99mTc-MIBI scintigraphy as a functional method for the evaluation of multidrug resistance in breast cancer patients.

PURPOSE: To evaluate the clinical application of (99m) Tc-methoxyisobutylisonitrile (MIBI) scintigraphy as a functional method for assessment of multidrug resistance (MDR) in breast cancer patients and the correlation of these results with P-glycoprotein (P-gp) overexpression and objective response to chemotherapy.

PATIENTS AND METHODS: 22 women, 35-68 years old with breast cancer, suitable for neoadjuvant chemotherapy were included onto this study. Two or three cycles of neoadjuvant chemotherapy were administered (FEC in 15 and CMF in 7 patients). Planar and SPECT (99m) Tc-MIBI scintigraphy was carried out before and after neoadjuvant chemotherapy. Focal (99m) Tc-MIBI uptake in breast cancer lesions was used as a scintigraphic criterion of abnormality. Tumor/background uptake (T/B Index) was calculated. Immunohistochemistry was carried out after surgery for P-gp detection in all cases. The degree of expression was evaluated according to semiquantitative score analysis from 0 to 4.

RESULTS: Planar imaging was true positive in 20 patients, false positive in 1 (with breast cancer and mastopathy), and false negative in 1 (with wide tumor necrosis and deep location in the breast). SPECT imaging was true positive in 21 patients and false positive in 1. In 3 patients with multifocal disease additional tumour masses were visualized using SPECT. Sensitivity was 95% (21/22) and 100% (22/22), respectively, for planar and SPECT detection of breast cancer. P-gp expression was positive in 40.8% of the patients and negative in 59.2%. Intense (99m) Tc-MIBI uptake was shown on the planar images in 21 patients independently of the P-gp expression. There was no significant relationship between T/B Index and P-gp detection. Objective response included 2 clinical complete remissions, partial response in 1 patient, minimal response in 12, and no change in 7. Some clinical results corresponded to (99m) Tc-MIBI scintigraphic data: after neoadjuvant chemotherapy T/B Index was reduced > or = 20% in 9 patients with objective response.

CONCLUSION: SPECT is an important diagnostic approach for identification of breast cancer with deep location and satellite tumour spots in multifocal disease. T/B Index did not correlate with P-gp overexpression on baseline (99m) Tc-MIBI scan. Objective clinical results after neoadjuvant chemotherapy corresponded to scintigraphic results in 75% of the patients with minimal response.