Can 99mTcO4- (Tc) replace 131I (I) for pre-ablation scintigraphy after thyroidectomy (TRD) for differentiated thyroid cancer? Our experience in 508 pts

424 Objectives To examine if Tc is adeguate for post TRD scan and verify if the lower radiation dose delivered to remnant thyroid tissue, reducing stunning can improve outcome after ablative I therapy. Methods 508 consecutive pts after TRD for non metastatic papillary or follicular cancer of the thyroid (TSH>30mIU/mL) entered the study and were divided into 2 groups (G): G1 (176 pts, from 2002 to 2004) had a scan with 185 MBq of I; G2 (318 pts, from 2005 to 2009) had a scan with 370 MBq of Tc. 14/508 pts had a Tc not detecting tissue remnant and were excluded. In case of scan evidence of residual thyroid tissue, after 72h from diagnostic dose of Tc or I, pts of both groups were treated with a standard dose of 3.7 GBq of I to ablate the remnant thyroid tissue. We considered the ablation was successfully if at 1 year a negative thyroid scan and an undetectable serum thyroglobulin (Tg) level ( 0.05 were considered of statistical consistency. Results The 2 G were comparable (Tab) as for age, sex ratio, tumour type, stage, Tg and AbTg (p > 0.05). Only 14/318 Tc scans did not detect remnant tissue, while among them in 7/14 pts a subsequent I scan did detect it. Based on 1y negative scan-Tg-AbTg successful ablation were obtained in 111/176 (63%) in G1 and in 270/318 (85%) in G2 and in 116/176 (66%) in G1 and 290/318 (91%) in G2, if just a negative scan was considered as successful ablation (Tab) (p Conclusions Tc could be the first choice for post TRD scan, reserving I only in case of a negative scan. The better post ablation outcome which follows Tc scintigraphy is suggestive of some thyroid radiation stunning after I scintigraphy and this should further push to replace I with Tc for post TRD scan