Determination of the radiological properties of materials: A new approximation method for calculation of the mass attenuation coefficients.

Dosimetric measurements of the medical LINAC device using phantoms are an important procedure to determine the correct delivery dose on the target volume. The percentage depth dose values of RW3, SW557, ABS, epoxy, PRESAGE phantom materials obtained from a simulation program were compared with those experimentally measured with water phantom at 6 and 18 MV photon energy spectrums. The values of energy spectrums of photons were taken from the database of Elekta Synergy Linear Accelerator device and defined in GATE/Geant4 simulation program. The mean percentage dose differences for all these materials were compared with water phantom and the closest phantom material to water was determined as SW557 at 6 and 18 MV photon energies. The mass attenuation coefficients, the effective atomic numbers and electron densities of water-equivalent phantom materials and some selected human organs were found for both monoenergetic photons and 6, 18 MV photon energy spectrums using GATE simulation code. Furthermore, a new approximation method was developed to calculate the mass attenuation coefficient for the photon energy spectrums. There was determined to be good agreement between the numerical calculations of the approximation method and the simulation results, with a difference of less than 4% between them.

Least Significant Changes and Reproducibility of 131I Uptake Test.

ABSTRACT: Radioactive iodine uptake (RAIU) is one of the important tests performed in the departments of nuclear medicine, testing thyroid function by measuring the amount of 131I uptake after oral administration. A RAIU value measured 4 and 24 h after administration has been widely used for differential diagnosis of thyroid function and for the calculation of treatment dose. This study was performed to define practical methods for reproducibility and least significant change (LSC) values replicating thyroid measurements. In the study were 119 patients referred to the nuclear medicine department for examination of thyroid gland function with the diagnosis of Graves' disease (60), toxic multinodular goiter (29), thyroiditis (10), thyroid cancer (6), and unknown etiology (14). The level of thyroid stimulating hormone (TSH) among the patients was 2.07 ± 6.74 µIU mL-1. RAIU measurements were carried out by two different technicians, who have performed an equal number of measurements from each diagnosis in reference to the clinical diagnosis. Measurement of each patient was performed twice at 4 and 24 h after the administration under stable geometry and counting conditions using a standard procedure. Data were evaluated using statistical methods. For assessment of the reproducibility, three parameters were used: reproducibility coefficient (RC), the root-mean-square standard deviation (SDRMS), and the least significant change values. The average RAIU values of the first and second measurements were found as 23.71 ± 16.52% and 23.94 ± 16.64% at 4 h (p >0.05), and 35.33 ± 19.22% and 35.49 ± 19.19% at 24 h (p >0.05), respectively. For thyroid uptake values repeated at 4 and 24 h after radioiodine administration, the mean difference was found to be -0.24 ± 0.62% [limits of agreement (%); -1.44 to 0.97] at 4 h and -0.16 ± 0.44% [limits of agreement (%); -1.02 to 0.70] at 24 h. Confidence intervals were within the limits of agreement (d̅-1.96 SD and d̅+1.96 SD). When the correlation between the repeated RAIU measurements was examined taking the differences and averages into account, there was a negative correlation between 4-h measurement pairs (r=-0.203, p <0.05). On the other hand, a significant correlation was not found between the 24-h measurement pairs (r=0.074, p >0.05). RC, SDRMS, and LSC were calculated as 0.70%, 0.46%, and 1.29% for the 4 h measurements and 0.54%, 0.33%, and 0.91% for the 24-h measurements, respectively. Although there was a minor difference between measurement pairs at 4 and 24 h post-administration, the difference appeared to be insignificant without an apparent effect in the clinical settings.

The efficacy of patient-dependent practices on exposure rate in patients undergoing iodine-131 ablation.

The authors investigated the efficacy of the patient-dependent practices (micturition, defecation, and taking a shower) after I ablation therapy in patients with thyroid cancer. Exposure rates were measured from a 1-m distance at 2, 4, and 24 h after the I administration. The comparisons for 2-4 h and 2-24 h time intervals were carried out among the patients with good and poor compliance, according to the relative amount of the patient-dependent practices, over mean values calculated for an effective half-life (h) or exposure rate decline ratio (%). In the 2-4 h interval, the mean values for good versus poor compliance were found to be: 4.16 h versus 8.14 h and 31% versus 17% in micturition; 5.70 h versus 6.84 h and 26% versus 21% in defecation; and 6.68 h versus 5.69 h and 24% versus 24% in taking a shower. Among patients with good versus poor compliance during a 2-24 h interval, the mean values were: 11.63 h versus 16.62 h and 74% versus 61% in micturition; 12.75 h versus 15.88 h and 71% versus 63% in defecation; and 13.77 h versus 14.21 h and 68% versus 67% in taking a shower.

Importance of bladder radioactivity for radiation safety in nuclear medicine.

OBJECTIVE: Most of the radiopharmaceuticals used in nuclear medicine are excreted via the urinary system. This study evaluated the importance of a reduction in bladder radioactivity for radiation safety. METHODS: The study group of 135 patients underwent several organ scintigraphies [40/135; thyroid scintigraphy (TS), 30/135; whole body bone scintigraphy (WBS), 35/135; myocardial perfusion scintigraphy (MPS) and 30/135; renal scintigraphy (RS)] by a technologist within 1 month. In full and empty conditions, static bladder images and external dose rate measurements at 0.25, 0.50, 1, 1.5 and 2 m distances were obtained and decline ratios were calculated from these two data sets. RESULTS: External radiation dose rates were highest in patients undergoing MPS. External dose rates at 0.25 m distance for TS, TKS, MPS and BS were measured to be 56, 106, 191 and 72 µSv h-1 for full bladder and 29, 55, 103 and 37 µSv h-1 for empty bladder, respectively. For TS, WBS, MPS and RS, respectively, average decline ratios were calculated to be 52%, 55%, 53% and 54% in the scintigraphic assessment and 49%, 51%, 49%, 50% and 50% in the assessment with Geiger counter. CONCLUSION: Decline in bladder radioactivity is important in terms of radiation safety. Patients should be encouraged for micturition after each scintigraphic test. Spending time together with radioactive patients at distances less than 1 m should be kept to a minimum where possible. CONFLICT OF INTEREST: None declared.