Correction Factors for More Accurate Estimates of Exposure Rates Near Radioactive Patients: Experimental, Point, and Line Source Models

Abstract

Radioactive patients may expose others after radiopharmaceutical administrations, and evaluation of the absorbed dose or exposure rates close to patients is important in keeping radiation doses as low as reasonably achievable. Two theoretical exposure models, point source and line source models, are frequently used to calculate exposure or dose rates without the support of actual measurements. If measurements of exposure rates were performed near patients, an experimental exposure model could be implemented. When measurements of exposure rates are performed, these measurements are made inside therapy rooms or other confined places, in which case scattered radiation may significantly influence the measurements. In this study we measured exposure rates from radioactive patients without the influence of scattered radiation and determined correction factors for the theoretical exposure models. The exposure rates from a total of 110 radioactive patients were measured at 1.0 h after oral administration of Na131I for thyroid therapy; the results +/-1 SD at distances of 0.5, 1.0, 1.5, 2.0, 3.0, and 4.0 m in front of the patients were (29 +/- 6), (9.9 +/- 1.7), (4.6 +/- 0.9), (2.7 +/- 0.5), (1.31 +/- 0.25) and (0.74 +/- 0.12) x 10(-10) C kg(-1) MBq(-1) h(-1) [1.0 x 10(-10) C kg(-1) MBq(-1) h(-1) = 14.34 x 10(-6) R mCi(-1) h(-1)], respectively. To obtain more accurate estimates of the actual exposure rates from patients using the theoretical exposure models, we found that correction factors should be applied; the functions CFEM = 1.19 + 32.80e(5.92D) and CFLS = 0.022LnD + 0.639 describe these correction factors for distances less than or equal to 1.0 m from the patients for experimental and line source exposure models, respectively. The function that describes the correction factors to the point source model is CFPS = 0.224LnD + 0.638 at the same distances; applying these correction factors leads to a reduction from 56% to 1% in the difference between measured exposure rates and theoretical exposure rates calculated by the point source exposure model at a distance of 1.0 m from patients. The results given here provide more accuracy in evaluation of exposure rates and consequently absorbed doses near radioactive patients and allow for more effective radiological protection procedures during patient management.

Comparing Dose Rates Near a Radioactive Patient Evaluated Using Various Source Models

“…The result revealed that the scattered radiation from the concrete walls, floor, and ceiling of the room contributed 27.5% of the total dose rate at a distance of 1 m from the 131 I patient in the room, thus to a large extent explaining the systematic underestimation of the line source predictions. This roomscattering contribution was greater than that reported by Willegaignon et al (2007) because the measurements were performed near a corner of the room. The results of this study also indicated that the contribution of room scattering can be reduced to only approximately 3.8% if all the inner surfaces of the room are covered with a sheet of lead.…”

“…De Carvalho et al (2011) presented a comparison of point, line, and phantom dose rate estimates around radioactive patients. For three therapy types, the point source model overestimated the phantom dose rate source model at ratios of 1.45-2.10 at 1 m, and the line source model exhibited improvements and overestimated by approximately 20-70% the dose rates at 1 m. According to Willegaignon et al (2007) and Yi et al (2013), irrespective of the source model used, the calculated dose rates typically exceeded the measured results. They have suggested "correction factors" or "self-shielding factors" for correcting the overestimation and increasing the dose estimates' accuracy.…”

“…Siegel et al did not compare the accuracy of the line source model with more sophisticated and realistic models or measurements. Based on comparisons between measurements and theoretical models, Willegaignon et al (2007) proposed a set of correction factors for the point and line source models to increase the accuracy of estimates of the dose rates near nuclear medicine patients. At a distance of 1 m, the correction factors were determined to be 0.638 and 0.639 for theoretical evaluations conducted using the point and line source models, respectively.…”

“…They have suggested "correction factors" or "self-shielding factors" for correcting the overestimation and increasing the dose estimates' accuracy. For two theoretical models, Willegaignon et al (2007) determined correction factors of 0.6 at 1 m. Yi et al (2013) reported that the self-shielding factors of the point and line source models for three radionuclides ( 99m Tc, 18 F, and 131 I) at 1 m ranged between 0.44-0.60 and 0.55-0.73, respectively.…”

Radiation Safety Considerations for the Use of 223RaCl2 DE in Men with Castration-resistant Prostate Cancer

“…As expected, this is less than the conservative theoretical exposure rate constant for an unshielded point source of 223 Ra in equilibrium with progeny of approximately 0.05 μSv h −1 MBq −1 (Smith and Stabin 2012). The measurement of dose rates in this low range can be affected by statistical variability, background fluctuations (especially in an active nuclear medicine department), and geometry (Willegaignon et al 2007; de Carvalho et al 2011). For example, in one patient, the normalized exposure rate at 1 m was measured as 0.1 μSv h −1 injected MBq −1 immediately following administration, a value that was likely artificially high due to background fluctuations in the nuclear medicine injection area during the measurement.…”