Biomedical subjects

P H Vos

Publications and source records attributed to P H Vos.

6 recordsLinked to original sources

PubMed1996

High-precision prostate cancer irradiation by clinical application of an offline patient setup verification procedure, using portal imaging.

PURPOSE: To investigate in three institutions, The Netherlands Cancer Institute (Antoni van Leeuwenhoek Huis [AvL]), Dr. Daniel den Hoed Cancer Center (DDHC), and Dr, Bernard Verbeeten Institute (BVI), how much the patient setup accuracy for irradiation of prostate cancer can be improved by an offline setup verification and correction procedure, using portal imaging. METHODS AND MATERIALS: The verification procedure consisted of two stages. During the first stage, setup deviations were measured during a number (Nmax) of consecutive initial treatment sessions. The length of the average three dimensional (3D) setup deviation vector was compared with an action level for corrections, which shrunk with the number of setup measurements. After a correction was applied, Nmax measurements had to be performed again. Each institution chose different values for the initial action level (6, 9, and 10 mm) and Nmax (2 and 4). The choice of these parameters was based on a simulation of the procedure, using as input preestimated values of random and systematic deviations in each institution. During the second stage of the procedure, with weekly setup measurements, the AvL used a different criterion ("outlier detection") for corrective actions than the DDHC and the BVI ("sliding average"). After each correction the first stage of the procedure was restarted. The procedure was tested for 151 patients (62 in AvL, 47 in DDHC, and 42 in BVI) treated for prostate carcinoma. Treatment techniques and portal image acquisition and analysis were different in each institution. RESULTS: The actual distributions of random and systematic deviations without corrections were estimated by eliminating the effect of the corrections. The percentage of mean (systematic) 3D deviations larger than 5 mm was 26% for the AvL and the DDHC, and 36% for the BVI. The setup accuracy after application of the procedure was considerably improved (percentage of mean 3D deviations larger than 5 mm was 1.6% in the AvL and 0% in the DDHC and BVI), in agreement with the results of the simulation. The number of corrections (about 0.7 on the average per patient) was not larger than predicted. CONCLUSION: The verification procedure appeared to be feasible in the three institutions and enabled a significant reduction of mean 3D setup deviations. The computer simulation of the procedure proved to be a useful tool, because it enabled an accurate prediction of the setup accuracy and the required number of corrections.

Feasibility Studies↗

PubMed1994

The accuracy of field shape definition using standard shielding blocks and the consequences for field shape verification.

The accuracy of field shaping using standard shielding blocks has been investigated in 157 fields in the thoracic region, the majority for treatment of lung cancer. Accuracy was defined as the width of the tolerance margin around the prescribed field outline that would be needed to accept the full megavoltage field boundary. The use of several positioning aids was evaluated, including a cardboard template that was created with the data already specified for field shape verification of megavoltage fields. Block positioning via tattoos on the skin gives large random field shape errors: assuming a tolerance margin of +/- 5 mm, approximately 50% of these fields would have to be rejected. When a transparent sheet with a drawing of the required field outline was used as positioning aid on the tray without an explicit quality control of the sheet itself approximately 24% of the fields would not be accepted because of an error in the sheet (systematic error) and approximately 20% because of a discrepancy in block position relative to the actual prescription of the sheet (random error). If block positioning is performed via a cardboard template of which the prescription was checked to be better than 3 mm measured in the plane of the isocentre, approximately 96% of the fields will be accepted assuming a tolerance margin of +/- 5 mm: the risk of misplacement of blocks appeared to be very low. Integration of prescription, preparation of the cardboard template, quality control of the template and verification of the megavoltage field shape results in an easy, accurate and reliable way of field shape definition using standard blocks.

Equipment Design↗

PubMed1986

Thallium myocardial perfusion scintigraphy: influence of perfusion, scatter, and photon energy on the detection of lesions.

