Biomedical subjects

A S Beddar

Publications and source records attributed to A S Beddar.

14 recordsLinked to original sources

PubMed2006

Intraoperative radiation therapy using a mobile electron linear accelerator: field matching for large-field electron irradiation.

Intraoperative radiation therapy (IORT) consists of delivering a large, single-fraction dose of radiation to a surgically exposed tumour or tumour bed at the time of surgery. With the availability of a mobile linear accelerator in the OR, IORT procedures have become more feasible for medical centres and more accessible to cancer patients. Often the area requiring irradiation is larger than what the treatment applicators will allow, and therefore, two or more adjoining fields are used. Unfortunately, the divergence and scattering of the electron beams may cause significant dose variations in the region of the field junction. Furthermore, because IORT treatments are delivered in a large single fraction, the effects of underdosing or overdosing could be more critical when compared to fractionated external beam therapy. Proper matching of the fields is therefore an important technical aspect of treatment delivery. We have studied the matching region using the largest flat applicator available for three different possibilities: abutting the fields, leaving a small gap or creating an overlap. Measurements were done using film dosimetry for the available energies of 4, 6, 9 and 12 MeV. Our results show the presence of clinically significant cold spots for the low-energy beams when the fields are either gapped or abutted, suggesting that the fields should be overlapped. No fields should be gapped. The results suggest that an optimal dose distribution may be obtained by overlapping the fields at 4 and 6 MeV and simply abutting the fields at 9 and 12 MeV. However, due to uncertainties in the placement of lead shields during treatment delivery, one may wish to consider overlapping the higher energy fields as well.

Electrons↗

PubMed2006

Water equivalent plastic scintillation detectors in radiation therapy.

A review of the dosimetric characteristics and properties of plastic scintillation detectors for use in radiation therapy is presented. The detectors show many desirable qualities when exposed to megavoltage photon and electron beams, including water equivalence, energy independence, reproducibility, dose linearity, resistance to radiation damage and near temperature independence. These detectors do not require the usual conversion and/or correction factors used to convert the readings from common dosemeters to absorbed dose. Due to their small detecting volume, plastic scintillation detectors exhibit excellent spatial resolution. Detector performance, in certain specific cases, can be affected by radiation-induced light arising in the optical fibres that carry the scintillator signal to a photodetector. While this effect is negligible for photon beams, it may not be ignored for electron beams and needs to be accounted for.

Algorithms↗

PubMed2005

Preliminary evaluation of implantable MOSFET radiation dosimeters.

In this paper, we report on measurements performed on a new prototype implantable radiation detector that uses metal-oxide semiconductor field effect transistors (MOSFETs) designed for in vivo dosimetry. The dosimeters, which are encapsulated in hermetically sealed glass cylinders, are used in an unbiased mode during irradiation, unlike other MOSFET detectors previously used in radiotherapy applications. They are powered by radio frequency telemetry for dose measurements, obviating the need for a power supply within each capsule. We have studied the dosimetric characteristics of these MOSFET detectors in vitro under irradiation from a 60Co source. The detectors show a dose reproducibility generally within 5% or better, with the main sources of error being temperature fluctuations occurring between the pre- and post-irradiation measurements as well as detector orientation. A better temperature-controlled environment leads to a reproducibility within 2%. Our preliminary in vitro results show clearly that true non-invasive in vivo dosimetry measurements are feasible and can be performed remotely using telemetric technology.

Calibration↗

PubMed2004

Calibration of low-energy electron beams from a mobile linear accelerator with plane-parallel chambers using both TG-51 and TG-21 protocols.

A new approach to intraoperative radiation therapy led to the development of mobile linear electron accelerators that provide lower electron energy beams than the usual conventional accelerators commonly encountered in radiotherapy. Such mobile electron accelerators produce electron beams that have nominal energies of 4, 6, 9 and 12 MeV. This work compares the absorbed dose output calibrations using both the AAPM TG-51 and TG-21 dose calibration protocols for two types of ion chambers: a plane-parallel (PP) ionization chamber and a cylindrical ionization chamber. Our results indicate that the use of a 'Markus' PP chamber causes 2-3% overestimation in dose-output determination if accredited dosimetry-calibration laboratory based chamber factors (N(60Co)(D,w,) Nx) are used. However, if the ionization chamber factors are derived using a cross-comparison at a high-energy electron beam, then a good agreement is obtained (within 1%) with a calibrated cylindrical chamber over the entire energy range down to 4 MeV. Furthermore, even though the TG-51 does not recommend using cylindrical chambers at the low energies, our results show that the cylindrical chamber has a good agreement with the PP chamber not only at 6 MeV but also down to 4 MeV electron beams.

Calibration↗

PubMed2001

A new approach to intraoperative radiation therapy.

Intraoperative radiation therapy (IORT) is an adjuvant treatment in which a large single dose of radiation is delivered during a surgical procedure to resected tumor beds or to an unresectable tumor. This article discusses the implementation of an IORT program and highlights the successful collaboration needed between the OR and radiation oncology departments. A better understanding of IORT in the OR setting will contribute to smooth program implementation.

