Iodine 123 for scanning in thyroid cancer

Full text: There is evidence that the outcome of 131I therapy for thyroid cancer is adversely affected by the radiation dose from the prior diagnostic 131I study, an effect called stunning . Reducing the diagnostic dose of 131I from 185 MBq to 74 MBq has been recommended, but reduces the sensitivity of the procedure. As an alternative approach we have investigated the use of 12I, an isotope with a half life of 13 hours and a gamma ray of 159 keV which is better suited to imaging. Although the the decay of 123I results in some local energy deposition from electrons the radiation dose to thyroid tissue is much less than from the beta rays of 131I: the dose to the normal thyroid from 131I is quoted as 720 to 7900 mGy per MBq administered for uptakes ranging from 5 to 55%, while the dose from 123I is two orders of magnitude less. High purity 123I was supplied as sodium iodide in an aqueous solution ol 0.1M NaOH, and adjusted to pH range 8.4 -10 for oral administration. 123I and 131I diagnostic studies were performed on 10 patients with suspected metastatic disease, but no evidence of residual or recurrent disease in the neck on the previous post-therapy 131I scan. Whole body anterior and posterior scans were performed 24 and 48 hours after one GBq of 123I. Patients were then given 185 MBq of 131I and scanned 72 hours later. Medium energy collimation and a scan speed of 7.5 cm/min were used for both 123I and 131I scans. In five patients, more foci were seen with 123I than with 13I. In another patient, a focus was clearly seen with 131 but not with 123I. Overall image quality was significantly better with 123I than 131I. We conclude that 123I is as good or better than 131I in detecting and localizing thyroid cancer, and should reduce the risk of stunning. 123I has also proved useful for the investigation of thyroid cancer patients with other clinical problems, such as dementia and incontinence, where 13I would pose a greater radiation hazard to staff