PubMed Health⌕ Search

Biomedical subjects

B R Paliwal

Publications and source records attributed to B R Paliwal.

At least 19 recordsLinked to original sources
PubMed1992

High dose rate intracavitary brachytherapy for carcinoma of the cervix: the Madison system: II. Procedural and physical considerations.

The loss in therapeutic ratio accompanying a conversion from low dose-rate (LDR) to high dose-rate (HDR) intracavitary brachytherapy (ICR) requires increased attention to the precision and accuracy of dose distribution calculations and treatment delivery. While the HDR-ICR treatment unit allows better custom-tailored dose distributions compared to LDR, it also requires more attention to detail to achieve the distribution desired. Because the relative biological effectiveness of different isodose levels in a dose distribution varies with the absolute dose (as described in Part 1 of this article), the relative dose distribution used with LDR must be modified for HDR to produce the same expected biological effect. Because of the difference in the radiobiology and physical positioning, simply duplicating applications as performed with LDR misses opportunities for dose distribution improvement as well as opens possibilities for significant complications. Due to differences in positioning the applicator (e.g., retraction of the cervix low in the pelvis instead of packing the applicator high), traditional definitions of points of interest (such as point A) apply poorly with HDR-ICR, compelling new systems of dose specification. With HDR-ICR, irreparable mistakes can happen very quickly, and quality assurance for the treatment plan and calculated dwell times prove much more important than with LDR. Key features of the dose distribution and constant relationships involving doses and dwell times help screen planned treatments for mistakes. This paper details the procedural and physical consideration of the Madison system for HDR-ICR brachytherapy for carcinoma of the cervix.

Brachytherapy↗
PubMed1991

A three-dimensional volume visualization package applied to stereotactic radiosurgery treatment planning.

A comprehensive software package has been developed for visualization and analysis of 3-dimensional data sets. The system offers a variety of 2- and 3-dimensional display facilities including highly realistic volume rendered images generated directly from the data set. The package has been specifically modified and successfully used for stereotactic radiosurgery treatment planning. The stereotactic coordinate transformation is determined by finding the localization frame automatically in the CT volume. Treatment arcs are specified interactively and displayed as paths on 3-dimensional anatomical surfaces. The resulting dose distribution is displayed using traditional 2-dimensional displays or as an isodose surface composited with underlying anatomy and the target volume. Dose volume histogram analysis is an integral part of the system. This paper gives an overview of volume rendering methods and describes the application of these tools to stereotactic radiosurgery treatment planning.

Brain Diseases↗
PubMed1991

Hyperthermia and irradiation for locally recurrent previously irradiated breast cancer.

From March 1981 to June 1989, 44 patients with locally recurrent, previously irradiated adenocarcinoma of the breast were treated with hyperthermia and irradiation. Treatment sites were chest wall (35) or nodal (nine), with an average tumor area of 29.44 cm2 (range 0.16 to 252 cm2) prior to treatment. Concurrent radiation doses varied from 16 to 56 Gy (mean = 29.4 Gy). Externally applied microwave hyperthermia was given twice weekly, aiming at 43 degrees C for 60 min. Evaluation at one month post treatment revealed 41% complete response (CR), 23% partial response (PR), and 36% no response (NR), and 67% of patients with a CR sustained that response for greater than 12 months. Tumors heated to a mean thermal dose (equivalent-minutes at 42.5 degrees C) greater than 50 had a 53% CR rate, significantly better than the 14% CR rate observed in patients whose tumors received a mean thermal dose less than 50. Among patients with tumors less than or equal to 6 cm2 in area, 65% achieved CR, significantly better than the 26% CR rate noted for patients with tumors greater than 6 cm2 in area. Only four patients (7.4%) experienced complications: one developed a catheter-related infection, two had ulcerated infections, and one had a severe blister. In summary, higher thermal doses delivered and smaller tumor areas were associated with more favorable tumor responses.

Adenocarcinoma↗
PubMed1990

Monte Carlo and convolution dosimetry for stereotactic radiosurgery.

The dosimetry of small photon beams used for stereotactic radiosurgery was investigated using Monte Carlo simulation, convolution calculations, and measurements. A Monte Carlo code was used to simulate radiation transport through a linear accelerator to produce and score energy spectrum and angular distribution of 6 MV bremsstrahlung photons exiting from the accelerator treatment head. These photons were then transported through a stereotactic collimator system and into a water phantom placed at isocenter. The energy spectrum was also used as input for the convolution method of photon dose calculation. Monte Carlo and convolution results were compared with the measured data obtained using an ionization chamber, a diode, and film.

Brain Neoplasms↗
PubMed1989

Hyperthermia: principles and quality assurance.

The methods of energy deposition, the power absorbtion properties of biological tissues and the basic components of a typical hyperthermia system are described. In addition, the clinical requirements of hyperthermia treatment are discussed. A perspective on treatment planning and quality control is presented.

Diathermy↗
PubMed1989

Microwave hyperthermia. Principles and quality assurance.

This article discusses some of the basic principles involved in the deposition of microwave power in the tissue. Insight is offered into the characteristics of the instrumentation, its limitations, and the necessary quality control needs.

Diathermy↗
PubMed1988

Hyperthermia thermometry evaluation: criteria and guidelines.

