REF018 MIBG Tumour Imaging

The purpose of this document is to assist clinicians in deciding on the appropriateness of the Nuclear Medicine procedure and to give guidance on the mechanism of referral.

Iodine-123 labelled metaiodobenzylguanidine (MIBG) is a radiopharmaceutical of similar structure to noradrenaline and is used in the diagnosis and localisation of active catecholamine secreting tumours, such as phaeochromocytomas, and their metastases.

This investigation is not a screening procedure for Phaeochromocytoma and should only be used after elevation of catecholamines on biochemical assay suggests the disease. Most adrenal phaeochromocytomas are visualised on CT. The role of iodine-123 labelled MIBG is in the localisation of extra-adrenal or metastatic lesions, as well as detection of bilateral disease, which is of great importance preoperatively. The investigation also provides an assessment for the potential of treating tumours with therapeutic doses of iodine-131 labelled MIBG in patients with metastatic and unresectable malignant disease. Iodine-123 labelled MIBG is also the investigation of choice in suspected recurrence of disease when a whole body study can be used to detect areas of increased uptake of radiopharmaceutical. In addition to phaeochromocytomas, neuroblastomas produce high levels of catecholamines and are receptive to MIBG imaging. Although they can normally be diagnosed using CT, iodine-123 labelled MIGB can be used to provide a more specific diagnosis and for the evaluation of tumour spread, particularly to bone marrow metastases.

SPECT/CT is likely to be acquired

The referrer may be any clinician holding a current valid medical qualification and currently employed in Secondary Care. Referring clinicians are reminded of the requirement to avoid unnecessary exposure to ionising radiation. The investigation may already have been requested at this hospital or elsewhere and clinicians are asked to be particularly vigilant to ensure that their request is not a double request.

Requests will only be accepted if the diagnostic question fulfils one of the referral criteria detailed in the following paragraph. Appropriate clinical information must be supplied to allow the investigation to be justified. Requests will be returned to the referrer if:

• The diagnostic question is unclear.
• The diagnostic question cannot be answered by the investigation.
• Insufficient clinical information is supplied to justify the investigation.

Requests should be submitted on the orange stripe NUCLEAR MEDICINE (ISOTOPE SCAN) request form. Patient details, investigation required and reason for test/diagnostic question should be fully and appropriately completed. The form MUST be signed.

Appropriate biochemical markers and cross sectional imaging eg CT should have been performed prior to requesting MIBG. If biochemical markers are negative but a clinical suspicion remains then the request must be discussed with the ARSAC licence holder. The clinical indications are detailed below.

• Investigation and localisation of suspected phaeochromocytoma
• Investigation and localisation of suspected extra-adrenal phaeochromocytoma
• Investigation and localisation of suspected phaeochromocytoma metastases
• Assessment of the unilateral or bilateral nature of the disease
• Assessment of the possible therapeutic use of iodine-131 MIBG
• Investigation and localisation of suspected neuroblastoma
• Investigation and localisation of suspected neuroblastoma metastases
• Investigation and localisation of suspected carcinoid tumour
• Assessment of suspected post-operative recurrence of disease

130mg potassium iodide tablets per day for a total of 3 days, starting the day before the injection of I-123 MIBG and continuing for 2 further days (i.e. the night before the injection, the night of the injection and the following night)^[1][2]

• If potassium iodide is unavailable, alternatives such as Lugol's solution can be used (0.8ml contains 104mg of iodine). Can be given with milk or water/cordial.
• See the ARSAC Notes for Guidance (section 8) for further details and alternatives^[3].

When thyroid blocking agents are administered to children, consideration should be given to reducing the dosage^[3]. i.e.

• children of 3 to 12 years 50% of adult dose
• children of 1 month to 3 years 25% of adult dose
• neonates (birth to under 1 month) 12.5% of adult dose

If iodine is contraindicated, thyroid blockade can be carried out with potassium perchlorate (200 mg adult dose). If used, this should be administered 1 hour^[3] prior to the administration of I-123 MIBG and repeated at 8 hour intervals^[3] for 2 days^[1].

It should be noted that currently potassium perchlorate is not licensed in the UK.

Care must be taken to ensure that drugs which interfere with MIBG uptake are discontinued (if possible) for an adequate time prior imaging. See http://www.heynm.org.uk/pdfs/mibg_drug_stop_list.pdf. Patients with metabolically active catecholamine secreting tumours (i.e. phaechromocytoma, paraganglioma) often receive alpha- or beta-blocking treatment. Therefore, drug interruption should be decided by consultation between the referring physician and the ARSAC licence holder, to enable evaluation of the patient’s condition. If it is decided to proceed without changing the medication, it should be noted that this could impair diagnostic accuracy^[1][2]