Factors affecting ¹³¹I therapy radiation dose

Purpose: Administration of the same empiric amount of activity of ¹³¹I to all pts to treat thyroid cancer may inadvertently deliver more than 200 cGy radiation dose (D) to blood, which is considered excessive. Reasons for pts to be referred for ¹³¹I dosimetry include: impaired renal function, abnormal pathology findings, age < 16 yrs, rising thyroglobulin levels, & metastatic disease to lungs, to bones or to both. Our study was undertaken to determine which of these factors had the highest likelihood of excessive radiation dose if an empiric amount of ¹³¹I were to be administered, & to determine if the presence of multiple factors increased that likelihood.

Methods: Data were examined retrospectively of 123 pts (age = 58±18 years; 62 female pts; 61 male pts) with thyroid cancer who underwent blood sample collection over 4-7 days following ingestion of 37-148 MBq (1-4 mCi) ¹³¹I, & whole-body ¹³¹I scanning 48 hrs after administration of ¹³¹I. Impaired renal function was defined as BUN & creatinine values exceeding normal limits. ¹³¹I blood-measurement-only dosimetry was used to compute ¹³¹I D. The % of pts for whom D > 200 cGy for an empiric amount of administered activity of 7.4 GBq (200 mCi) ¹³¹I were tabulated. Results: Among the 123 pts, 62 had only one factor, for whom reasons for referral had significantly different D values (ANOVA F-factor = 4.2, p = 0.003). The single factor with greatest D was impaired renal function (D = 210±65 cGy, N = 7) & the one with lowest D was rising thyroglobulin levels (87±31 cGy, N = 14). Among all pts increasing numbers of factors were significantly associated with different D values (ANOVA F-factor = 5.6, p = 0.001), & were significantly correlated with increasing D (Rank correlation ρ= 0.40, p < 0.001). Pts with more than 1 factor had significantly higher D than those with only 1 factor (199±109 versus 125±90 cGy, p = 0.0001). Conclusion: Pts referred for ¹³¹I dosimetry have significantly greater risk of exceeding blood dose limits if administered empiric activity with increasing numbers of involving factors, for which impaired renal function constitutes the single factor associated with greatest excessive radiation dose.