Experimental studies have been made of lesion detectability in myocardial perfusion studies using thallium-201. A series of images (AP view) was generated mathematically. Images were simulated with 100k, 200k, and 300k counts for the complete image and either non- or half-perfused lesions (lesion volume 1-6 1/2% of total myocardial volume). In addition thallium images were generated excluding scatter; images were also generated with a radiopharmaceutical using 140, 184 and 296 keV, assuming the biological distribution of thallium. All images were interpreted by seven observers. Count density did not affect the true-positive fraction of the interpretations. However, the false-positive fraction decreased significantly when count density increased. Lesions located far from the camera were detected worst. The intraobserved variability was largest for interpretation of these regions. Small lesions were poorly detected. Perfusion of lesions significantly affected their detection, independent of the count density, lesion volume, and lesion location. Scatter did not affect the detection of lesions. No significant difference was found when the performance of the observers was compared for 80 keV and 140 keV. The use of 184-keV and 296-keV photons resulted in a lower detection rate probably because of the use of a high energy collimator.

Coronary Circulation↗

PubMed1984

Observations on computerized quantitative bone scintigraphy in renal osteodystrophy.

Skeletal radiotracer (99mTc-HEDP) uptake was quantitated with and without the aid of a computer in 30 chronic dialysis patients with histologic evidence of renal osteodystrophy. Before scintigraphy, elevated soft-tissue activity due to the absence of renal radiotracer excretion was reduced by hemodialysis. The results were compared with those of a normal group and with the results of the biochemical and the bone morphometric studies of these patients. In all patients the radiotracer uptake was elevated, often markedly. In several patients with minimal histologic bone disease, however, soft-tissue activity could not be normalized by hemodialysis although its influence on the quantitative data could be further reduced (but not excluded) by computer evaluation of skeletal radiotracer uptake. Since the latter technique clearly distinguished the majority of the patients from the normals, it appears that computerized quantitative skeletal analysis is a potentially accurate scintigraphic method for detecting renal osteodystrophy. The significant relationship between skeletal radiotracer uptake, in particular at the bone biopsy site, and only the histologic features of increased bone turnover suggest that hyperparathyroidism is the major cause of this increased tracer uptake in renal osteodystrophy.

Adolescent↗

PubMed1983

Quantitative assessment of wall motion in multiple-gated studies using temporal Fourier analysis.

We described a method for quantification of left-ventricular wall motion in multiple-gated studies, based on approximation of local time-activity curves by the first harmonic of the corresponding Fourier spectrum. The Fourier transform is adapted to the global left-ventricular time-activity curve by shortening the length of the base period. The degree of shortening depends on the corresponding power in the second and higher harmonics; the base period that resulted in minimal power in the higher harmonics was selected for a Fourier transform of the regional curves. This usually results in exclusion of both the diastasis phase and the left-atrial filling phase from the calculations. With a model of the left-ventricular volume curve it is shown that important advantages of this adaptation are (a) a better approximation of the maximum difference in the curves, and (b) less spread in the phase determination. Wall motion was related to the Fourier parameters. The combination of the mean amplitude and the standard deviation of the phase histogram gives the best results for global analysis of the motion of the wall.

Diastole↗

PubMed1982

Detection of lesions in thallium-201 myocardial perfusion scintigraphy.

Experimental studies have been made of the lesion detectability in myocardial perfusion studies using thallium-201. A series of images (AP-view) was generated using a convolution of a mathematical model of the left ventricular myocardium and an experimentally determined point spread function. Background was added. Images were simulated with 100 k, 200 k, and 300 k counts for the complete image. Each image contained a lesion with either 0% or 50% of the normal tracer concentration. All images were interpreted by five experienced observers, independently of each other. Their interpretations were analysed using the Kolmogorov-Smirnov two sample test. The true positive fraction (TPF) was hardly affected by changes in count density. The TPF decreased significantly if the lesion tracer concentration changed from 0% to 50% of the normal myocardial tracer concentration. The decrease was independent of the count density. The false positive fraction (FPF) decreased significantly if the count density increased; no difference in FPF was found for a change in lesion tracer concentration. In addition, plots were generated with the TPF versus lesion volume. The TPF was lowest for locations far from the camera; FPF was high for these segments. Increase of the count density mainly improved the FPF in these segments. Small lesion were more difficult to detect.

Heart Diseases↗