Exposure Rate in Hot Waiting Area of Small but Busy Nuclear Medicine Department: “Meter Matters”

Abstract

Background: Time, distance and shielding are the 3 effective methods to keep radiation dose to radiation workers. However, in small but busy nuclear medicine department, efficacy of distance from injected patients staying in hot waiting area is questioned. The aim of this study was to find out the exposure rate in hot waiting area of a busy nuclear medicine department and measuring of radiation dose to NM technologists them during study period.

Material and Methods: This was a prospective study conducted from 1^st November 2015 till 31^st December 2015 at Nuclear Medicine Department of Dr Ziauddin Hospital Karachi, Pakistan, having an area of 240 square meters and equipped with 01 gamma camera. Exposure rate was measured on all patients at 1 meter distance from anterior mid trunk in sitting position using digital rate meter (Radiation Alert Inspector, S.E. international, INC). Observed measurements of exposure rate were carried out first at 1 meter distance from sitting patients in hot waiting area followed by second measurements by doubling the distance at 2 meters to verify the inverse square law in injected patients. We also measured a distance (in meter) at which exposure rate 1 meter declined to 50% from initial values. In order to estimate agreements of observed measurements with expected values, the expected measurements of exposure by doubling distance and increase in distance for 50% reduction in exposure from initial values were also calculated by commercially available calculators of inverse square law (Rad Pro Calculator). During the study period we have also measured the personal dosimetry of 03 technologists using film badges.

Results: Study consisted of 150 consecutive patients referred for Nuclear Medicine procedures with mean age 50 ± 19 (0.25-87 Yr) years, F: M, 60%:40% with average body mass index (BMI) of 26.962 ± 6.861 Kg/m2. Average dose of the procedures was 434 ± 207 with a range of 37-851 MBq. The procedure distribution was 29% stress first myocardial perfusion scintigraphy, 53% skeletal, 15% thyroid and 03% renal scintigraphy respectively. The average exposure rates were 9.784 ± 5.761 (range: 1.477-27.808) and 2.329 ± 1.355 (range: 0.347-6.778) uSv/hr at 1 meter and at 2 meters by doubling the distance respectively.  The distance measured at which 50% reduction in exposure rate from initial value was an average 1.321 ± 0.169 meters. Comparative analysis of expected (by Inverse Square Law; ISL) and measured values for exposure and distance were found non-significant difference (p=0.452) in exposure rates by doubling the distance with 75% reduction in exposure rates in both expected and measured values.  23% ± 10% increase in a distance was measured for 50% reduction in initial exposure rate which significantly lower than expected values i.e. 29% ± 03% (average 1.421 ± 0.082) as calculated by ISL (p <0.0001). The Bland Altman’s analysis illustrated a good agreements between expected and measured values of exposure rate by doubling the distance to follow ISL and by increasing the distance to decline exposure to 50% respectively. The personal dosimetry record of 03 technologists during study period was i.e. average 0.140 ± 0.055 mSv over two months period

Conclusion: We conclude that radiation exposure from injected patients in nuclear medicine follows Newton’s inverse square law although patients are not point source. By practicing all cardinal rules of ALARA, including distance even in a small but busy department, radiation dose to NM technologists will remain well within statutory limits.

Key words: inverse square law; radiation exposure; waiting area; distance; nuclear medicine