Initial radiation exposure levels X (0) at 1 m from the navel of thyroid cancer patients were measured for 165 individuals at the time of ingestion. Some 61 patients had previously signed informed consent so only those patients could be assayed with regard to body parameters. While the activity was in the stomach, resultant X (0) values were seen to be linearly correlated with the total (131)I activity (A) given orally. Yet large differences in X (0) were seen; e.g., at A = 7.4 GBq, variations of a factor of four were found between the largest and smallest exposure rates. Correlation analyses were performed between normalized rate X (0)A-1 and several patient physical parameters. These included age, sex, height, weight, and BMI (body mass index). Only weight and BMI had significant linear correlation (p < 0.05) with normalized exposure rate. In the former case, the correlation coefficient ρ (weight) was -0.296 (p = 0.02). Using BMI as the independent variable, ρ (BMI) was -0.386 (p = 0.0021). With further analysis of the BMI variation, 95% confidence intervals could be determined at various BMI levels. For example, at 28 kg m(-2), the normalized rate varied between 0.039 and 0.0446 μGy h(-1) MBq(-1)-approximately a ±6.5% variation on the mean value of 0.0419 μGy h(-1) MBq(-1) at this BMI. Given such clinical information, differences in normalized exposure rate can be reduced to values on the order of ±10% or less for BMI values over the clinically relevant interval 20 to 40 kg m(-2).

Patients receiving ¹³¹I-based therapies are generally restricted in leaving the medical institution. In the U.S., the U.S. Nuclear Regulatory Commission (U.S. NRC) has developed the rule that a ≤ 7 mR h⁻¹ reading at 1 m from the patient (or 33 mCi) is sufficient to allow unrestricted release. Because of home situations and other constraints, it is preferable that a patient-specific release level be determined by the radiation safety staff. Locally, the City of Hope has instituted a general release criterion of ≤ 2 mR h⁻¹ at 1 m. While contributing to a reduction in public exposure, this as low as reasonably achievable (ALARA) approach is difficult to justify on a cost basis due to the expense of maintaining the radioactive individual in a hospital room. Instead, it was determined that a motel-type room already on the campus be modified to allow the patient to remain on-site until at or below a locally permitted release level. By adding lead to the bathroom area and sealing the tile surfaces, the room may be converted for less than $5,000. Daily cost for the patient is $65. In comparing the use of this facility for thyroid cancer patients from 2006 to 2010, it was found that the public exposure at 1 m was reduced by approximately 70% as compared to release at the 7 mR h level. In addition, controlling the release reduces the likelihood of a radiation incident in the public environment such as on public transportation or in a hotel.

Average initial exposure rates at one meter from 165 NaI-131 patients have been determined. The biological clearance was seen to be much more rapid than the physical decay constant for I-131 (0.0036/h). At a given activity level, variation of exposure rates was approximately +/- 40% over the corresponding patient population.

Purpose: Thyroid cancer (papillary and follicular) patients were evaluated for initial external radiation exposure rate (r) at the time of ingestion of their I‐131 capsule. It is important to measure this rate (mR/h) and establish its variation with individual physical parameters such as age, sex, height, weight and body mass index (BMI). In this way, staff and public exposures can be estimated a priori for any future I‐131 therapy given the parameters of the prospective patient. Methods: Over the 2006 to 2010 interval, 156 consecutive individuals were measured for r values. A calibrated ion chamber was used to determine the initial exposure rate at one meter from the navel. Because of HIPPA requirements, only 61 of these patients could have their medical records reviewed for personal information. For this subset of individuals, correlations were made of r/activity with age, sex, height, body mass and body mass index (BMI). Results: The average female initial exposure rate was 0.150 +/− 0.044 mR/(h*mCi) with the male value being 0.154 +/− 0.033 mR/(h*mCi). There was no significant correlation on initial rate/activity with age, sex or height. Significant negative correlation of r/activity was found (p < 0.05) for patient weight (−0.296) and BMI (−0.386). In the former case, the probability was 0.021 and in the latter 0.00214 that the correlation was due to random causes. Conclusions: Mean initial exposure rate/activity for male (24) and female (37) patients were not significantly different. Largest correlation of r/activity was seen with the BMI index with weight being the second strongest correlation. Average patient initial exposure rate/activity at 1 meter decreased from 0.17 mR/(h*mCi) to 0.13 mR/(h*mCi) as BMI increased from 20 to 40 Kg/(m*m). BMI has significant effect on clinical exposure rates in thyroid cancer therapy.