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Biomedical subjects

H K Leetz

Publications and source records attributed to H K Leetz.

At least 19 recordsLinked to original sources
PubMed1995

The radiation exposure of the patient from stable-xenon computed tomography.

For stable-xenon computed tomography (CT), an X-ray examination for measurement of cerebral blood circulation in the brain, the radiation exposure of the patient was determined in order to estimate the risk of inducing cancer. Organ doses of brain, eyelenses, thyroid and gonads have been calculated using the measured air kerma free-in-air on the axis of rotation and organ-specific conversion factors calculated with the Monte Carlo method. Dose measurements with TLD-100 rods using a humanoid Alderson phantom were carried out for verification of the calculated organ doses. In the case of brain partially located in the region of primary radiation a mean organ dose of 39 mSv was calculated. The dose measurements showed dose equivalents between 6 and 68 mSv in different regions of the brain and consequently an inhomogeneous dose distribution. From an estimation of the radiation-induced risk using the effective dose of 1.6 mSv it follows that one additional fatal cancer per 12,500 stable-xenon CT examinations has to be expected. The organ doses of eyelenses and thyroid located in the region of scattered radiation are so low that biological effects are hardly to be expected. The calculated dose equivalents of 6.5 mSv and 0.5 mSv, respectively, are in good agreement with measurements. The organ dose of gonads amounted to less than 0.07 mSv.

Brain↗
PubMed1994

[The determination of the ovarian dosage in defecography on a digital C-arm unit].

When using defaecography as a radiological tool for diagnosing the anorectal function in female patients, the ovaries are inevitably directly exposed. With the aim of minimising the ovarian dose applied both the area-dose product and the surface dose above the ovaries were measured during investigations of female patients with a digital C-arm unit. These values were converted into ovarian doses using tissue-air ratios. From the surface dose measured with TLD-100 a mean ovarian dose of 15.6 +/- 8.6 mGy was derived. The contributions of screening and film exposures were similar. A reduction of screening time by 10 seconds or omission of 60 film exposures results in a decrease of ovarian dose by values between 0.7 and 2 mGy. Employing a greater focus-skin distance during defaecography, however, made it possible to reduce the ovarian dose by 26%.

Anal Canal↗
PubMed1990

[Monte Carlo calculations and TLD measurements to determine the two-dimensional dosage distribution of 125-iodine seed, Model 6702].

The angular and radial dose distribution around a type 6702 125-Iodine-seed was measured in water and air using thermoluminescence dosimetry. The radial distribution could be confirmed using Monte Carlo simulation techniques. The simulation produced enough data to allow the derivation of a new analytical function describing the radial dose distribution.

Computer Simulation↗
PubMed1989

[Uncertainties in the localization of radiation sources in brachytherapy].

While investigating ways to solve the problem of locating radiation sources applied in brachytherapy a device was constructed which enables a precise assessment of their or their applicators' position from two only roughly positioned orthogonal radiographs. The overall accuracy (including exposure and evaluation) achieved is +/- 1 mm. The constructional elements and the evaluation algorithm are described and the influence of the location uncertainty on the uncertainty of the dose applied are discussed.

Algorithms↗
PubMed1988

Plastic material used to optimize radiotherapy of head and neck tumors and the mammary carcinoma.

In order to improve head and neck tumor therapy, face masks were developed. The physical and mechanical properties of 11 apparently suitable materials were tested using a phantom. According to our studies "HEXCELITE" (supplied by Medimex, Hamburg, F.R.G.) proved to be the best material. Plastic breast molds were made to optimize the dose distribution for radiation therapy with fast electrons of post-mastectomy breast tumors. Here too the mechanical and physical properties of nine different materials were tested. The most suitable of these proved to be the gel mat "PRIMAMED" (supplied by Schülke and Mayr, Norderstedt, F.R.G.). The two materials mentioned have been well tolerated by more than 200 patients.

Bandages↗
PubMed1988

[A simple device for positioning and fixing the head in radiotherapy of brain tumors in frontal and lateral position].

In radiotherapy of cerebral tumors in the occipital, central, and parietal area it can be favourable to fix the patient's head in frontal or lateral position. It is true that this problem can be solved by manufacturing individual face masks of PVC with the vacuum deep-drawing method, but this method is very time- and cost-consuming. The present study suggests a simplified method using the thermoplastic polyester "Orfit". Such a mask provides a good immobilization and can be manufactured in 10 to 15 minutes without bothering much the patient. Simulation and computed tomography are not substantially affected by the material.

Brain Neoplasms↗
PubMed1987

[Systematic research on the dose distribution in mantle field irradiation with Co60 gamma radiation in the Alderson phantom].

A mantle field is localized according to the patient's data in an inhomogenous Alderson phantom. After having established the irradiation scheme, the dose distributions measured in the Alderson phantom are compared with those measured and calculated in an homogenous water phantom. The possible side effects of this irradiation technique can be assessed on the basis of the radiation exposure of heart, lung and spinal marrow. The dose differences found on the central ray are within the margin of error amounting to 5 to 10% which is indicated in the calculation program. In case of a total reference dose of 40 Gy, the radiation exposures of heart and lung do not reach the tolerance limit. A spinal marrow dose of 50 Gy is found in the upper neck marrow. In case of an exact therapy planning, a sufficient dose can be directed to the Waldeyer's tonsillar ring.

