[The usefulness of 99mTc-tetrofosmin SPECT in the detection of lung metastases from extrapulmonary primary tumors]

Angela Spanu, Maria Elisabetta Solinas, Antonio Farris, Antonio Arru, Francesca Chessa, Giordano Madeddu, Antonio Falchi, Giuseppe Madeddu
La Radiologia Medica 2004, 107 (1-2): 113-27

PURPOSE: The appearance of pulmonary metastases in neoplastic patients previously submitted to radical operation of an extrapulmonary primary tumor and with no recurrences and/or secondary distant localizations can markedly worsen disease prognosis if the lesions are in advanced stages and no longer removable by surgical procedures. In this study, we evaluated the usefulness of 99mTc-tetrofosmin scintigraphy, both planar and SPECT, in the detection of pulmonary metastases from previously treated extrapulmonary primary tumors, comparing the results with those obtained with CT.

MATERIALS AND METHODS: We studied 73 patients, 33 M and 40 F, aged 24 to 79 yrs, had who undergone previously surgical resection extrapulmonary primary tumors; 31 patients had mammary, 13 gastrointestinal, 13 genitourinary, 13 thyroid and 3 laryngeal carcinomas. In all patients CT ascertained single (42 cases) or multiple (31 cases) pulmonary lesions. 99mTc-tetrofosmin scintigraphy, always performed within a week of CT, was acquired 10 min after 740 MBq i.v. tracer injection, using a rectangular, large field of view, dual head gamma camera, equipped with low energy, parallel-hole and high resolution collimators. Conventional planar acquisition, with the patient in the supine position, was followed by SPECT over 360 degrees with body contouring system, using a zoom factor of 1-1.3, a 64x64 matrix size, a 3 degrees angular step and an acquisition time of 30 sec/frame; image reconstruction was performed with the Back Projection Filtered Method using a Metz filter. In the coronal slices of SPECT images a semiquantitative analysis of the lesion was also made and the tumor-to-background (T/B) ratio was calculated; the calculation of the latter, considered indicative for tumor when >1.4, was performed in the lesion with the highest radiotracer uptake in patients with multiple lung lesions. The results of SPECT qualitative images were compared with those of planar scintigraphy and both of these with the data obtained with CT, and statistical difference was calculated with McNemar's test. The definitive diagnosis was achieved after scintigraphy by the analysis of cytologic or histologic specimens obtained by thoracotomy, percutaneous thoracic needle biopsy or bronchoscopic biopsy, and in some cases by the confirmation of instrumental exams at 6-8 month follow up. Pulmonary metastases were ascertained in 65/73 cases, while benign lesions were present in the remaining 8 patients.

RESULTS: CT was true positive in 60/65 (92.3%) patients with metastases and false negative in the remaining five, four with a single lesion and one with two lesions in the same lung. SPECT detected metastases in 62/65 (95.4%) patients, including the five CT false negative cases, while it was false negative in three cases with single lesions, all positive at CT, with a size of 1.0, 1.0, 2.0 cm, respectively. Planar scintigraphy was only positive in 23/65 (35.4%) patients, with a sensitivity value significantly (p<0.0005) lower than both SPECT and CT. In the patients with multiple metastases, SPECT, positive in all cases and capable of correctly differentiating the patients with metastases only in one lung from those with bilateral involvement, underestimated the number of micronodular lesions in respect of CT in six patients with >6 lesions < or =0.5 cm in size, while it was more sensitive in three other cases; CT and SPECT were concordant in macronodular metastases detection, except in one case in which SPECT depicted more lesions. Planar scintigraphy had 42 false negative results and, when positive, underestimated the number of multiple lesions in respect of both SPECT and CT; it was positive but CT negative in only one case with two lesions. Specificity was higher for both SPECT and planar scan (87.5%) than CT (62.5%) but not significantly; accuracy was higher for SPECT (94.5%) than both planar scan (42%) and CT (89%), but the difference was significant only in respect of planar. The combined use of SPECT and CT achieved 100% sensitivity and accuracy values. The T/B ratio was >1.ratio was >1.4 in all SPECT positive cases (range: 1.6-5.3), indicating a suspected tumor, while it was 1.2 in the only benign case which turned out to be false positive at qualitative SPECT imaging.

CONCLUSIONS: 99mTc-tetrofosmin SPECT seems to be a reliable diagnostic tool in the detection of pulmonary metastases from extrapulmonary primary tumors, in particular in the cases in which CT is indeterminate or false positive, and therefore contributes to a more correct patient classification. Moreover, SPECT image semiquantitative analysis can give additional information for a differential diagnosis between malignant and benign lesions as well as offering a more accurate selection of the lesions to be subjected to biopsy in patients with multiple metastases. However, only the combined use of CT and 99mTc-tetrofosmin SPECT can achieve the highest sensitivity and accuracy values and obtain the correct staging in most patients, thus suggesting that these two procedures must be employed together.

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