treatment of malignant disease to exclude or to detect tumor recurrence. The time interval between both studies ranged from 2 to 22 d (mean interval ± SD, 9.3 ± 7.2 d). Patients were not treated with unlabeled somatostatin analogs within 1 mo of the imaging studies being performed.

View this table:
TABLE 1

Patient Characteristics

Radiopharmaceuticals

99mTc-TOC was prepared as described (14). Briefly, 20 μg HYNIC-TOC were heated with 10 mg EDDA, 20 mg Tricine, 15 μg stannous chloride dihydrate, and 1 GBq 99mTc-pertechnetate in 2 mL of 0.05 mol/L phosphate buffer, pH 6, at 100°C for 10 min. The solution was purified using a Sep-Pak mini cartridge (Waters, Milford, MA) eluted with 80% ethanol and diluted with 5 mL saline. The purified radiolabeled peptide was sterilized by filtration, and 350–400 MBq of the resulting solution were used for each patient study. Radiochemical purity was >95% using analytic techniques based on high-performance liquid chromatography and thin-layer chromatography (TLC) as described elsewhere (16).

111In-OCT was prepared from a commercially available kit (Octreoscan; Mallinckrodt Medical). The injected dose per patient was 150 MBq. Radiochemical purity exceeded 95% analyzed by TLC using the recommended instant TLC method with 0.1 mol/L citrate buffer, pH 5, as solvent.

Imaging

Whole-body imaging was performed with a double-head camera (Helix; Elscint, Haifa, Israel). For 99mTc whole-body studies, the camera was equipped with a low-energy, all-purpose, parallel-hole collimator, window setting 140 keV, width 10%. 111In images were obtained using a medium- or high- energy, parallel-hole collimator, window setting over both 111In peaks at 172 and 246 keV with a window width of 20%. For 99mTc tomographic acquisition, the same double-head gamma camera, as described above, was used. Acquisition parameters were 60 projections, 25 s per projection, 64 × 64 matrix, and zoom, 1. SPECT of 111In scans was performed on a single-head camera (ZL3000; Siemens Medical Systems, Erlangen, Germany) equipped with a medium-energy, parallel-hole collimator using 60 projections, 35 s per projection, and 64 × 64 matrix. 99mTc-TOC studies were only performed 4 h after injection; 111In-OCT scintigraphy was performed 4 and 24 h after injection. In 5 (patients 8, 11, 13, 30, and 38) of 41 cases, patients were late for their schedule and could be imaged only 2–3 h after injection. In 3 cases (patients 6, 10, and 15), planar imaging of the abdomen was performed 48 h after injection to clarify doubtful findings. SPECT imaging of areas of interest was performed 4 h after injection and, for 111In-OCT, SPECT was also performed 24 h after injection. Abdominal SPECT was performed on 40 patients 4 and 24 h after injection with 111In-OCT and 4 h after injection with 99mTc-TOC. In addition, SPECT of the chest was performed on 35 patients and SPECT of the head was performed on 14 patients 24 h after injection of 111In-OCT and 4 h after injection of 99mTc-TOC. In patient 38, who was monitored after resection of a pituitary adenoma, only 1 SPECT study of the cerebrum 24 h after injection of 111In-OCT and 4 h after injection of 99mTc-TOC was available. Because many SPECT acquisitions were performed, primarily for reasons of patient convenience, given the length of the procedure, complementary scintigraphic planar images were not acquired except delayed 111In images of the abdomen. For SPECT analysis, raw data were transferred to a HERMES system (Nuclear Diagnostics, London, U.K.) and filtered (Wiener filter) before reconstruction.

CT and MRI

Abdominal, chest, and head CT scanning (HiSpeed Advantage CT scanner; General Electric Medical Systems, Milwaukee, WI) was performed with 5-mm contiguous sections using a 512 × 512 matrix, before and after rapid intravenous infusion of contrast medium. All MRI examinations were performed on a 1.5-T whole-body scanner (Magneton Vision; Siemens) by using a phased-array surface coil. T1- and T2-weighted spin-echo images were obtained with and without fat suppression.