Combined Modality Therapy↗

PubMed2001

Mobile linear accelerators for intraoperative radiation therapy.

Intraoperative radiation therapy (IORT) is becoming an increasingly common procedure for treating gross tumors or tumor beds after resection. Traditionally, IORT delivery required either heavily shielded ORs or transporting anesthetized patients to the department of radiation oncology. The availability of a self-shielded mobile electron linear accelerator has made this treatment modality accessible to institutions that otherwise would not consider performing IORT. This article describes IORT equipment and supplies and addresses perioperative nursing issues, as well as the roles of other team members involved in the delivery of IORT.

Combined Modality Therapy↗

PubMed2001

In vivo dosimetry using a single diode for megavoltage photon beam radiotherapy: implementation and response characterization.

The AAPM Task Group 40 reported that in vivo dosimetry can be used to identify major deviations in treatment delivery in radiation therapy. In this paper, we investigate the feasibility of using one single diode to perform in vivo dosimetry in the entire radiotherapeutic energy range regardless of its intrinsic buildup material. The only requirement on diode selection would be to choose a diode with the adequate build up to measure the highest beam energy. We have tested the new diodes from Sun Nuclear Corporation (called QED and ISORAD-p--both p-type) for low-, intermediate-, and high-energy range. We have clinically used both diode types to monitor entrance doses. In general, we found that the dose readings from the ISORAD (p-type) are closer of the dose expected than QED diodes in the clinical setting. In this paper we report on the response of these newly available ISORAD (p-type) diode detectors with respect to certain radiation field parameters such as source-to-surface distance, field size, wedge beam modifiers, as well as other parameters that affect detector characteristics (temperature and detector-beam orientation). We have characterized the response of the high-energy ISORAD (p-type) diode in the low- (1-4 MV), intermediate- (6-12 MV), and high-energy (15-25 MV) range. Our results showed that the total variation of the response of high-energy ISORAD (p-type) diodes to all the above parameters are within +/-5% in most encountered clinical patient treatment setups in the megavoltage photon beam radiotherapy. The usage of the high-energy buildup diode has the additional benefit of amplifying the response of the diode reading in case the wrong energy is used for patient treatment. In the light of these findings, we have since then switched to using only one single diode type, namely the "red" diode; manufacturer designation of the ISORAD (p-type) high-energy (15-25 MV) range diode, for all energies in our institution and satellites.

Feasibility Studies↗

PubMed1996

Characteristics of a scintillator-based daily quality assurance device for radiation oncology beams.

A plastic scintillator based device marketed as a "DCT 444" is investigated for daily quality assurance (QA) of radiation beams. Variations in DCT responses were studied for three identical devices with beam, energy, distance, dose, dose rate, and orientations. The effects of gantry, collimator, and patient support assembly (PSA) rotations were investigated at photon energies of 6, 10, and 18 MV and electron energies in the range of 5-20 MeV from three different linear accelerators. The short and long term linearity, reproducibility, and radiation damage were also investigated in kilovoltage and megavoltage beams. Results indicate that the DCT response is linear with dose and dose rate, but shows directional dependence for all beams. The response is dependent on its orientation and the angle of rotations of collimator, gantry, and PSA of the linear accelerator. When the collimator is rotated through 0-36 degrees, the DCT response varied within +/- 4.5% and +/- 7.2% for the 6 MV and 18 MV beams, respectively. With PSA rotation, deviations up to +/- 11% were noted. The DCT response is symmetric across 0 degrees and peaks at +/- 60% PSA angle for all accelerators and beam energies. The effect of backscattering medium on DCT response is noted for all beams but significant for kilovoltage beams. A deviation as high as 30% was observed when DCT was placed on a thick polystyrene phantom. The long term stability studied over 25 months showed varied signal losses possibly due to radiation damage. However, there is no noticeable signal loss over a short period of time (2-4 weeks). The lightweight DCT device is linear with dose and dose rate, stable, and easy to operate. It stores ten readings that can be downloaded to a personal computer. This study indicates that DCT-444 is a suitable QA tool for central axis dose measurement. However, proper precaution is needed as this device exhibits directional and positional dependence. For a meaningful QA result of a radiation unit, the daily QA setup geometry should be identical to that used for calibration of the DCT device.

Biophysical Phenomena↗

PubMed1994

Description and evaluation of a new 3-D computerized treatment planning dose compensator system.

Evaluation has been performed of compensators generated by means of a computerized three-dimensional treatment planning system that can utilize either digitized slice profiles or CT scans. Two methods of calculating compensator thickness are used: the modified Batho power law (dSAR) method for digitized profiles and the equivalent TAR (eqTAR) method for CT scans. This system not only compensates for patient surface contours but also compensates for internal inhomogeneities. In addition, any required wedging will be incorporated in the compensator generation. This system has been tested for a number of extreme cases with inhomogeneities and sloping contours. Good agreement was obtained between the measured and computer calculated dose profiles especially along the central axis of the beam. A "Profile Uniformity Index" was defined to quantify the goodness of dose compensation in three dimensions. Compensation using this system can achieve good dose uniformity within the target volume in all clinical cases and is definitely an improvement over systems based solely on tissue deficit.

