A study to establish international diagnostic reference levels for paediatric computed tomography.

The article reports results from the largest international dose survey in paediatric computed tomography (CT) in 32 countries and proposes international diagnostic reference levels (DRLs) in terms of computed tomography dose index (CTDI vol) and dose length product (DLP). It also assesses whether mean or median values of individual facilities should be used. A total of 6115 individual patient data were recorded among four age groups: <1 y, >1-5 y, >5-10 y and >10-15 y. CTDIw, CTDI vol and DLP from the CT console were recorded in dedicated forms together with patient data and technical parameters. Statistical analysis was performed, and international DRLs were established at rounded 75th percentile values of distribution of median values from all CT facilities. The study presents evidence in favour of using median rather than mean of patient dose indices as the representative of typical local dose in a facility, and for establishing DRLs as third quartile of median values. International DRLs were established for paediatric CT examinations for routine head, chest and abdomen in the four age groups. DRLs for CTDI vol are similar to the reference values from other published reports, with some differences for chest and abdomen CT. Higher variations were observed between DLP values, based on a survey of whole multi-phase exams. It may be noted that other studies in literature were based on single phase only. DRLs reported in this article can be used in countries without sufficient medical physics support to identify non-optimised practice. Recommendations to improve the accuracy and importance of future surveys are provided.

Poster - Thur Eve - 68: Evaluation and analytical comparison of different 2D and 3D treatment planning systems using dosimetry in anthropomorphic phantom.

PURPOSE: The aim of this study was to evaluate and analytically compare different calculation algorithms applied in our country radiotherapy centers base on the methodology developed by IAEA for treatment planning systems (TPS) commissioning (IAEA TEC-DOC 1583). MATERIAL & METHODS: Thorax anthropomorphic phantom (002LFC CIRS inc.), was used to measure 7 tests that simulate the whole chain of external beam TPS. The dose were measured with ion chambers and the deviation between measured and TPS calculated dose was reported. This methodology, which employs the same phantom and the same setup test cases, was tested in 4 different hospitals which were using 5 different algorithms/ inhomogeneity correction methods implemented in different TPS. The algorithms in this study were divided into two groups including correction based and model based algorithms. RESULTS: A total of 84 clinical test case datasets for different energies and calculation algorithms were produced, which amounts of differences in inhomogeneity points with low density (lung) and high density (bone) was decreased meaningfully with advanced algorithms. The number of deviations outside agreement criteria was increased with the beam energy and decreased with advancement of the TPS calculation algorithm. CONCLUSION: Large deviations were seen in some correction based algorithms, so sophisticated algorithms, would be preferred in clinical practices, especially for calculation in inhomogeneous media. Use of model based algorithms with lateral transport calculation, is recommended. Some systematic errors which were revealed during this study, is showing necessity of performing periodic audits on TPS in radiotherapy centers.

3D calculation of absorbed dose for 131I-targeted radiotherapy: a Monte Carlo study.

Various methods, such as those developed by the Medical Internal Radiation Dosimetry (MIRD) Committee of the Society of Nuclear Medicine or employing dose point kernels, have been applied to the radiation dosimetry of (131)I radionuclide therapy. However, studies have not shown a strong relationship between tumour absorbed dose and its overall therapeutic response, probably due in part to inaccuracies in activity and dose estimation. In the current study, the GATE Monte Carlo computer code was used to facilitate voxel-level radiation dosimetry for organ activities measured in an (131)I-treated thyroid cancer patient. This approach allows incorporation of the size, shape and composition of organs (in the current study, in the Zubal anthropomorphic phantom) and intra-organ and intra-tumour inhomogeneities in the activity distributions. The total activities of the tumours and their heterogeneous distributions were measured from the SPECT images to calculate the dose maps. For investigating the effect of activity distribution on dose distribution, a hypothetical homogeneous distribution of the same total activity was considered in the tumours. It was observed that the tumour mean absorbed dose rates per unit cumulated activity were 0.65E-5 and 0.61E-5 mGY MBq(-1) s(-1) for the uniform and non-uniform distributions in the tumour, respectively, which do not differ considerably. However, the dose-volume histograms (DVH) show that the tumour non-uniform activity distribution decreases the absorbed dose to portions of the tumour volume. In such a case, it can be misleading to quote the mean or maximum absorbed dose, because overall response is likely limited by the tumour volume that receives low (i.e. non-cytocidal) doses. Three-dimensional radiation dosimetry, and calculation of tumour DVHs, may lead to the derivation of clinically reliable dose-response relationships and therefore may ultimately improve treatment planning as well as response assessment for radionuclide therapy.

Doses to the scanned individual and to the operator from an X-ray body scanner system.

The X-ray body scanner (BS) is going to find common use as a body-checking equipment at the entrance borders of countries, to find illicit drugs or forbidden items which have been hidden inside the body cavities, or attached to the body parts of the passengers. Considering the tissue weighting factors of the sensitive organs, the total effective dose due to the scanning by the system was estimated to be 3.8 µSv per scan. The ambient dose equivalent rates within the distance range of 300-40 cm from the X-ray generator were measured to be 4.5 up to 50 µSv h(-1). It is concluded that, in general, BS systems could be a safe device for the operators and people who are being scanned. But using such systems should be justified for everybody and special care should be taken for children and pregnant ladies.

Evaluation of patient dose in some mammography centres in Iran.

High diagnostic sensitivity and specificity while maintaining the least dose to the patient is the ideal mammography. The objective of this work was to evaluate patient dose and image quality of mammograms to propose corrective actions. The image quality for 1242 patient in 7 mammography facilities in Tehran city was evaluated based on selected image quality criteria using a three-point scale. Clinical image quality, the entrance surface air kerma, the average glandular dose and optical density of films for standard PMMA phantom of 4.5 cm thickness were evaluated. The results showed that up to 72 % of mammograms were in good condition to be diagnosed, and only about 3.4 % of the images were unacceptable or with suboptimal quality. The entrance surface air kerma values were in the range of 3.8-10.5 mGy, average glandular dose 0.5-1.8 mGy and optical density of films 0.74-2.03. The image quality evaluation after correction actions, periodic image quality evaluation and using the correct equipment certainly will improve patient dose.

Criteria for patient release according to external dose rate and residual activity in patients treated with 131I-sodium iodide in Iran.

External dose rate (ED) and residual activity (RA) of patients treated with (131)I sodium iodide are two main factors, to consider before release of patients. In this study, six nuclear medicine centres out of total seven centres in Iran were selected and measurements have been done on 330 patients. ED of patients was measured by physicists of the centres for 6 months (May-November 2009) at a 1 m distance from the thyroid of each patient on the first, second and third days after administration by a calibrated survey meter. The maximum and minimum values of ED were 21 (SD = 18) and 11 (SD = 4) µSv h(-1), respectively. Furthermore, the maximum and minimum values of RA during release of patient were 720 and 250 MBq, respectively. According to the study, we recommend a release activity limit of 500 MBq (14 mCi) or a dose rate level of 20 µSv h(-1) at 1 m from the patient to be set instead of a release activity limit of 1100 MBq (30 mCi) as it is now for the country.

An investigation on the response of PADC detectors to neutrons.

The response of PADC was calculated by Monte Carlo simulation using MCNPX code. H, C and O recoils as well as nuclear reactions have been taken into account. The calculated results show important contribution to detection efficiency of PADC due to H, C and O recoils. The irradiated detectors with (241)Am-Be source and (241)Am-Be softened with a spherical polyethylene moderator were electrochemically etched. The paper presents the results of the experimental and Monte Carlo simulations, which are in a good agreement.