Image Analysis

Evaluation of each study was performed by independent visual interpretation by 2 nuclear medicine physicians who were experienced in the interpretation of 111In-OCT studies. The statistical design included a randomization of the sequence of the studies. Readers of the scans were unaware of the underlying pathology and the results of the standard staging procedures. Corresponding studies were compared for the final analysis lesion by lesion and ruled as matching or mismatching. Any focal tracer accumulation exceeding normal regional tracer uptake was rated as a pathologic finding (tumor uptake). Linear, nonfocal limited intestinal uptake was rated as nonspecific, nonpathologic uptake. In relevant areas, SPECT images were available to the viewer. All data were analyzed on a HERMES system using a region-of-interest (ROI) technique for semiquantitative analysis of major organ and tumor uptake. ROIs were drawn over the whole body, sites of tumor uptake, kidney, liver, spleen, heart, and a right thigh area (as an ROI for muscle) on both the anterior and the posterior whole-body views of the 111In and 99mTc images. For evaluation of kidney uptake, the left kidney was selected to avoid interference from liver superimposition. Parts of organs showing tumor infiltration or superimposition were excluded from evaluation of organ uptake. Total counts and counts/pixel in all ROIs were calculated. For the evaluation of uptake in malignant tissue, the lesion with the maximum uptake in counts/pixel was selected and tumor-to-organ ratios for this lesion were calculated from the respective counts/pixel values in normal organs. The projection with the highest tumor-to-organ ratios was selected (anterior or posterior) and used for a direct comparison between 111In-OCT and 99mTc-TOC in the same patient.

Because not all lesions could be histologically assessed and the different imaging procedures frequently showed discrepancies, the consensus was based on the sum of all conventional imaging procedures excluding scintigraphic data. For evaluation of previously unconfirmed scintigraphic abnormalities, 3 mo after the scans, repeated clinical examinations with CT or MRI of the abdomen, chest, and neck were performed. So far, all patients underwent imaging with either CT (n = 34), MRI (n = 3), or both modalities (n = 4). CT and MR scans were interpreted by experienced radiologists who were unaware of the scintigraphic result. A positive diagnosis was based on the specific appearance of malignant disease derived from neuroendocrine tumors as described elsewhere (17). If the clinical setting required further information, endoscopic procedures (e.g., coloscopy and second-look surgery) were also performed. Attention has been directed at still unproven findings of the scans.

Statistical Analysis

For statistical analysis the tumor-to-organ ratios were expressed as medians and ranges. Paired t tests and Wilcoxon tests were used to determine statistically significant differences in the semiquantitative analyses of the groups, as appropriate. The McNemar test of correlated properties was used to statistically compare the scintigraphic results of 99mTc-TOC and 111In-OCT. Analysis was done on a lesion basis and on a patient basis. Two-sided P values < 0.05 were considered significant. Cohen’s κ with confidence intervals of 95% was calculated to show the degree of association between the 2 techniques. A randomization procedure was used for determination of the order of the studies being evaluated.

RESULTS

Analysis per lesion from scintigraphy of all 41 patients studied are summarized in Table 2. Results per patient are reported in Table 3. True-positive results were obtained in 27 patients with 99mTc-TOC (65.9%) and in 21 patients with 111In-OCT (51.2%). In 11 cases (26.8%) scintigraphy was negative with 99mTc-TOC and in 19 cases it was negative (46.4%) with 111In-OCT. Of the 11 negative scintigraphic studies using 99mTc-TOC in 9 patients (21.9%), pathologic findings were detected on the basis of conventional procedures, whereas 111In-OCT was false-negative in 15 cases (36.6%). In 3 patients (7.3%) a false-positive scan result was obtained with 99mTc-TOC, whereas 111In-OCT was false-positive in only 1 case. Overall, 99mTc-TOC and 111In-OCT scintigraphy produced total agreement in 32 cases (78.1%). In 11 patients (26.8%) no findings suggestive of abnormal pathology were detected in both studies. This group included 5 patients (12.2%) who were investigated for localization of a suspected neuroendocrine tumor, in 1 patient (2.4%) tumor staging was performed, SSTR status was undertaken in 3 patients (7.3%), and in 2 patients (4.9%) a suspected recurrence was excluded by SSTR scintigraphy after successful therapy. In the remaining 30 patients (73.2%), at least 1 scintigraphic study (99mTc-TOC or 111In-OCT) revealed scintigraphic abnormalities. Overall, 105 hot spots were detected with 99mTc-TOC, whereas 111In-OCT found 91. Site-related findings are listed in Figure 1.

FIGURE 1.
FIGURE 1.

Number of abnormal findings revealed by 99mTc-TOC and 111In-OCT in 31 patients.