Algorithms↗

PubMed1994

A new scintillator detector system for the quality assurance of 60Co and high-energy therapy machines.

A new single-channel detector system has been developed to perform routine quality assurance of 60Co and high-energy therapy machines. This detector is composed of an orange plastic scintillator, optically coupled to a radiation-resistant polycarbonate light pipe and a shielded silicon photodiode imbedded in a hollow solid water phantom block. No temperature and pressure corrections are required. Stability results were consistent with standard deviations fluctuating from 0.03% up to 0.09% for 60Co and from 0.05% up to 0.18% for other high energies. This device provides a quick, easy and reliable beam output check remotely, using an automatic reset based on a radiation triggering system device, storing multiple sequential readings. The reproducibility of this detector was checked on a daily and weekly basis at different energies (60Co, 6 MV and 18 MV x-rays and 6, 9, 12, 16 and 20 MeV electron beams). These results were found to be consistent with those obtained using an ion chamber. Other characteristics of this detector, including the consequences of the radiation-induced light in the light pipe (stem effect) and the radiation damage on this system are briefly discussed.

Cobalt Radioisotopes↗

PubMed1994

Absorbed dose perturbation caused by diodes for small field photon dosimetry.

Uncertainties in dose measurements which may be considered acceptable for standard radiotherapy techniques may not be acceptable for radiosurgery or other precision radiotherapy techniques. One example of this type of uncertainty are the small perturbations to measured dose distributions which are caused by the use of diode detectors. In this article the effect of using diodes to measure small field penumbra is assessed and comments are made about diode orientation and diode construction.

Biophysical Phenomena↗

PubMed1992

Water-equivalent plastic scintillation detectors for high-energy beam dosimetry: I. Physical characteristics and theoretical consideration.

A minimally perturbing plastic scintillation detector has been developed for the dosimetry of high-energy beams in radiotherapy. The detector system consists of two identical parallel sets of radiation-resistant optical fibre bundles, each connected to independent photomultiplier tubes (PMTs). One fibre bundle is connected to a miniature water equivalent plastic scintillator and so scintillation as well as Cerenkov light generated in the fibres is detected at its PMT. The other 'background' bundle is not connected to the scintillator and so only Cerenkov light is detected by its PMT. The background signal is subtracted to yield only the signal from the scintillator. The water-equivalence of plastic scintillation detectors is studied for photon and electron beams in the radiotherapy range. Application of Burlin cavity theory shows that the energy dependence of such detectors is expected to be better than the commonly used systems (ionization chambers, LiF thermoluminescent dosimeters, film and Si diodes). It is also shown that they are not affected by temperature variations and exhibit much less radiation damage than either photon or electron diode detectors.

Fiber Optic Technology↗

PubMed1992

Water-equivalent plastic scintillation detectors for high-energy beam dosimetry: II. Properties and measurements.

The properties of a new scintillation detector system for use in dosimetry of high-energy beams in radiotherapy have been measured. The most important properties of these detectors are their hgh spatial resolution and their nearly water-equivalence. Measurements have shown that they have excellent reproducibility and stability, and a linear response versus dose-rate. It is also shown that they have better spatial resolution than ionization chambers and have much less energy or depth dependence in electron fields due to the removal of the influence of the polarization effect. Dose distributions in water, using miniature plastic scintillation detectors, have been measured for different high-energy photon and electron beams.

Fiber Optic Technology↗

PubMed1991

Influence of detector size in photon beam profile measurements.

Correction is necessary to account for the detector size in clinical dosimetry of photon and electron beams. This correction is due to the absorbed dose gradient present in a finite-size detector. Further corrections are necessary when the detector and phantom materials are not the same. These corrections are due to the perturbation in the charged-particle fluence. Generally these corrections are applied to measurements along the central axis of the beam. Cross beam profile measurements, however, are not usually corrected for detector size. The ionization profile is also usually assumed to be equivalent to the absorbed dose profile. We have corrected the ionization chamber size effect by two approaches: extrapolation of measurements to zero detector size and deconvolution of measurements using a simple model for the detector response function. We have measured absorbed dose profiles to water using a small water-equivalent plastic scintillation detector. Film profile measurements were also studied. The ionization profile corrected for detector size and absorbed dose profile were not equal, probably due to loss of charged-particle equilibrium in the beam edges. For ionization chamber measurements, knowledge of the charged-particle spectrum is needed to convert ionization to absorbed dose to water. This is not necessary for relative absorbed dose measurements under charged-particle equilibrium. Film has been shown to be a straightforward and reliable method for cross beam profile measurements.

Film Dosimetry↗