Application of a dosimetry model based on the whole body retention at 48hrs

Objectives Evaluate prospectively the validity of a simple regression dosimetry model.

Methods Previously we reported on a bi-exponential model (jnm 2007:48:268P) to estimate the maximum tolerated activity (MTA) of I-131 (based on a limit of 2 Gy to the blood) for the treatment of metastatic well-differentiated thyroid cancer (WDTC). It uses just the patient’s body surface area (BSA) and the whole body retention of a dx dosage of I-131 at 48 hrs post administration. The following regression model was developed between the MTA normalized to the patient’s BSA and the whole body retention as a % of the administered dose at the 48 hr time point (R): MTA/BSA = [376.0·e^-0.0387R+1144.4·e^-0.8522R]. This study applied the model prospectively to all adult patients referred for dosimetry and possible radioiodine treatment of WDTC under conditions of thyroid hormone withdrawal. The MTA predicted (MTA_p) using the model shown above was compared to the measured MTA (MTA_m) with statistical analysis performed using ProStat v4.5.

Results A total of 81 dosimetries were performed in 71 patients (42F;29M) with an MTA_m range of [2.3-23.2 GBq] over which the linear correlation between the MTA_p and MTA_m was excellent (r=0.96) with an avg dev of only +1.2%. However, to avoid overdosing a patient on the basis of the MTA_p a weighting factor (< 1.0) should be applied, e.g. using a value of 0.7 would result in only 1 patient receiving a prescribed activity of I-131 that exceeded the MTA_m (<3%).

Conclusions A simplified dosimetry model could be used as an alternative method of estimating the MTA for treatment of metastatic WDTC for those facilities without the resources to perform a standard dosimetry protocol or for patients unable or unwilling to.