View this table:
TABLE 2

Scintigraphic Results of 99mTc-TOC and 111In-OCT: Analysis per Lesion

View this table:
TABLE 3

Scintigraphic Results of 99mTc-TOC and 111In-OCT: Analysis per Patient

In 21 patients (51.2%) true-positive findings were observed with both tracers, leading to an equivalent scan result (Fig. 2). Of these 21 patients 12 (29.3%) had been referred for tumor staging, 7 (17.1%) were under investigation to determine the SSTR status, and follow-up was being performed in 2 patients. There was 1 false-positive finding in both studies (patient 18). This patient (underwent surgery for a medullary thyroid carcinoma) presented with an elevated serum calcitonin level. In both studies enhanced tracer accumulation was shown in the right hip, so that a bone metastasis could not be excluded. Other radiologic imaging modalities, including CT and MRI, confirmed arthritis of the hip. In the same patient an SSTR-expressing metastasis in the upper lumbar spine was poorly depicted on 99mTc-TOC scintigraphy but was confirmed by MRI. This focus was not visualized with 111In-OCT. However, this case was classified as false-positive for both studies in Table 3. Discrepancies between the 2 studies were observed in a further 8 patients (19.5%; patients 6, 14, 17, 25, 27, 29, 34, and 35). A suspicious finding in the upper part of the abdomen was detected with the 99mTc-TOC scan in patient 29, who presented with a slightly elevated serum gastrin level and clinical signs of a gastrinoma. This abdominal focus could not be delineated on the 111In-OCT scan. This patient also suffered from a high-grade non-Hodgkin’s lymphoma of the small bowel but was in complete remission after 5 cycles of cyclophosphamide, hydroxydaunomycin, oncovin, and prednisone at the time of the scans. 18F-FDG PET, CT, and endoscopy were negative and several serum gastrin assays did not show any significant change during 9 mo of follow-up. This 99mTc-TOC study was, therefore, considered false-positive. Five patients (12.2%) who were being evaluated for tumor staging showed discrepant scan results (patients 6, 17, 25, 27, and 35). Patient 17 had a carcinoid of the papilla of Vater and 2 metastases in the liver that were positive on both studies. The 99mTc-TOC scan detected an additional retroperitoneal lymph node metastasis that was confirmed by CT. In patient 27, 111In-OCT was negative for a solitary liver metastasis after the surgical removal of a small bowel carcinoid, whereas 99mTc-TOC was positive. This result was confirmed by MRI.

FIGURE 2.
FIGURE 2.

A 51-y-old man (patient 23) with multiple liver metastases of small bowel carcinoid. Matching tracer accumulations are shown by 99mTc-TOC and 111In-OCT on anterior views (99mTc-TOC 4 h after injection [A] and 111In-OCT 24 h after injection [B]) as well as on SPECT (99mTc-TOC 4 h after injection [C] and 111In-OCT 24 h after injection [D]).

In the remaining 3 cases of this group, further clinical management was influenced by positive findings on the 99mTc-TOC scan. In patient 6, with hepatic metastases due to a small bowel carcinoid, 99mTc-TOC scintigraphy additionally showed 2 paraaortal lymph node metastases in the abdomen that were confirmed by CT 3 mo later. In patient 35, with a neuroendocrine tumor of the tail of the pancreas, a solitary metastasis of the liver was distinctly delineated with 111In-OCT as well as with 99mTc-TOC. However, a small residual tumor in projection to the cranial pole of the left kidney was visualized only by 99mTc-TOC. A second-look procedure confirmed this finding. Both patients were spared from extensive surgical intervention to remove liver metastases.

In patient 25, who suffered from a neuroendocrine tumor of the pancreas with multiple liver metastases, 99mTc-TOC scintigraphy showed 3 small metastases in the right liver lobe and 2 small metastases in the left liver lobe, all of which were in the range of 1.0 cm. Although the 111In-OCT scan showed a patchy pattern of uptake in the liver, the metastases could not be delineated. All findings were confirmed by CT. This patient was subsequently treated with 90Y-dodecanetetraacetic acid (DOTA)-TOC leading to partial remission.

The treatment regime was also adapted in patient 14, who was referred to our department to determine the SSTR status of a solitary pulmonary metastasis of a papillary carcinoma of the thyroid gland. Radioiodine uptake was negative. CT-controlled biopsy of the lung lesion confirmed the thyroid carcinoma origin of the metastasis. On planar whole-body views as well as on SPECT, 99mTc-TOC showed an enhanced tracer accumulation in this lesion, whereas the 111In-OCT scan was negative (Fig. 3). The size of this metastasis was 1.3 × 1.4 cm. Because the patient refused further surgical intervention we started a treatment regime using a long-acting SST analog. Follow-up during 8 mo showed a slight increase in serum thyroglobulin but the lung metastasis did not show an increase in size.

FIGURE 3.
FIGURE 3.

A 66-y-old woman (patient 14) with papillary thyroid cancer. SSTR scintigraphy with 99mTc-TOC (A) shows clear uptake in solitary metastasis in right lung. 111In-OCT scan was negative 4 and 24 h after injection (B).

