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P Kneschaurek

Publications and source records attributed to P Kneschaurek.

32 records · Page 2Linked to original sources
PubMed1994

The influence of repopulation kinetics on isoeffect doses for permanent implants.

PURPOSE: Determination of isoeffect doses for permanent implants with different half-life times. METHOD: To calculate the isoeffective doses a model is used which describes the number of clonogenic tumor cells as a function of time. The effective irradiation time for permanent implants depends on the half-life T1/2 of the used isotope. During the irradiation tumor cells may repopulate, an effect which is well known in fractionated radiotherapy. The longer the radiation treatment lasts the more dose will be needed to kill these repopulating tumor cells. A differential equation has been constructed where cell kill is determined by an alpha-beta term and cell proliferation by a constant tumor cell doubling time Tpot. The solution of the differential equation gives the number of clonogenic tumor cells as a function of time. RESULTS: For our model an analytic solution has been found. The number of repopulating tumor cells shows a minimum if the dose rate is sufficiently high. Otherwise cell kill by radiation is overcompensated by growing tumor cells. The minimum value Nmin of tumor cells depends on the initial cell number N0, the total dose Dtot, the half-life T1/2 of the implant, the half-life Tr for repair of the sublethal damage, the alpha and beta values of the tumor cells, and the effective clonogen doubling time Tpot of the tumor. Assuming that the tumor is cured if no clonogenic cell survives, the tumor control probability (TCP) is determined by the Poisson statistics TCP = exp(-Nmin). Isoeffective doses are doses with constant TCP. Isoeffective doses Dtot have been calculated for different Tpot as a function of the half-life T1/2 of the isotope. CONCLUSIONS: The model allows the calculation of isoeffective doses for permanent implants. Care must be taken to use the results in clinical practice unless all of the radiobiological parameters are known for a specific tumor.

Brachytherapy↗
PubMed1992

[High-dose-rate brachytherapy of prostatic carcinoma with iridium 192].

In the therapy of localized prostatic cancer the radical prostatectomy shows good results within a five-year interval with no evidence of disease in nearly 90%. An important alternative is the radiotherapy by external beam or interstitial technique with iodine, gold or iridium. We use the high dose rate technique with Ir-192. In this technique five to seven hollow needles are placed in the prostate from perineal punctures under transrectal sonographic control. The three-dimensional brachytherapy planning is done according the actual needle position. A computer program calculates the radiation dose and distribution for each needle by adjustment of time and stops of the Ir-implants. The Ir-192 is temporary loaded twice with 9 Gy supplemented with external beam radiation (18 x 2 Gy). Since 1985 29 patients with localized tumor (T1-T3, N0, M0) have been irradiated. 21/29 had a pelvic lymphadenectomy before. In 85% of the patients were seen no side effects. Only one patient had a serious complication through a recto-vesical fistula. Out of the 21 followed-up patients 16 were in full remission, three had an androgen deprivation because of progression. A local tumor control could be demonstrated by cytology in 70% of the patients. This rational technique seems to be an alternative for patients not eligible for a radical prostatectomy.

Aged↗
PubMed1989

First experiences with fission neutrons in the radiation therapy of cancer patients.

16 patients have been treated until October 1987 at the mixed reactor beam of the RENT-facility in Munich-Garching. All of them received a single dose of 200 to 250 cGy after complete conventional radiotherapy and in same cases also surgery because of the lack of complete tumor resection or local tumor control after radiation control. The ages ranged from 40 to 84 years. The follow-up amounted between two and 28 months, 8.5 months on the average. The tumors selected appeared to be particularly suitable because of their growth in poorly perfused tissues, after the previous surgery and/or radiotherapy. All cases are briefly reported. The general experience from the very limited study is promising. The local effects to the tumor tissue so far are impressive. No severe side effects which could be related to the RENT-session were observed until now.

Adult↗
PubMed1987

[Computer-assisted system for interstitial hyperthermia].

The combination of interstitial radiotherapy and interstitial hyperthermia is more promising in the treatment of tumors than one of these methods alone. The unit developed by us uses the afterloading needles for heating up the tumor tissue with ohm current and for controlling the distribution of temperature in the target volume. Up to twelve needles are provided by one commutator with the R.F. current controlled by the computer. The temperature is measured by three thermistors per needle which are arranged at an axial distance of 2 cm each. The linearization of the thermistor characteristics and the control of cummutator and R.F. generator is performed by the computer over an interface constructed by us. In order to achieve a homogeneous distribution of temperature in the target volume and to avoid hot spots, we have examined several needle configurations by measuring in an homogeneous phantom.

