Dosimetric and Contrast Noise Ratio Comparison of Three Different Digital Imaging Technologies in Mammography.

PURPOSE: The purpose of the study was to compare the three most common technologies available in digital mammography based in the evaluation of average glandular dose and contrast noise ratio (CNR). METHOD: The average glandular dose was estimated with a direct digital detector (aSe) with a pixel size of 0.85 µm, a photon counter with a pixel size of 50 µm and a computerized radiography (CR) system. A semiconductor detector was used to measure the input kerma to the detector, and the average glandular dose was calculated from the conversion factors dependent on the anode filter combination, half value layer and breast glandularity. RESULTS: The average glandular dose obtained with polymethyl methacrylate and CNR of mean thickness 4.5 cm using direct digital equipment was 1.02 mGy and CNR = 7.4; using the photon counter it was 0.43 mGy and CNR = 4.7 in C100 mode, and 0.64 mGy and CNR = 5.7 in C120 mode; and using the photostimulable CR, the estimated value was 1.65 mGy and CNR = 5.1. CONCLUSIONS: The photon counter offers a lower average glandular dose than the other two devices with adequate image quality (CNR). The CR equipment offers a similar CNR value but delivers a higher dose than the new generations of available mammograms.

Clinical impact of the detector size effect in 3D-CRT.

OBJECTIVE: The detector size artificially increases the measured penumbra of radiotherapy fields. The aim of this work is to determine the influence of the detector size when planning three-dimensional conformal radiation therapy (3D-CRT) treatments. MATERIAL AND METHODS: Two anatomical sites of interest in 3D-CRT were studied: prostate and hypophysis chordoma. Conventional 3D-CRT treatments for two cases in these locations were planned with a FOCUS 4.0.0 (Computerized Medical Systems, USA) treatment planning system (TPS) equipped with Fast Fourier Transform Convolution (FFTC) and Multigrid Superposition (MGS) algorithms, making use of beams modelled from radiation profiles measured either with a 2.0 mm diameter detector (PFD(3G) diode) or with a 5.5 mm diameter detector (PTW-31002 ionisation chamber). These detectors cover up the range of detector sizes commonly used to measure radiation profiles for 3D-CRT. Dose-volume histograms (DVHs), radiobiological indexes, tumor control probability (TCP) and normal tissue complication probability (NTCP) were analysed and compared for planning target volumes (PTVs) and organs at risk (OAR) studied. RESULTS: Important differences in DVH were found. OAR received higher dose levels when a 5.5 mm detector was used to measure profiles compared to the case in which a 2.0 mm detector was used. A 2 Gy increment in the mean rectal dose was found when the larger detector was used. In the same way, NTCP of brain stem in hypophysis chordoma treatments was doubled when this detector was used. CONCLUSION: The current use of ionisation chambers of about 5 mm active diameter to get the necessary data to model treatment machines in radiotherapy treatment planning systems (TPS) implies a significant overirradiation of OAR close to the PTV in 3D-CRT treatments due to errors in the measured penumbra of beam profiles. To avoid this overirradiation, the measured profiles should either being acquired with a suitable detector size (2-3 mm active diameter) or being deconvoluted.