How the detector resolution affects the clinical significance of SBRT pre-treatment quality assurance results.

PURPOSE: Aim of this work was to study how the detector resolution can affect the clinical significance of SBRT pre-treatment volumetric modulated arc therapy (VMAT) verification results. METHODS: Three detectors (PTW OCTAVIUS 4D 729, 1500 and 100 SRS) used in five configurations with different resolution were compared: 729, 729 merged, 1500, 1500 merged and 1000 SRS. Absolute local gamma passing rates of 3D pre-treatment quality assurance (QA) were evaluated for 150 dose distributions in 30 plans. Five different kinds of error were introduced in order to establish the detection sensitivity of the three devices. Percentage dosimetric differences were evaluated between planned dosevolume histogram (DVH) and patients' predicted DVH calculated by PTW DVH 4D® software. RESULTS: The mean gamma passing rates and the standard deviations were 92.4% ±â€¯3.7%, 94.6% ±â€¯1.8%, 95.3% ±â€¯4.2%, 97.4% ±â€¯2.5% and 97.6% ±â€¯1.4 respectively for 729, 729 merged, 1500, 1500 merged and 1000 SRS with 2% local dose/2mm criterion. The same trend was found on the sensitivity analysis: using a tight gamma analysis criterion (2%L/1mm) only the 1000 SRS detected every kind of error, while 729 and 1500 merged detected three and four kinds of error respectively. Regarding dose metrics extracted from DVH curves, D50% was within the tolerance level in more than 90% of cases only for the 1000 SRS. CONCLUSIONS: The detector resolution can significantly affect the clinical significance of SBRT pre-treatment verification results. The choice of a detector with resolution suitable to the investigated field size is of main importance to avoid getting false positive.

AEC set-up optimisation with computed radiography imaging.

Phototimer set-up is a critical procedure for dose and image quality optimisation in computed radiography (CR) systems. While a conventional radiography automatic exposure control device (AEC) can be calibrated in order to gain a constant optical density on the film independent of beam quality and patient size, CR detectors present a high dynamic range which allows a much larger dose interval, but with different image quality levels. CR leads to a less frequent exam repetition, but may produce quite noisy images if the exposure level on the plate is not correct. The aim of this work is to evaluate the performance of a CR plate (Agfa MD40) in order to optimally calibrate an AEC device. The plate response has been characterised in terms of digital signal, exposure on the plate and signal-to-noise ratio for different beam qualities, in a patient of standard size.

Use of TLD in evaluating diagnostic reference levels for some radiological examinations.

The European Council Directive 97/43 introduces diagnostic reference levels (DRL) for all medical examinations involving ionising radiation. Each department has to evaluate patient dose and to compare that value with the DRL adopted by its member state. Italian law, applying the Directive, states that reference levels must be measured every 2 years. Quantities that must be measured are entrance surface dose or air kerma, or other dosimetric quantities. In our work, clinical measurements on patients were made by positioning a thermoluminescence dosemeter (TLD) over the skin of a statistically significant number of patients for each projection of each examination. As there is no national guideline for these measurements in Italy, the aim of this work was to establish a method based both on European publications and on clinical experience. Three different modalities were considered: conventional radiography, computed radiography and mammography. Accordingly, differently shaped types of TLD were required, especially for mammography where the beam energy is lower.

[Dosimetry in dental radiology: comparison of spiral computerized tomography and orthopantomography]. / Dosimetria nella radiologia odontostomatologica: confronto tra tomografia computerizzata spirale e ortopantomografia.

INTRODUCTION: Dental diagnosis still depends largely on diagnostic imaging for correct anatomical and radiological assessment. Many studies confirm the risk of ionizing radiations, especially if used in pediatric populations and with suboptimal control. We compared the doses absorbed by the dentomaxillary area in Spiral CT and panoramic radiography examinations. MATERIAL AND METHODS: Doses were measured at critical organs in neck, ocular and intracranial regions with lithium fluoride dosimeters calibrated on the national standard and then positioned on an anthropomorphic Rando phantom made of tissue-equivalent material covering a skeleton. Multiple measurements were made during Spiral CT with the Dentascan software and panoramic radiography, to calculate mean absorbed doses for both examinations. Acquisition technical parameters were similar to those used in vivo. RESULTS: The parotid, cerebellum and thyroid gland were the most irradiated organs with panoramic radiography, with the addition of the mandible with Spiral CT. The gonads did not receive major doses. CONCLUSIONS: Our dose measurements demonstrate that patients receive smaller doses with panoramic radiography than with Spiral CT with Dentascan. After allowing for some variations from instrumental differences, they are in substantial agreement with literature data. Further investigations are needed considering the radiobiological risk related to the growing spread of Dentascan examinations.

Possible sources of error in the calculation of ferrokinetic parameters using Ricketts' triexponential model.

The triexponential model developed by Ricketts et al. in 1975 for the calculation of ferrokinetic parameters has been widely used to obtain quantitative information about effective and ineffective erythropoiesis. However, a calculation error in one of the formulae of this method may have a considerable effect on those results which depend on the value of the third exponential constant, gamma 3. Moreover, the analysis of plasma 59Fe activity needs to be based on good counting statistics, which lambda-counting seldom provides after the first 24 h; therefore, 59Fe beta-counting is considered to be the best method.