Adsorption of 99mTc-sestamibi onto plastic syringes: evaluation of factors affecting the degree of adsorption and their impact on clinical studies.

Over the last 3–5 y, there has been increased scrutiny of the radiation burden associated with many diagnostic procedures. In 2010, the Food and Drug Administration announced an initiative to reduce unnecessary radiation exposure from 3 types of medical imaging procedures, one of which was nuclear medicine (1). Achieving a meaningful dose reduction requires that we look at all aspects of the imaging procedure to determine where reductions in patient dose can be achieved while maintaining the diagnostic quality of the image data. Molecular breast imaging is a promising screening technique for the detection of breast cancer. This technique uses the radiopharmaceutical 99mTc-sestamibi and a dedicated direct conversion semiconductor-based γ camera for the detection of breast lesions (2). A key consideration in the use of molecular breast imaging for screening is the radiation dose associated with the procedure. Recent studies of direct-conversion molecular breast imaging have used the lowest reported doses to date of approximately 300 MBq (~8 mCi) of 99mTc-sestamibi (3). Efforts to further reduce doses below this level have been confounded by wide interpatient variation in the measured uptake of 99mTc-sestamibi in breast tissue. In evaluating this issue, we have frequently observed substantial retention of the radiopharmaceutical in the injection syringe, meaning that in many patients who receive a dispensed dose of 300 MBq, the actual injected dose is much lower, leading to inadequate image count density. The issue of adsorption of radiopharmaceuticals to syringes has received sporadic attention over the years, with contradictory findings. Although some particulate preparations, such as 99mTc-macroaggregated albumin, are known to be susceptible to settling and require agitation before injection, most 99mTc-labeled radiopharmaceuticals are not considered to be problematic with regard to adhesion to the syringe. The most recent evaluation of retention of 99mTc-based radiopharmaceuticals in syringes by Mushtaq et al. (4) showed insignificant adhesion (<5%) of all tested radiopharmaceuticals (including 99mTc-sestamibi) to the plastic syringes. The primary cause of residual activity is usually thought to be dead space near the needle hub or poor technique on the part of the technologist performing the injection (5). Ponto (6) noted that enhanced retention of lipophilic myocardial perfusion radiopharmaceuticals, such as 99mTc-sestamibi, has been reported in syringes constructed with elastomeric plunger tips. This enhanced retention appears to be related to greater adsorption to the elastomeric component of the plunger. For 99mTc-sestamibi, flushing the syringe with normal saline has been reported to remove up to 70% of this retained activity (7). Earlier studies by Jannson et al. (8) and Hurless et al. (9) have indicated that high adsorption of 99mTc radiopharmaceuticals can occur and appears to be highly dependent on the particular radiopharmaceutical and type of syringe. High residual activity in syringes can lead to several problems, including the delivery of a suboptimal dose to the patient, resulting in poor image quality or extended imaging times, and can confound efforts to lower the recommended minimum administered dose and reduce the radiation burden to the patient. The purpose of this study was to document the extent of adhesion of 99mTc-sestamibi to syringes in our molecular breast imaging procedures, determine factors that influence the degree of adhesion in syringes, and evaluate alternatives to our current practice that would either result in a more reproducible degree of adhesion or, ideally, eliminate adhesion.