REF006 - Parathyroid Localisation Imaging

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

The diagnosis of hyperparathyroidism is made biochemically by the presence of hypercalcaemia and elevated parathyroid hormone levels in the serum. The contribution of nuclear medicine imaging is in the pre-surgical localisation of the source of abnormal production in patients with hyperparathyroidism and not in the primary diagnosis. Parathyroid visualisation depends upon the size of the gland. A normal parathyroid gland is too small to be localised by Nuclear Medicine techniques. It is also important to realise that ectopic parathyroid glands occur in 25% of patients, with 5% of patients having greater than four glands. The use of Nuclear Medicine as a functional modality can be combined with MRI to define the precise anatomic location.

The ⁹⁹Tc^m-sestamibi radiopharmaceutical is distributed in parathyroid and thyroid tissue in proportion to blood flow. Differential washout of sestamibi from the thyroid and parathyroid can allow localisation on washout imaging. Na¹²³I (sodium iodide) thyroid scanning allows subtraction imaging to improve localisation.

The dual isotope subtraction method should be employed routinely. Sestamibi washout method should be employed only if the patient is known to have iodine allergy, or is taking thyroid medication and cannot have it withdrawn for the required period of time.

Referrals will only be accepted from Endocrinologists, ENT/Thyroid surgeons or Renal Physicians as per the Trust’s “Pathway of Parathyroid Surgery and Management of Thyroid Nodule” agreed 29th April 2014.^[1]

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 fulfills one of the referral criteria as shown below. It must be stressed that this technique is not intended to establish a diagnosis. It is a localisation technique when the diagnosis of primary hyperparathyroidism has been established. Appropriate clinical information must be supplied to allow the investigation to be justified. It is not acceptable to simply request the investigation without appropriate clinical information. Requests will be returned to the referrer if:

1. The diagnostic question is unclear.
2. The diagnostic question cannot be answered by the investigation.
3. 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 and the form signed.

Sufficient clinical information must be provided to justify the investigation proceeding.

The clinical indications are detailed below.

• Avoid iodine based contrast agents (e.g. those used in x-ray/CT) for 4-6 weeks prior to the scan.^[2]
• Stop antithyroid drugs (e.g. carbimazole, methimazole or propylthiouracil) for 1 week prior to the scan (if agreed by the referring clinician).^[2]

NB: Patients with secondary hyperparathyroidism may need to stop other medications ^[2]. This must be discussed with an appropriate ARSAC licence holder.

• The patient should be well hydrated at the time of administration of the radiopharmaceutical.
• Patients with secondary hyperparathyroidism may need to stop some medications ^[2]. This must be discussed with an appropriate ARSAC certificate holder.