Methods: It was analyzed retrospectively by two independent specialists, 78 scintimammographies performed after the pedal injection of 370 MBq of MIBI[^99mTc]. Planar images were acquired during 10 minutes on lateral and oblique posterior views with prone dependent breast over a table overlay and on anterior view, with a gamma-camera (Orbiter, Siemens) fitted with a low energy and high resolution collimator. The degree of MIBI[^99mTc] uptake was classified as: 1-mild uptake, 2-moderate uptake, 3-intense uptake. One male and 77 female patients with mean age of 58.5 years (30 to 80 years) were operated and 79 breast carcinomas were found. Seventy-eight was ductal carcinoma and one was duct-lobular carcinoma. The tumor size ranged from 0.7 to 7.0 cm (mean=2.96 cm). All tumors were classified according to the Bloom-Richardson (B-R) histological grading: I-well differentiated, II-moderately differentiated, III-poorly differentiated.

Results: Seventeen specimens were classified as B-R grade I, 47 as grade II and 15 as grade III. According to the degree of MIBI[^99mTc] uptake, 15 tumors were classified as grade 1, 43 as grade 2, 11 as grade 3 and 10 were considered false-negative. It was observed a positive correlation between the histological grading and the degree of MIBI[^99mTc] uptake by the Spearman's rank correlation test (r=0.4033; p=0.0002). This study, that was done on a not independent sample, also reveals that the MIBI[^99mTc] uptake and the B-R tumor grading did not difer regarding the capability on discriminating the tumor aggressiveness calculated by the proportion test for not independent populations (U2=2.91; p>0.05). The size of the false-negative cases ranged from 1.0 cm (1 case) to 3.5 cm and only three tumors were located on external quadrants. The histological diagnosis was ductal carcinoma in 9 cases and duct-lobular carcinoma in one. Several factors may influence the MIBI[^99mTc] uptake on scintimammography. The histologic type of the tumor and the size of the lesion were described as variables that may affect the radiotracer uptake. We did not identify the tumor size as a prominent factor influencing the MIBI[^99mTc] uptake on our study. By the other hand, our results confirm that the histological grading is a very important influence to the degree of MIBI[^99mTc] uptake observed on scintimammography. The B-R tumor grading classification takes into account the degree of tubule formation, the regularity in the size, shape and staining character of the nuclei, the nuclear hyperchromasia and mitotic activity. The degree of the mitotic activity has been identified as one of the mecanisms of uptake of MIBI[^99mTc] on breast cancer cells.

Conclusions: MIBI[^99mTc] uptake correlates with the histological tumor grading and can be used, together with other variables, as an indicator of prognosis in patients with breast carcinoma.