Determination of residual activity of 99 metastable(99m Tc) in Urine Samples of Patients at University Teaching Hospital, Lusaka, Zambia

Abstract

INTRODUCTION :Ionizing radiation is the largest contributor to human exposure from manmade sources of radiation. Diagnostic and therapeutic procedures are routinely carried out in Zambia and patients are allowed home without measurement of the remaining amount of tracer in the body. Pharmacokinetic profiles of tracers in the body differ between individuals and in between 99metastable Technetium (99mTc) containing tracers. The pharmacokinetics of radioactive isotopes is dependent on the rate of decay which is characteristic to the isotope. Decay of radioactive elements in a biological system occurs by both biological and physical means and each of these have a distinct biological (Tb) and physical half-life (Tp). The effective half-life (Te) is a combination of both the biological and physical half-lives. The effective half life is used to determine whether to allow home or to quarantine the patient after scanning with 99mTc containing tracers. The aim of the study was to determine the residual activity of 99mTc in urine samples of patients at the University Teaching Hospital Department of Nuclear Medicine. The study also sought to establish whether the biological and effective half-life of 99mTc was affected when different pharmaceuticals were added to 99mTc for various scanning procedures. METHODOLOGY: A prospective cross-sectional study that involved measuring amount of radioactivity from an on the spot urine sample of 22 patients with age ranging from 33 to 95 years who had just been scanned with 99mTc-MDP for bone metastases due to cancer of the prostate and cancer of the breast (15 males and 7 females respectively) and 13 patients (all females with age ranging from 22 to 51 years) scanned with sodium pertechnetate (NaTcO4-) for thyroid disorders. Pearson’s correlation coefficient (r) was used to establish the relationship between independent and dependent variables (i.e. administered dose and biological and effective half life respectively).A p value < 0.05 was considered statistically significant. PROCEDURE AND DATA COLLECTION 99mTc was eluted from the Molybdenum/Technetium generator and activity was measured after each elution. A pharmaceutical was added using aseptic technique according to the scan required i.e. methylene diphosphonate added to 99mTc solution, saline added to 99m-Tc solution and dimercapto succinic acid added to 99mTc solution. Patients were encouraged to hydrate before and after scanning process with 500-750ml of water. The activity to be administered was then withdrawn from the eluate vial and administered aseptically via the intravenous route. Patients submitted on the spot urine samples immediately after each procedure and the activity measured instantly. Subsequent activity measurements were done at 30 minutes, 1 h, 1.5 h, 2 h, and 2.5 h using the same sample. Date of collection and volume of urine were also recorded. Biological half-life was determined using the first order reaction equation; Tb= 0.693/ kd. Where, Tb is the biological half-life and kd is the biological decay constant obtained from the slope of the log activity vs. time curve which was linear. The slope which represented the decay constant (kd) was determined from x and y values on the log (ln) residual activity vs. time graph as follows; kd= y2-y1/x2-x1 Effective half-life was then calculated using the formula; 〖 T〗_e=(Tb×Tp)/(Tb+Tp) RESULTS: The mean residual activity for 99m Tc-MDP and NaTcO4- was (14.56 MBq and 3.84 respectively) The mean administered dose of 99mTc-MDP was correlated with the mean biological half life Tb (r = 0.096) and mean effective half life Te (r = -0.39). The mean administered dose of NaTcO4- was correlated with mean biological half life Tb (r = 0.073) and mean effective half life Te (r = 0.227).The mean Tb of 99mTc-MDP (5.57 hrs, SD 0.962) was 5.4% longer than that of Na-TcO4-(5.27 hrs) and the mean Te (2.88 hrs, SD 0.32) was 3.5 % longer than that of Na-TcO4-(2.78hrs) The mean Te of 99mTc-MDP(5.81 hrs, SD 0.618, 95% CI 4.57-7.04) in patients with bone metastases due to cancer of the prostate was 9.3% longer than in patients with thyroid disorders ( 5.27 hrs, SD 1.33 95%CI 2.66-7.87). The mean Tb in males with prostate cancer was longer than mean Tb in females with breast cancer. CONCLUSION :The mean residual activity of 99mTc-MDP and NaTcO4- in urine was low (14.46 and 3.84 MBq respectively) and it varied between patients and was independent of the administered dose. Tb varied between individual patients and in between 99mTc-MDP and NaTcO4- tracer groups Te and Tb was affected by the addition of different pharmaceuticals to 99mTc In patients with thyrotoxicosis (Grave’s disease) some doses of sodium pertechnetate were undetectable in urine samples. This was due to complete uptake of the NaTcO4- by the toxic thyroid gland.