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A very interesting and worthwhile article regarding the patient effective dose received from sentinel lymph node imaging has been published in the British Journal of Radiology [1]. The paper highlights the low doses received during the study compared with other radiological examinations.
Law et al describe using a 100 MBq (2.7 mCi) Co-57 flood source for a total of 5 min for each position to produce each transmission scan image with an average effective study dose of 3 µSv (Table 4, reference 1). From a practical, clinical standpoint, we would just like to note that the dose can be reduced and a time saving realised if dual energy windows are employed. We use two simultaneously acquired energy windows, the first optimized for the 140 keV photon of 99Tcm (135–150 keV) and the second for the primary, most abundant, 122 keV photon of Co-57 (112–132 keV), at a 16–20% window. The optimized energy windows for Co-57 provide over an 8 fold gain in relative sensitivity. At our institution, a Co-57 flood source as weak as 11 MBq (0.3 mCi) has been used with exposures of 1 min per position in the right patients. This will further decrease the patient effective dose markedly and save time. Changing the transmission source scanning time from 5 min per position as described by Law et al, to 1 min per position using the same 100 MBq source but with Co-57 windows, will produce superior outlining results compared with 5 min at the 99Tcm energy window as used in the article [1]. When producing final images we combine the data from the Co-57 window of the transmission images and the data from the 99Tcm window of the emission images to produce an accurate, well-delineated outline of the patient's body in relation to the sentinel lymph nodes. Images without the transmission source, utilizing only the 99Tcm window, are also always provided [2].
Our experience is that this saves a considerable amount of time, reduces exposure even further, and potentially saves money. By shortening the scan time to 1 min rather than 5 min per view, potential time savings of 12–25 min per study are possible depending on the number of views. With this additional time, the technologist is free to perform another study. By allowing the use of old, decayed sources, the time between replacements can be increased and a cost saving can potentially be realised.
Yours etc.,
The Mount Sinai Medical Center, 1 Gustave L Levy Place, New York, NY 10029-6574, USA
Received for publication January 5, 2004. Accepted for publication April 16, 2004.
References
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