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Department of Radiology, Harbor UCLA Medical Center, Torrance, California; Department of Radiology, The University of Texas Medical Branch at Galveston, Galveston, Texas; Women's Imaging Center of Delaware, Newark, Delaware; Departments of Pathology, Radiology, and Surgery, Beth Israel Deaconess Medical Center, Boston, Massachusetts; and DuPont Pharmaceuticals Company, Wilmington, Delaware, and North Billerica, Massachusetts
| ABSTRACT |
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= 0.820.99). Overall institutional sensitivity and specificity for 99mTc-sestamibi breast imaging were 75.4% and 82.7%, respectively. In this population with a 40.1% disease prevalence, the positive predictive value was 74.5% and the negative predictive value was 83.4%. The negative predictive value was 94% in patients with a 40% or lower mammographic likelihood of breast cancer. Sensitivity was higher for palpable abnormalities; specificity was higher for nonpalpable abnormalities. Sensitivity was decreased for tumors <1 cm in largest dimension but appeared not to be affected by patient's age. Conclusion: As an adjunct to current procedures, 99mTc-sestamibi breast imaging may contribute to patient management decisions in selected populations, including women with dense breasts, mammographically indeterminate lesions >1 cm, and palpable abnormalities.
Key Words: scintimammography 99mTc-sestamibi breast cancer breast
| INTRODUCTION |
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99mTc-sestamibi is a radiopharmaceutical that was originally developed as a myocardial perfusion imaging agent. However, results from in vitro (46) and in vivo (710) preclinical studies have suggested that it may have potential as a tumor imaging agent. Several investigators have reported results of breast imaging with 99mTc-sestamibi that have generally supported this potential (1120). The objectives of our study were to extend these earlier results by determining in a multicenter trial the diagnostic accuracy of 99mTc-sestamibi in women with suspected breast cancer and to investigate factors influencing diagnostic accuracy.
| MATERIALS AND METHODS |
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Mammography
Mammography was performed using standard craniocaudal and mediolateral oblique views with additional views obtained as clinically indicated. Each participating site in the United States was accredited by the American College of Radiology Mammography Accreditation Program. The Canadian site had full accreditation in medical imaging by the Diagnostic Accreditation Program in British Columbia. Additionally, phantom images and representative mammograms from each site were evaluated by the mammography core center. Deficiencies noted by the core center were corrected before enrolling patients. Collection of mammography results was based on the American College of Radiology Breast Imaging Reporting and Data System (21). Although it is not a common practice, for this trial the mammographer at each site assigned a mammographic probability of malignancy to each lesion (palpable or nonpalpable) that was to be biopsied. The parenchymal patterns "heterogeneously dense" and "extremely dense" were defined to represent dense breasts, whereas "almost entirely fat" and "numerous vague densities" were defined to represent fatty breasts.
Scintigraphy
Planar imaging with high-resolution collimation was performed using the gamma cameras available in the investigators' departments. Subjects received a 740- to 1110-MBq (2030 mCi) bolus intravenous injection of 99mTc-sestamibi in the arm contralateral to the suspicious breast abnormality; subjects with bilateral abnormalities were injected in a dorsalis pedis vein. Five minutes after injection, a 10-min lateral view of the breast scheduled for biopsy was obtained with the subject positioned prone on an imaging table overlay (Bodfish Research and Design, Inc., Bodfish, CA) so that the breast being imaged was pendent (11). The subject was then repositioned to obtain a lateral view of the contralateral breast followed by a supine anterior view. Lateral views were repeated 1 h after injection.
Scintigraphic images were read at the sites by the investigators (institutional results). Because most investigators in the trial had minimal experience interpreting 99mTc-sestamibi breast images, a set of 18 images, scored on a 5-point scale (0 = normal; 1 = equivocal; 2 = focal uptake, low intensity; 3 = focal uptake, medium intensity; and 4 = focal uptake, high intensity) but without histopathologic correlation, was provided for image scoring training.
For the calculation of diagnostic statistics, scores of 24 were considered positive, scores of 0 were considered negative, and scores of 1 were considered uninterpretable and were not analyzed. Overall, institutional readers scored 6.4% of biopsied scintigraphic abnormalities as 1. In addition to the image interpretation by the investigators, two groups of three independent nuclear medicine physicians who had no knowledge of the subjects' clinical history, institutional scintigraphic results, or other test results (blinded readers) interpreted the images. Thus, the blinded readers were not told whether they were reading images from subjects with nonpalpable abnormalities or with palpable abnormalities. One group of blinded readers read the images from subjects with nonpalpable abnormalities, and the other group read the images from subjects with palpable abnormalities. For the blinded reading, digital data were converted to a common image display format, and images were randomized and read from the computer display.
