Molecular Breast Imaging: Administered Activity Does Not Require Adjustment Based on Patient Size

At our institution, molecular breast imaging (MBI) is performed with 300 MBq of ^99mTc-sestamibi for all patients. For some nuclear medicine procedures, administered activity or imaging time is increased for patients of larger size to obtain adequate counts. Our objective was to assess whether uptake of ^99mTc-sestamibi in the breast is influenced by patient size. Methods: Records from patients who underwent a clinical MBI examination between July and November 2016 were reviewed. Those in whom our standard injection and preparation techniques were followed were included in the analysis. Patients were injected with approximately 300 MBq of ^99mTc-sestamibi. Residual activity was measured to allow calculation of exact administered activity for each patient. Breast images were acquired at 10 min/view using a dual-head cadmium zinc telluride-based γ-camera. Breast thickness was measured as the distance between the 2 detectors. Patient height, weight, body surface area, and body mass index were obtained from records. Lean body mass with the James equation (LBM_James) and Janmahasatian correction (LBM_Janma) was calculated. Count density in the breast tissue was measured by drawing a region of interest around the central breast tissue of the right breast mediolateral-oblique view of the lower detector. Count density was expressed as cts/cm²/MBq of administered activity. Spearman correlation coefficient (r_s) was calculated. Results: A total of 200 patients were analyzed. No dose infiltration was suspected at any injection. Average administered activity was 292 MBq (SD, 13.8 MBq; range, 247-326 MBq). Average count density was 7.2 cts/cm²/MBq (SD, 2.7 cts/cm²/MBq; range, 3.1-17.8 cts/cm²/MBq). MBI count density was weakly negatively correlated with height (r_s = -0.18; P = 0.01), weight (r_s = -0.23; p = <0.001), body mass index (r_s = -0.16; P = 0.02), body surface area (r_s = -0.22; P = 0.002), LBM_James (r_s = -0.23; P = 0.001), and LBM_Janma (r_s = -0.23; P = 0.001). No correlation was observed between count density and breast thickness (r_s = 0.06; P = 0.37). Conclusion: Our results suggest a lack of relationship between uptake of ^99mTc-sestamibi in breast tissue and body size or compressed breast thickness. Altering from the standard 300 MBq of administered activity for larger patients is likely unnecessary.