Volume 29, Issue 12, December 2002
Index of content:
- PH. D. THESES ABSTRACTS
29(2002); http://dx.doi.org/10.1118/1.1525011View Description Hide Description
This thesis presents methods to define and improve the accuracy of treatment planningdose and volume calculations for conformal radiotherapy, especially in regions with the highest expected benefits—the edges and shielded parts of the beams. The context of this thesis is a verification program of a treatment planning system. It is shown that an appropriately designed “combinatorial verification” makes such a program feasible. Results of this thesis apply to megavoltage photon beams in homogeneous and heterogeneous geometries, to proton beams, and to dose–volume representations. A “field accuracy” concept is shown to facilitate simultaneous checks of dose and distance criteria. This concept is applied to pencil beam calculations in partially blocked photon beams. In a lungtumor phantom large dose discrepancies were found between photon pencil beam calculations and film measurements. This is due to improper scatter handling, as is shown by comparison to results of a point spread kernel algorithm. Methods have been developed for high-resolution measurements of photon beam edges with ionization chambers, thus in particular suited for a program of routine verification measurements. Improvements in accuracy of up to 2 mm in the penumbra have been achieved. Dose distributions and penumbras in small proton beams have been investigated with Monte Carlo calculations and a 2D scintillating screen detector. Accuracy of grid-based volume sampling has been studied, yielding the required grid sizes for specific uncertainties in the volume representation of dose–volume histograms. Finally, bringing the concepts of combinatorial verification and field accuracy together, the feasibility of a comprehensive presentation of calculation accuracy in a conformal prostate treatment plan has been demonstrated.
29(2002); http://dx.doi.org/10.1118/1.1510515View Description Hide Description
A radiochromic gel dosemeter in which ionizing radiation causes a color change in the visible wavelength region was produced. The roles of the dosemeter’s active components were quantified with special consideration given to their effect on the system sensitivity and stability. Optimal composition was found to be 0.5 mM ferrous sulfate, 0.1 mM xylenol orange, and 25 mM sulfuric acid. The dose response is linear in the range 0.1–30 Gy and the dosemeter sensitivity was where A is the optical absorbance measured at a wavelength of 585 nm. An optical tomographyscanner with parallel light beam and charged coupled device detector was designed, built, and used to interrogate cylindrical samples of the dosemeter. The current version of the scanner can cope with samples up to 10 cm in diameter limited by the width of the light beam. A maximum of 512×512 pixels per slice images are obtainable with this scanner. Slice thickness as small as 0.14 mm was achieved that is much narrower than the value of about 6 mm achieved when using the MRI method of three-dimensional (3D) readout. The time to acquire a stack of 100 cross-sectional images with a resolution of 128×128 pixels per slice is about 20 min. The 3D imaging performance was tested with collimated x-ray beams, and the scanner high-resolution presentation demonstrated.
29(2002); http://dx.doi.org/10.1118/1.1525012View Description Hide Description
Coughing is a reflex-generated perturbation of the respiratory function and an important clinical symptom in many respiratory diseases. The information contained in the cough-sound is limited due to the lack of an understanding of the origin of the cough sound. This thesis focuses on (1) automated interpretation, (2) quantification, and (3) sound-production. “In vivo” coughing was registered from individual animal (domestic pig Sus Scrofa) and human subjects in normal and pathological respiratory health conditions. The cough-sound is mathematically presented in the frequency domain. Clustering of the mean spectral features results in automated subject-independent cough-sound-recognition. In addition normal and pathological cough-sounds are distinct for both species. Therefore the simple and noninvasive nature of acquiring information using free field cough-sound-registration makes soundanalysis attractive for on-line follow-up and clinical diagnosis. The vibratory activity of the vocal folds is known to be an important issue in sound production. A three-phase pattern is shown to be a generic cough-feature. A global fundamental frequency was determined. Modeling the acoustic cough waveform aims to parametrize the cough-sound-signal for analysis and physical interpretation. Cough formant frequencies are estimated from the model parameters. A first physical interpretation in terms of a concatenate tube model of the vocal tract is presented. All investigated issues, either parametric or nonparametric, show evidence that the cough-sound character is related to the state of both the supra- and subglottal respiratory system. This finding physically and objectively motivates the statement the “cough-sound is a lung-sound.”