Nuclear Medicine
CLINICAL DECISION SUPPORT
mIBG is an aralkylguanidine produced from 2 anti-hypertensives. This is achieved by combining the benzyl group of bretylium and the guanidine group of guanethidine with iodine at the meta position. The uptake mechanism is based on specific characteristics of abnormal neural crest cells, namely an active, sodium-dependent, saturable absorption mechanism in the cell membrane as well as storage in neurosecretory granules in the cytoplasm. Because of the specific retention of mIBG in these cells, the abnormality will, over time, become more clearly distinguishable from normal tissue which does not have this mechanism.
mIBG-scintigraphy and CT scanning are both very sensitive means of demonstrating an intra-adrenal pheochromocytoma. CT shows more anatomical detail, but the mIBGscintigram allows a more specific statement about the nature of the disease.
mIBG scintigraphy is preferred in the analysis of extra-adrenal and malignant pheochromocytoma (metastases). In general, the findings on an mIBG scintigram correspond to the excretion of catecholamine degradation products in the urine. However, this is not the case for neuroblastomas. The pathological findings on the mIBG scintigram of a neuroblastoma not always correspond to the observed excretion of catecholamines or catecholamine metabolites in the urine and are not always consistent with the findings of the bone marrow aspiration.
Confirmation of lesions found using ultrasound/CT scan/MRI is desirable and aids in tumour volume determination for dosimetric purposes.
For carcinoid, the findings of the mIBG scintigram are not clearly correlated with the amount of 5-HIAA excretion in urine. Correlation with somatostatin receptor scintigraphy (higher sensitivity) may determine the choice of therapy.
Well differentiated neuroendocrine tumours will procure more reliable (or more specific) results from mIBG and somatostatin receptor scintigraphy as compared to [18F]FDG PET.
New PET tracers, such as [68Ga]Ga-DOTATATE, [68Ga]Ga-DOTANOC, [68Ga]Ga-DOTATOC and [18F]FDOPA, are now available for detection of neuroendocrine tumours. Both sensitivity and specificity are much higher than those reported for mIBG or Octreotide scintigraphy. In the perspective of a treatment, the advantage of mIBG scintigraphy over the newer techniques is the close correlation with catecholamine production and, thereby, allowing image guided therapy. The combination of mIBG scintigraphy and other tracers or techniques such as MRI and CT is of value in patients with metastasized neuroendocrine tumours to assess whether therapy with [131I]-mIBG is indicated. In this respect, multifocality and knowledge of uptake at all sites is important to assess the therapeutic effect and overall outcome.
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 37 MBq [123I]-mIBG or 35 MBq [131I]-mIBG
The effective dose for [123I]-mIBG is 13 µSv/MBq [109]. The organ with the highest absorbed dose is the liver: 67 µGy/MBq.
The effective dose for [131I]-mIBG is 140 µSv/MBq [109]. The organ with the highest absorbed dose is the liver: 830 µGy/MBq (1).
The range in effective dose in adults for [123I]-mIBG is: 2.4-4.8 mSv per procedure. The effective dose in children ranges between 6.3 mSv in neonates to 5 mSv at >1 year old.
The range in effective dose in adults for [131I]-mIBG is: 5.6-11.2 mSv per procedure.
The radiation exposure related to a CT scan carried out as part of an mIBG SPECT/CT study depends on the intended use of the CT study and may differ from patient to patient.
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."
Normal structures into which mIBG is absorbed are: salivary glands, myocardium, adrenal medulla and liver, while as a result of excretion, activity in the bowel and bladder can also be seen.
Concentrations outside the normal structures should be interpreted as suspicious for a tumour of neural crest origin.
One should be mindful of contamination, hypertrophy of the remaining adrenal medulla after unilateral adrenalectomy, hydronephrosis, pathology in or near liver, bowel and bladder.
False-negative results can be obtained when certain medications are used (see patient preparation).
The results of myocardial uptake quantification have to be interpreted according to the proposal of the European Association of Nuclear Medicine (EANM) for standardization of [123I]-mIBG scintigraphy [320].
The following medication should be discontinued in any case: reserpine, cocaine, tricyclic antidepressants, calcium channel blockers, labetalol, tranquilizers (especially phenothiazines). In the literature, more potential sources of interference are mentioned which should be taken into account.
Blocking of unintentional radioactive iodine uptake in the thyroid by administration for 5-10 days ([131I]-mIBG) or 2-3 days ([123I]-mIBG) of sodium or potassium iodide (100-150 mg per day) or sodium or potassium perchlorate (200-400 mg per day) starting the day before administration of the radiopharmaceutical.
The detailed recommendations are available in the 131I/123I-Metaiodobenzylguanidine (mIBG) scintigraphy: procedure guidelines for tumour imaging [321].