Lesion Correlation
Because breast tissue is highly mobile and because mammographic and scintigraphic imaging used different views and techniques (compressed versus noncompressed), it was necessary to review the scintigraphic findings to establish lesion correlation. Thus, a radiologist not associated with the trial reviewed the scintigraphic images and mammograms in conjunction with the blinded readers' image interpretations and the investigational sites' mammographic data reporting to determine whether the scintigraphic abnormality corresponded to the same tissue as the mammographic finding. Similarly, a breast surgeon not associated with the trial reviewed the scintigraphic images, blinded readers' image interpretation, and the investigational sites' physical finding reporting to determine whether the scintigraphic abnormality corresponded to the same tissue as the palpable abnormality. All calculations of diagnostic statistics were based on histopathologically determined malignancy or benignity of biopsied tissue. Because a scintigraphic abnormality that did not correspond to a mammographic abnormality or physical finding was not biopsied, such abnormalities could not be assigned as malignant or benign and thus were excluded from analysis. Characterization of these scintigraphic abnormalities as either false-positive or true-positive findings is the objective of a follow-up study.
Biopsy and Histopathology
Biopsy decisions were based on clinical presentation independent of scintigraphic findings. After needle localization, excision of nonpalpable mammographically detected abnormalities was confirmed by specimen radiography. The institutional histopathologists determined tumor size from excised tissue specimens. Histopathologic diagnosis by the core laboratory was based solely on excisional biopsy or mastectomy specimens. Evaluation was performed on original slides or recuts of the tissue blocks if originals were unavailable. Ductal carcinoma in situ (DCIS), infiltrating ductal carcinoma, and infiltrating lobular carcinoma were classified as malignant. Cases of lobular carcinoma in situ were classified as nonmalignant.
Statistical Analysis
The primary analysis in evaluating 99mTc-sestamibi for detecting the presence of malignancy compared the blinded reader's image interpretation with the histopathologic diagnosis. Sensitivity, specificity, positive and negative predictive values, and accuracy were calculated. Comparisons between two groups were performed using a two-sample test of proportions. Group differences were considered significant at P < 0.05. Inter-reader agreement was assessed using the
statistic.
< 0.40 indicates poor agreement, whereas
= 0.400.75 and
> 0.75 indicate fair-to-good and strong agreement, respectively (22).
To further investigate factors influencing diagnostic results, a multivariable regression analysis was performed. Variables entered into the model included patient age dichotomized as
50 or >50 y, mammographers' likelihood of malignancy, mammographic finding categorized as mass or calcification, lesion palpability, and tumor size dichotomized as <1 or
1 cm.
| RESULTS |
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50 y old with nonpalpable abnormalities had a disease prevalence of 21.2%, whereas the group of younger women with palpable abnormalities had a prevalence of malignancy of 34.0%. The overall diagnostic sensitivity for 99mTc-sestamibi breast imaging was comparable in women
50 y old and women
50 y old (75.3% and 75.5%, respectively; P = not significant). When the results were considered separately for women with palpable abnormalities and for women with nonpalpable abnormalities, there was a trend for higher sensitivity in older women, although these differences were not statistically significant (Table 2). Imaging specificity was higher in the older population, although this difference was not statistically significant for institutional results for women with palpable abnormalities.
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1 cm than for tumors <1 cm (74.2% and 48.2%, respectively; P < 0.05). Similar results were observed for the blinded readers.
Likelihood of Malignancy
The prevalence of histopathologically determined disease was well tracked by the mammographers' estimate of a lesion's likelihood of being malignant (r = 0.98; P < 0.005). The institutional sensitivity and specificity of 99mTc-sestamibi imaging were similar in women with all likelihoods of disease, 64%82% and 73%87%, respectively (Table 3). Consequently, in women with
40% mammographic likelihood of malignancy, the negative predictive value for 99mTc-sestamibi imaging was 94%.
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Time of Imaging
We were interested in evaluating the impact of time of imaging after 99mTc-sestamibi injection on diagnostic accuracy because 99mTc-sestamibi tumor efflux could lead to decreased diagnostic sensitivity for delayed imaging. Overall institutional results showed a modest decrease in sensitivity for delayed imaging and a corresponding increase in specificity. However, diagnostic accuracy (Table 4) and area under the receiver operating characteristic curve were comparable for early and delayed imaging. Consequently, to ensure highest sensitivity for the identification of malignant abnormalities, we recommend early scintigraphic imaging only.
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= 0.930.99). Inter-reader agreement for evaluation of nonpalpable abnormalities ranged from 95% to 96% (
= 0.820.89).