Analog-Digital Conversion↗
PubMed1987

[Combination of interstitial hyperthermia and afterloading therapy at a high-dosage rate].

The study was supposed to develop a method allowing to combine interstitial hyperthermia with high dose rate afterloading therapy. For hyperthermia, metallic hollow needles, which are also used in brachytherapy, are connected with the RF source by a computer-controlled relay switch. The temperature is measured by thermistors incorporated into the needles. Several heating methods are investigated in a homogenous phantom and compared by means of the experimentally determined temperature/time evolutions and isotherms (in a stationary state).

Brachytherapy↗
PubMed1986

The r.b.e. of fast fission neutrons (2 MeV) for chronic radiation damage of the large bowel of rats after single dose and fractionated irradiation.

Late damage in the rectum of rats was studied after local irradiation with 2 MeV fast fission neutrons and the results were compared with those for 300 kV X-rays. A similarity between latency times, and between clinical, gross pathological and histological appearances of the rectal obstruction after the two types of radiation was observed. The r.b.e. evaluated from ED 50/200 days values was 1.8 for single doses, 2.1 for two fractions and 3 for five fractions and these are similar to r.b.e.s from other investigations of chronic intestinal radiation damage. Using linear quadratic analysis, a limiting r.b.e. of 5 was obtained. The beta-values are similar for both radiation qualities. Because of the high gamma-contamination (25 per cent) in the RENT I beam a possible correction of the r.b.e. for neutrons only, assuming interaction, is discussed.

Animals↗
PubMed1986

[First results of high-dose-rate 192-Iridium afterloading method applied alone or associated with laser therapy in tumors of the respiratory and digestive tracts].

High-dose-rate short-term brachytherapy with 192iridium, preceded by laser therapy if practicable, has been applied in an interdisciplinary collaboration for more than one year at our hospital. Until April, 1985, 127 applications have been made in 49 patients suffering from tumors of the respiratory and upper digestive tract. The palliative effects obtained (74% and 86%, respectively) correspond with those achievable by percutaneous radiotherapy. The local control rates can possibly be improved by an 192iridium boost in the course of a curative radiotherapy.

Adult↗
PubMed1985

[Dose and dose distribution close to Ir-192 sources].

A report is given on dose measurements performed in the radiation field close to Ir-192 sources. The dose was measured at a distance of 2 to 8 cm in the air and in a phantom with ionization chambers. Radiachromic foils were used in order to determine the dose distribution surrounding the sources immediately. The measurement results were compared to model calculations and show a good conformity.

Brachytherapy↗
PubMed1983

[Experimental and mathematical check of the moving-strip irradiation technic].

The moving-strip irradiation of the total peritoneal region plays an important role in the adjuvant therapy of the ovarian carcinoma. TLD dosimetric and mathematic investigations of this radiotherapeutic method show that 1. the best dose homogeneity in the longitudinal profile is achieved by 60Co units, if the patients are not too stout, 2. a higher relative depth dose has to be estimated than for an open field of the same total field size, 3. an additional 2,5 cm strip has to be inserted in order to achieve an abrupt drop on the border of the total irradiation field, 4. this method offers a good guarantee against false adjustment, if this is not too grave, 5. the conformity between the dose distribution curves calculated and those measured by TLD is excellent.

Cobalt Radioisotopes↗
PubMed1983

[Dose distribution in a linear moving-field irradiation].

The authors discuss the effects of a linear moving field irradiation. It is demonstrated that the depth dose curves and the longitudinal field profiles of this irradiation method are considerably modified with respect to a stationary field of the same superficial field size. Especially the depth dose attenuation becomes less abrupt. The reason therefore is of merely geometrical nature and does not depend on the type of radiation. For example, a cobalt field with a focus-skin distance of 60 cm and a superficial field size of 10 x 10 cm2 has a half-value depth of 10.3 cm in case of a stationary field and of 12.0 cm in case of a linear moving field.

Cobalt Radioisotopes↗
PubMedDate not supplied

Optimization of activity distribution in brachytherapy.

We investigated the problem of how to minimize the dose rate outside a spherical target volume for a given minimal dose rate inside the sphere for 1/r2 photon emitters. An integral equation for an activity distribution is derived from the demand that the dose rate is constant inside the sphere. The solution of this equation is given and it is proven that the resulting activity distribution gives the smallest dose rate at every point outside the target volume. Some other activity distributions are discussed; they are assessed by the required integral activity, which is proportional to the dose rate at some distance from the target volume. Compared with a single point source at the center of the sphere, one can save 50% of the total activity when using the optimal activity distribution. The benefit reduces to about 30% for the other continuous activity distributions and to a few % for symmetrical arrangements of a few point sources.

Brachytherapy↗