Cobalt Radioisotopes↗
PubMed1986

[Chest wall moulages for radiotherapy of resected breast cancer with fast electrons: comparative tests of different materials].

Irradiation of the thoracic wall with high-speed electrons is one of the standard methods of prophylaxis and therapy of local recurrences and cutaneous metastases of an operated mammary carcinoma. The surface dose, however, is only 85% of the maximum dose, due to the depth dose curve of the electron beams with the preponderantly applied energy of 7MeV. This is a poor value, since most of all recurrences appear near to the surface and so the risk of giving an insufficient dose is involved. The dose distribution could be essentially improved by the use of moulages on the chest. These moulages were made of different materials which were tested and compared with respect to their suitability for radiotherapeutic purposes. The best materials proved to be "Urgo-Plastan" (manufacturer: Holphar, Sulzbach) and "Orthoplast" (manufacturer: Johnson & Johnson, Düsseldorf). Both materials are synthetic substances which after heating can easily be adapted to the body shape and which offer a good stability, little inconvenience for the patient and a relative easy handling. With these moulage materials, the surface dose is increased to 98% ("Urgo-Plastan") and 99% ("Orthoplast") of the maximum dose.

Breast Neoplasms↗
PubMed1986

[Face masks for radiotherapy of head and neck tumors: comparative tests of different materials].

A most precise immobilisation of the patient's head is indispensable in order to reach a high degree of exactness and reproducibility in radiotherapy of malignant head and neck tumors. Face masks made of different synthetic materials have proved to be a simple and economical solution for this problem. Based on our own experiences with "Baycast Longuettes" (manufacturing firm: Johnson & Johnson, Düsseldorf), eleven substances have been tested in the phantom (compound of plaster and synthetic resin, thermoplast, polyurethane foam, compounds of cotton and synthetic resin, and fibre glass compounds). An appropriate material was "Hexcelite" (manufacturing firm: Medimex, Hamburg), a reticulated thermoplast which after warming up can be easily adapted to the patient's face and which guarantees a very good fixation of the head. As compared to solid masks, there is only a slight superposition of the depth dose of Co-60 gamma radiation by secondary electrons from the mask material, so that an increased rate of radiogenic dermatitides is not to be expected.

Casts, Surgical↗
PubMed1986

[Calculation of the dose distribution for ultrahard X-ray bremsstrahlung].

The dose distribution of the 42 MV X-ray bremsstrahlung of the betatron is represented by reference dose, depth dose and transverse dose. The method used to determine the parameters of a system of functions to approximate reference dose and depth dose is described and the coefficient schemes of the functions are indicated. The standardized equivalent field surface and the opening ratio of the useful radiation stop of the betatron serve as variables for the approximation.

Calibration↗
PubMed1985

[Experience in the use of face masks for radiotherapy of head and neck tumors].

By means of positioning and fixation aids, the precision and reproducibility of irradiation fields in radiotherapy of malignant tumors of the head and neck can be considerably improved. Face masks made of different synthetic materials have proved to be a practicable solution of this problem. In our hospital we have developed and tested a simple and not expensive possibility of manufacturing the masks with "Baycast" (producer: Bayer AG Leverkusen). The material is generally well tolerated by the patients, and the head is sufficiently fixed. An increased incidence of radiogenic dermatitides is caused by the overlapping of the depth dose of the Co-60 gamma radiation due to additional secondary electrons emanating from the mask material. This effect can be partly prevented by cutting out the irradiation fields in the masks.

Cobalt Radioisotopes↗
PubMed1983

[Deviations in the control of the monitor calibration of a circular accelerator].

The operation of an accelerator requires to perform at regular intervals a control measurement of the basic calibration. This measurement showed a deviation growing from week to week. Finally, the test measurements gave occasion to suppose a defect of the used ionizing chamber, but this was not confirmed by measurements at a gamma irradiation unit. When the chamber had been repaired by the manufacturer, there were no more deviations with respect to the basic measurement.

Particle Accelerators↗
PubMed1979

[An interactive system for recording, storage and processing of treatment data (author's transl)].

For correct radiation treatment and evaluation of treatment a complete recording of data is necessary. With this aim an interactive system was developed. Data recording for every patient is started from a terminal assigned to the physician. After completing the patient file with the data of the physical treatment plan, the file may be displayed on the terminal assigned to the particulr treatment unit, and additional data of the particular treatment session added. If a treatment is finished, a hardcopy of all data is generated by the system. Retrieval of patient data is always possible from the physicians terminal. The system is a contribution towards reducing the work in documentation the radiation treatment. It will be possible not only to carry out epidemiological studies, but also to investigate and perhaps optimate the results of treatment methods.

Computers↗