Another discrepant scan result was found in patient 34, who was investigated to exclude recurrence of a secretory inactive neuroendocrine tumor of the small bowel. A solitary focus was shown in projection to the small bowel on the right side with 99mTc-TOC, whereas 111In-OCT was negative (Fig. 4). The 99mTc-TOC finding could not be confirmed by repeated CT and an endoscopic procedure was also negative. This 99mTc-TOC study was, therefore, considered false-positive.

FIGURE 4.
FIGURE 4.

A 69-y-old woman (patient 34) monitored 2 y after surgical treatment of small bowel carcinoid. On 99mTc-TOC scan, focal uptake (A), highly suspicious for tumor recurrence, was observed in abdomen (arrow). 111In-OCT images were negative at 4 h (B) and 24 h (C) after injection. Further investigations could not confirm this abnormal finding; 99mTc-TOC scan result was considered false.

The delayed planar 111In images of the abdomen did not change the scan results in each of these 3 cases (patients 6, 10, and 15).

Figure 5 shows a summary of the semiquantitative ROI analyses from 21 patients with SSTR-expressing tumors and a matching 111In-OCT/99mTc-TOC scan result. In the images 4 h after injection, tumor-to-organ ratios obtained with 99mTc-TOC were higher than those obtained with 111In-OCT in all organs except the spleen. These differences were statistically significant (P < 0.001) for tumor-to-blood, tumor-to-liver, and tumor-to-kidney ratios. When 24-h 111In-OCT images were compared with 4-h 99mTc-TOC images, only the tumor-to-kidney ratios were statistically significant, with 99mTc-TOC again showing superior ratios.

FIGURE 5.
FIGURE 5.

Statistical analysis of tumor-to-organ ratios in matching studies with pathologic uptake (n = 21).

From the clinical point of view, 99mTc-TOC showed statistically significant better results in terms of detection and localization of pathologic sites (P < 0.001) using the McNemar test, and Cohen’s κ of 0.68 (0.52–0.83) revealed a moderate association between both techniques. An analysis per patient comparing the scan results emphasizes the improved diagnostic efficacy of 99mTc-TOC with a P value of 0.0078 and Cohen’s κ of 0.6 (0.37–0.83). The correlation coefficient was based on 41 observations.

DISCUSSION

Since the introduction of SSTR imaging in 1989 (18), scintigraphy with 111In-OCT has become a reliable, noninvasive method for diagnosing different SSTR-expressing tumors with several clinical implications (9,19–27). More recently, researchers have tried to develop 99mTc-labeled somatostatin analogs to improve availability and image quality of SSTR scintigraphy as well as to reduce the radiation burden to the patient (28,29). Only 1 analog, 99mTc-depreotide (P829 [NeoTECT; Amersham Health, Amersham, U.K.]), has so far been commercially introduced into clinical practice (30). However, this compound does not have imaging properties similar to those of 111In-OCT. Although it has proven sufficient diagnostic efficacy in the evaluation of thoracic nodules (31,32), the detection rate for 99mTc-P829 scintigraphy in patients with endocrine tumors was lower than that of 111In-OCT scintigraphy (33). 99mTc-TOC, based on an octreotide derivative, is a new compound that performed somewhat better than 111In-OCT in a pilot study done on 10 patients (15). In this study with 41 patients, these initial findings were confirmed not only by qualitative but also by semiquantitative image analysis. Altogether, no side effects were observed after intravenous injection (n = 51). Unlike NeoTECT, only minimal hepatobiliary clearance of 99mTc-TOC was observed. Although the kidneys are the predominant excretion organs for both 99mTc-TOC and 111In-OCT, significantly higher tumor-to-kidney ratios were obtained on the 4-h 99mTc-TOC images as compared with the 4- or 24-h 111In-OCT images. These results indicate that the advantage of the longer half-life of 111In, with potential imaging up to 48 h after injection, were compensated by the higher spatial resolution of 99mTc, better counting statistics, and the higher tumor uptake of 99mTc-TOC.