Multivariable Regression Analysis
The only independent predictors of a true-positive 99mTc-sestamibi image in the multivariable regression analysis were lesion palpability and tumor size (global
2 = 37.7). The analysis was also conducted excluding tumor size, as would occur for abnormalities detected solely as microcalcifications. Lesion palpability became the only predictive variable in that model.
| DISCUSSION |
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Improvements in mammography techniques and procedures can certainly overcome some of these interpretive challenges. However, an adjunctive technique such as 99mTc-sestamibi breast imaging, which relies on characteristics of tumor growth, has promise as a tool to assist in resolving difficult-to-interpret mammograms.
Earlier studies of 99mTc-sestamibi breast imaging reported diagnostic sensitivities ranging from 83% to 96% and specificities ranging from 83% to 94% in populations with disease prevalence ranging from 29% to 84% (1120). Although some of these reports evaluated 99mTc-sestamibi breast imaging of both palpable and nonpalpable abnormalities, any attempt to estimate diagnostic sensitivity from these 10 earlier studies would be based on results from only 13 nonpalpable cancers. Thus, although these single center studies were limited by their focus on palpable abnormalities and their referral bias as evidenced by the high prevalence of disease, they did suggest the value of more systematically studying the diagnostic performance of 99mTc-sestamibi breast imaging. More recently, reports have appeared from multicenter studies conducted in Europe. The European multicenter trial reported an overall blinded sensitivity of 71% in a population with 65% disease prevalence, a sensitivity for palpable malignancy of 83%, and a sensitivity for nonpalpable malignancy of 30% (24). The Spanish multicenter study reported an overall blinded sensitivity of 90% in a population with 59% disease prevalence, a sensitivity for palpable malignancy of 94%, and a sensitivity for nonpalpable malignancy of 75% (25). Similar to many of the single center reports, a high prevalence of disease represented a limitation to these studies. Additionally, the results for nonpalpable abnormalities are difficult to evaluate because only 38 nonpalpable malignancies were included in the European trial and, although 126 nonpalpable lesions were represented in the Spanish study, the number of nonpalpable malignancies is not reported.
We examined the diagnostic accuracy of 99mTc-sestamibi breast imaging in women suspected of having breast cancer and scheduled for excisional biopsy or mastectomy. Diagnostic accuracy was also evaluated as a function of patient age, tumor size, and mammographers' estimate of likelihood of malignancy. The overall institutional sensitivity and specificity were 75.4% and 82.7% with a 79.8% diagnostic accuracy. In this population with a 40.1% disease prevalence, the positive predictive value was 74.5% and the negative predictive value was 83.4%. The sensitivity was lowest for nonpalpable tumors <1 cm in their largest dimension.
Screening mammography shows decreased sensitivity for invasive cancer in younger women compared with that of older women. Initially attributed to a higher prevalence of radiographically dense breasts in younger women, recent studies suggest that more rapid tumor growth and the greater prevalence of interval cancer in younger women may be the main cause of lower sensitivity (26,27). Multivariable regression analysis in this study showed that patient age was not predictive of a true-positive 99mTc-sestamibi image. Additionally, previous reports have shown that the sensitivity of 99mTc-sestamibi breast imaging is comparable in patients with dense and fatty breasts (28,29). This suggests an adjunctive usefulness for 99mTc-sestamibi breast imaging in younger patients, particularly in those with radiographically dense breasts.
We also compared the performance of mammography and 99mTc-sestamibi scintimammography in the same population. The institutional 99mTc-sestamibi scintimammography sensitivity and negative predictive value for patients with palpable abnormalities were comparable with mammography; however, the specificity and positive predictive value were higher (Fig. 3).
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Finally, our study is limited by subject enrollment criteria. Because each subject enrolled in this study was already scheduled for biopsy, our results should not be generalized to a screening population. However, the high negative predictive value in subjects with a low-to-intermediate likelihood of malignancy (Table 3) does suggest an adjunctive value for 99mTc-sestamibi breast imaging.
| CONCLUSION |
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This multicenter study on 563 subjects with biopsy confirmation defined the diagnostic characteristics of planar 99mTc-sestamibi breast imaging. The institutional sensitivity and specificity for breast cancer detection in palpable lesions were 87% and 76% and were 61% and 87% for nonpalpable lesions. The negative predictive value was 94% in patients with a 40% or lower mammographic likelihood of breast cancer. Sensitivity is independent of patient age, breast density, and mammographically established likelihood of malignancy but is decreased for small nonpalpable tumors.
| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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For correspondence or reprints contact: Stephen B. Haber, PhD, DuPont Pharmaceuticals Company, 331 Treble Cove Rd., 500-2, North Billerica, MA 01862.
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