Modification of 99mTc-Thyroid Scan Protocol to Decrease the Radiation Dose to Salivary Glands

Abstract

Introduction: The pharmacokinetics pathway of 99mTcpertechnetate, used for thyroid scan, involves the absorption of 99mTc in normal non-pathologic salivary glands. Aim: To modify the original protocol used for thyroid scan for the purpose of decreasing radiation absorbed dose to the nontarget organ (Salivary Gland). Materials and Methods: Thirty-one patients of different age, gender, and thyroid function test were gathered and analysed. Each patient underwent two thyroid scans, the first scan was performed using the original protocol, and the second scan was obtained with a slight modification to the original protocol, where patients were instructed to chew a gum and drink a cup of water before acquiring the second thyroid scan which started 5 minutes after completing the original scan protocol. Images of the original and modified protocols were acquired using SPECT scanner with the first scan starting 20 minutes post 99mTc injection. Patients lay in a supine position while undergoing both scans. The percentage of the net administrated activity localised in the thyroid and salivary glands were measured. Results: The absorbed dose values of the thyroid and salivary glands from the original protocol and the modified protocol were estimated. There was no significant change in the thyroid gland absorbed dose between the two protocols, whereas a significant (t-test) reduction was found in the salivary gland absorbed dose using the original and modified protocol with values being 0.77±0.39 mGy and 0.28±0.09, respectively. Conclusion: Using the modified chew gum phase protocol in imaging of the thyroid gland with 99mTc result in decreasing the absorbed dose of the salivary gland.

Keywords

• absorbed doses
• effective doses
• single photon emission computed tomography
• thyroid glands
• 99mtc-pertechnetate