In this series of patients, the clinical information obtained with 99mTc-TOC using a 1-d, single-acquisition, 4-h after-injection protocol was at least comparable to the standard 2-d protocol of 111In-OCT and no advantage was gained from delayed imaging with the longer-lived radionuclide. However, in 9 of 41 patients, differences were observed between the 2 radiopharmaceuticals. In all of these cases, additional hot spots were detected on the 99mTc-TOC scans. This higher sensitivity was statistically significant according to the McNemar test of correlated properties. However, the use of 99mTc-TOC also resulted in a greater number of false-positive results. Despite a rapid background clearance and low hepatobiliary excretion, some nonspecific accumulation in the bowel can lead to false-positive interpretations with 99mTc-TOC when a single-acquisition protocol is used. This phenomenon was observed in 2 patients (patients 29 and 34). If the area of clinical interest is in the abdomen (e.g., staging of neuroendocrine gastroenteropancreatic tumors), additional imaging 1–2 h after injection could avoid such pitfalls, because the favorable pharmacokinetics allows accurate imaging at such early time points (15). Adequate bowel preparation, which can be advantageous for investigations with 111In-OCT, cannot be used to improve accuracy of the 99mTc studies considering the short time range available between tracer application and scanning.

Although 111In-OCT scintigraphy, especially with SPECT, can often provide additional valuable staging information when compared with other imaging modalities (34,35,36), some sites of metastases will be missed (37). In our study, the use of 111In-OCT resulted in 6 false-negative cases in which 99mTc-TOC was true-positive. In some of these cases, this higher sensitivity of 99mTc-TOC even resulted in a change of patient management. When taking into account the high number of false-negative results with 111In-OCT, we feel that the potential advantage of 111In-OCT with acquisition at later time points was compensated by the advantage of improved pharmcokinetics allowing earlier imaging with 99mTc-TOC.

Receptor-specific accumulation in the thyroid gland regularly resulted in a clearer delineation of this organ on the 99mTc-TOC study than on the 111In-OCT scan, even in the absence of thyroid disease. Uptake of free technetium-pertechnetate could be excluded because the amount of free pertechnetate present (as determined by HPLC) was <1% and no uptake was shown in stomach or salivary glands. This suggests that 99mTc-TOC may show advantages over 111In-OCT not only in the visualization of smaller lesions but also of lesions with a lower density of SSTRs, such as in thyroid tumors (38). In our series we studied 8 patients with differentiated nonmedullary thyroid cancer and 3 patients with medullary thyroid cancer. In these 11 patients, 99mTc-TOC and 111In-OCT showed equivalent scan results in 10 cases. However, in 1 patient (patient 14), with a papillary thyroid carcinoma showing a solitary radioiodine-negative metastasis in the right lung, tracer uptake was clearly visualized with 99mTc-TOC, whereas 111In-OCT was negative (Fig. 3), which formed the basis for treatment with an unlabeled SST analog.

This patient and another patient (patient 25), with multiple SSTR-expressing liver metastases on 99mTc-TOC, negative on 111In-OCT, who was successfully treated with 90Y-DOTA-TOC, demonstrate that a greater number of patients might benefit from an improved in vivo detection of SSTR-expressing tumor tissue. Patients who exhibit a positive scan result are, in principle, accessible to treatment options with either unlabeled SST analogs or analogs labeled with β−-emitting isotopes. Improved diagnostic accuracy might therefore also help to encourage new clinical applications for SSTR scintigraphy. The lower radiation dose received with 99mTc favors the use of repeated investigations—for example, for therapeutic monitoring or applications in children. The physical decay characteristics of 99mTc might also improve the use of surgical probes for detection of involved lymph nodes (39). In this series of patients several different kinds of tumor types and clinical indications were included. Although this study proves that 99mTc-TOC could be an alternative to the clinical use of 111In-OCT, further phase III trials are still necessary to assess the value of 99mTc-TOC in clinical nuclear medicine. Further improvements in SSTR scintigraphy are ongoing. Other 99mTc-labeled analogs have been developed using tetraamine-functionalized Tyr3-octreotate. Such analogs are expected to show even higher affinity for SSTR subtype 2 (7). Promising preclinical results have already demonstrated favorable pharmacokinetics and very high tumor uptake (40).

CONCLUSION

This study shows that the use of a single-acquisition protocol with 99mTc-TOC can provide improved clinical information when compared with a 2-d 111In-OCT protocol. 99mTc-TOC combines the advantages of favorable pharmacokinetics, higher spatial resolution, lower radiation dose, and improved availability of 99mTc with a simplified imaging procedure and could replace 111In-OCT for routine SSTR scintigraphy. Further clinical studies are required to assess the usefulness of 99mTc-TOC in more specific clinical settings.

Footnotes

  • Received Sep. 23, 2002; revision accepted Jan. 13, 2003.

    For correspondence or reprints contact: Clemens Decristoforo, PhD, Department of Nuclear Medicine, University of Innsbruck, Anichstrasse 35, A-6020 Innsbruck, Austria.

    E-mail: clemens.decristoforo{at}uibk.ac.at

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