Nuclear Medicine
CLINICAL DECISION SUPPORT
When erythrocytes (red blood cells, RBC) are damaged by incubation at higher temperature than circulating blood, specifically 49.5°C, they tend to become more rigid and spherical. Subsequently, such RBC will not be able to change their shape (deform) when passing through the tiny capillaries of the spleen filter. This mimics what is naturally happening in hereditary spherocytosis, a familial disorder that causes haemolysis. After i.v. injection, such HDRBC will be taken up by the liver, spleen, and bone marrow. Some less damaged RBC will circulate in the blood and cause some blood pool image.
[99mTc]Tc-HDRBC scintigraphy is useful in detecting accessory spleen before splenectomy and in patients with a suspicious mass, usually in the left upper abdomen, e.g. perisplenic, perirenal, pancreatic, or peri gastric. It is also very useful to detect spleen remnants after splenectomy, when the diseases that lead to splenectomy recur, e.g. immune thrombocytopenic purpura, or after unscheduled splenectomy for open or delayed spleen rupture, blunt trauma, since both situations could result in seeding of spleen tissue into the retroperitoneal or intraperitoneal spaces, following laparoscopic surgery, and even after diaphragmatic trauma in the lung or pleura. These conditions are known as splenosis. Less frequent indication is the evaluation of functional asplenia, particularly in sickle-cell anaemia.
The functional nature of [99mTc]Tc-HDRBC makes it a unique tool for functional assessment of abnormal structures in (usually) the left hemi-abdomen and lung that may correspond to splenic tissue. It is particularly useful after elective or unscheduled splenectomy, because the lack of uptake by the normal spleen allows more RBC to be taken up by abnormal tissue. Quantitative assessment of spleen function is possible using the peripheral blood clearance of the labelled HDRBCs. This is usually achieved using Chromium-51 (sodium chromate) as a label in a research setting.
The suggested activities to administer are
In paediatric nuclear medicine, the activities should be modified according to the EANM paediatric dosage card (https://www.eanm.org/publications/dosage-calculator/). The minimum recommended activity to administer is 80 MBq.
The effective dose for 99mTc-denaturated erythrocytes is 1.9 µSv/MBq [3]. The organ with the highest absorbed dose is the spleen: 560 µGy/MBq [1–3].
The range in effective dose for [99mTc]Tc-HDRBC is: 0.14-0.35 mSv per procedure.
Caveat:
“Effective Dose” is a protection quantity that provides a dose value related to the probability of health detriment to an adult reference person due to stochastic effects from exposure to low doses of ionizing radiation. It should not be used to quantify the radiation risk for a single individual associated with a particular nuclear medicine examination. It is used to characterize a certain examination in comparison to alternatives, but it should be emphasized that if the actual risk to a certain patient population is to be assessed, it is mandatory to apply risk factors (per mSv) that are appropriate for the gender, the age distribution and the disease state of that population."
After splenectomy, any focal uptake in the left hemi-abdomen (down the left iliac fossa) or in the left thoracic cavity should be considered as suspicious. Caution should be exercised in interpreting the shift of the left hepatic lobe to the spleen bed after splenectomy as well as renal uptake, related to blood pool, and sometimes, to some extent, the excretion of some eluted free 99mTc-pertechnetate.
Contrast-enhanced CT-scan should be preferred for the detection of accessory spleen(s) in view of splenectomy, because the physiological uptake by the normal spleen is very high and may hamper delineation of small tissues close to the spleen tissue (it is most likely, though, that a combination of SPECT and CT, or either technique separately, may prove just as useful).
No specific preparation. Some medications might interfere with labelling efficiency, but this is not a major issue, since HDRBC are washed with saline before reinjection. Autologous cells are used; therefore, blood group determination is not necessary.
99mTc-RBC labelling is performed using the in vitro or in vivo pyrophosphate method followed by incubation of the labelled RBC for 20 min at 49.5°C and saline washing before reinjection. A very precisely regulated water bath must be used and quality control performed regularly (usually before each examination in most centres, where the examination is infrequent).
Planar imaging (AP/PA) is performed 60-90 min post-injection and followed by SPECT/CT over the entire abdomen. If the assessment of (a) unusual thoracic nodule(s) is the indication, the lung field must be included.
Iterative reconstruction is advised and mandatory for the detection of accessory spleen due to the proximity of the very high uptake by the normal spleen.