Results of the evaluation of thermometry devices used during hyperthermia treatments at 14 different clinics in the USA are presented. Measurements were made by the Hyperthermia Physics Center (HPC, a national hyperthermia quality assurance program under NCI contract No. N01-CM-37512) according to a protocol. Our sample included thermocouples, fiberoptic thermometers, and high lead resistance thermistors. We found that only some but not all of the thermometers of each kind performed within the +/- 0.2 degrees C acceptability criteria of accuracy. The precision, stability, and response times achieved with each type of thermometer are presented. A summary of perturbations and artifacts typical for each system is presented together with suggested precautions to avoid them during clinical usage. We conclude that although the technology used with each thermometer system is capable of producing a temperature accuracy of 0.2 degrees C, this accuracy is clinically achievable only with a concerted effort and a constant alertness on the part of the investigator. Based on the combined experience of this survey, the clinical investigators we visited, and published reports, we present certain guidelines and procedures that can help to reduce the inaccuracies and improve the reliability of temperature data obtained in clinical hyperthermia trials.

Fiber Optic Technology↗
PubMed1988

Experimental derivation of beta for high-energy photons.

The absorbed dose in a medium for a given beam of megavoltage photons is related to the collision kerma by the energy dependent parameter beta. Some theoretical methods of estimating and calculating beta have been proposed in the past. The majority of the methods take into account only Compton interactions, with just one method taking into account the beam spectrum, coherent and incoherent scattering and pair production effects. Experimentally measured data, on the other hand, implicitly include appropriate contributions of all these processes. Experimentally derived beta values are tabulated and the rationale for their measurement is discussed. The beta value for a low-Z ion chamber calibration in free space with a 60Co gamma-ray beam is 1.002. The dependence of beta on several factors such as energy, field size, phantom material and depth has been studied.

Dose-Response Relationship, Radiation↗
PubMed1987

Effect of field irregularities on the dose distribution of 4 MV photon beam.

Complex irregular fields are often used for the treatment of tumors in radiation therapy. The dose distribution of irregularly shaped fields depend strongly on the shape of field irregularities. The effect of field irregularities on the uniformity of the dose distribution of a 4 MV photon beam has been studied. The uniformity index of the blocked field which is a measure of the dose uniformity of the field has been compared to its corresponding unblocked field. The measured and computer calculated dose are compared for points within 1 and 2 cm from the edge of the field irregularities at the depth of 10 cm. The discrepancies between the computer calculated dose and the measured dose are discussed. Some suggestions are made to improve the dose uniformity of the irregularly shaped blocked fields.

Radiation Dosage↗
PubMed1985

Determination of contamination-free build-up for 60Co.

Experimental verification of the difference between absorbed dose in tissue and the collision fraction of kerma requires precise knowledge of the absorbed dose curve, particularly in the build-up and build-down regions. A simple method for direct measurement of contamination-free build-up for 60Co, which should also be applicable for most of the photon energies commonly employed for treatment, is presented. It is shown that the contribution from air-scattered electrons to the surface dose may be removed by extrapolating measurements of build-up to zero field size. The remaining contribution to contamination from the collimators and other source-related hardware may be minimised by measuring these build-up curves sufficiently far from the source. These results were tested by measuring the build-up using a magnet to sweep scattered electrons from the primary photon beam and by measuring the surface dose in the limit of an evacuated beam path. The relative dose at zero depth in polystyrene was found to be approximately 8.9 +/- 0.3% of the dose at the depth of maximum build-up.

Cobalt Radioisotopes↗
PubMed1985

A proposed modification of the cavity theory for electrons.

An empirical expression of cavity theory for electron fields is developed in a fashion similar to Burlin's general theory of cavity ionization for photons. It incorporates a term that relates the absorbed dose ratio in the cavity and material medium to the differences in electron scattering. This new expression correlates very well with experimental observations for progressively increasing LiF cavity sizes embedded in different media, the energy response of LiF at high electron energies relative to 60Co and the absorbed dose conversion factors CE in the ionization chamber dosimetry of electron beams.

Electrons↗
PubMed1985

Chronic response of normal porcine fat and muscle to focused ultrasound hyperthermia.

Annular focused ultrasound (1.13 MHz) hyperthermia was used to evaluate chronic histologic effects of a range of high thermal dosages on normal porcine tissues. The effects of three peak temperatures (45, 47, and 49 degrees C) at a focal depth of 2 cm in thirty 4-cm-diameter sites were studied as a function of exposure time (10-60 min). Relative fat and muscle damage were histologically graded 1 month post-treatment. Unlike reports of radiofrequency hyperthermia, no necrosis or abscess formation was observed, even at 49 degrees C for 40 min. Fat sustained a greater percentage maximal tissue damage than muscle, although less than 4% of sections evaluated had histologic evidence of severe injury. Focused ultrasound provides a relatively uniform heat distribution in normal tissues. It should therefore be possible to raise normal tissues surrounding tumors to high temperatures using focused ultrasound, potentiating tumoricidal effects with minimal associated complications.

Adipose Tissue↗
PubMed1979

Magnetic modification of electron beam dose distributions.

A 10 kG magnetic field has been used to modify the conventional electron beam isodose distributions. Isodose curves of modified distributions for single and multiple beams measured in a polystyrene phantom are presented. The results suggest a potential use of this technique to achieve dose distributions adjusted to the various tumor contours and corrected for heterogeneity effects.

Electromagnetic Fields↗
PubMed1977

A tissue compensation source-to-skin measurement system.

An easy to use, low-cost system for measuring the source-to-skin distance used in determining tissue compensation requirements is described. These measurements can be performed with accuracy and precision for any treatment field size and at any treatment beam angle. The system can be used in conjuction with several different tissue compensation methods.

Humans↗