Improved dose homogeneity in scalp irradiation using a single set-up point and different energy electron beams.

Homogeneous irradiation of the entire or a large portion of the superficial scalp poses both technical and dosimetric challenges. Some techniques will irradiate too much of the underlying normal brain while other techniques are either complex and involve field matching problems or may require sophisticated linear accelerator (linac) add-ons such as intensity modulated radiation therapy (IMRT)/electron multileaf collimation. However, many radiotherapy facilities are not equipped with such treatment modalities.

Methods of bolusing the tracheostomy stoma.

Purpose: The tracheostomy stoma is a potential site of recurrence for patients who have subglottic cancer or subglottic spread of cancer. In these patients, it is important that the anterior supraclavicular field does not underdose the posterior wall of the tracheostomy stoma when using a 6-MV anterior photon field. Conventionally, this problem is surmounted with placement of a plastic tracheostomy tube, which is uncomfortable for the patient, potentially traumatic, and can interfere with vocalization via a tracheal esophageal puncture.

Clinical experience with routine diode dosimetry for electron beam radiotherapy.

Purpose: Electron beam radiotherapy is frequently administered based on clinical setups without formal treatment planning. We felt, therefore, that it was important to monitor electron beam treatments by in vivo dosimetry to prevent errors in treatment delivery. In this study, we present our clinical experience with patient dose verification using electron diodes and quantitatively assess the dose perturbations caused by the diodes during electron beam radiotherapy.

The relationship between dose heterogeneity ("hot" spots) and complications following high-dose rate brachytherapy.

Purpose: It is generally believed that "hot" spots should be avoided in radiotherapy because they lead to complications. Dose homogeneity within the target volume is much more difficult to achieve during brachytherapy than during external beam irradiation, and implants are rarely geometrically perfect. To not underdose some parts of the target volume, therefore, it may be necessary to accept hot spots in other parts of the target volumes, but it is not at all clear from the literature how much dose heterogeneity should be considered excessive.

Comparison of the transit dose components and source kinematics of three high dose rate afterloading systems.

High dose rate (HDR) afterloading systems are using a high activity Ir-192 source that stops at programmed dwell positions to deliver a prescribed dose. The treatment planning systems are based on the dose calculation model that does not take into account the transit part of a dose resulting from a source while in motion. In this presentation the transit dose components as well as the source kinematics of three commercially available HDR systems were examined using a previously established technique1 based on film dosimetry.

Close or positive margins after surgical resection for the head and neck cancer patient: the addition of brachytherapy improves local control.

Purpose: Microscopically positive or close margins after surgical resection results in an approximately 21-26% local failure rate despite excellent postoperative external radiation therapy. We sought to demonstrate improved local control in head and neck cancer patients who had a resection with curative intent, and had unexpected, microscopically positive or close surgical margins.

Methods And Materials: Twenty-nine patients with microscopically close or positive margins after curative surgery were given definitive, adjuvant external radiation therapy and 125I brachytherapy.

Parotid gland tumors: a comparison of postoperative radiotherapy techniques using three dimensional (3D) dose distributions and dose-volume histograms (DVHS).

Purpose: To compare different treatment techniques for unilateral treatment of parotid gland tumors.

Methods And Materials: The CT-scans of a representative parotid patient were used. The field size was 9 x 11 cm, the separation was 15.

Total body irradiation with an arc and a gravity-oriented compensator.

Purpose: To deliver uniform dose distributions for total-body irradiation (TBI) with an arc field and a gravity-oriented compensator. This technique allows the patient to be treated lying on the floor in a small treatment room.

Methods And Materials: Through the sweeping motion of the gantry, a continuous arc field can deliver a large field to a patient lying on the floor.

Uncertainty of calibrations at the accredited dosimetry calibration laboratories.

The American Association of Physicists in Medicine, through a subcommittee (formerly Task Group 3) of the Radiation Therapy Committee, has accredited five laboratories to perform calibrations of instruments used to calibrate therapeutic radiation beams. The role of the accredited dosimetry calibration laboratories (ADCLs) is to transfer a calibration factor from an instrument calibrated by the National Institute of Standards and Technology (NIST) to a customer's instrument. It is of importance to the subcommittee, to physicists using the services of the ADCLs, and to the ADCLs themselves, to know the uncertainty of instrument calibrations.

Customization of a radiation management system to support in vivo patient dosimetry using diodes.

Considerable effort is generally made in the quality control of radiation therapy units and in patient chart checking to ensure accurate delivery of treatment to the patients. Record and verify (RV) systems have been implemented which will inhibit the beam from being turned ON if the parameters set on the medical linear accelerators do not match a preprogrammed file. It should be emphasized, however, that RV systems are designed only to ensure proper setup of parameters on the therapy units and bear no direct link with proper patient position.

Radiation therapy of breast carcinoma: confirmation of prescription dose using diodes.

Purpose: To quantitate the dose delivered during tangential breast radiation therapy and measure the scatter dose to the contralateral breast for three different breast setup techniques.

Methods And Materials: A commercial semiconductor diode system is used for dose measurements. The diode characteristics were studied by comparing the diode response against a standard ionization chamber response in a reference configuration.

The use of diode dosimetry in quality improvement of patient care in radiation therapy.

The purpose of this work is to improve the quality of patient care in radiation therapy by implementing a comprehensive quality assurance (QA) program aiming to enhance patient in vivo dosimetry on a routine basis. The characteristics of two commercially available semi-conductor diode dosimetry systems were evaluated. The diodes were calibrated relative to an ionization chamber-electrometer system with calibrations traceable to the National Institute of Standards and Technology (NIST).

Effect of ocular implants of different materials on the dosimetry of external beam radiation therapy.

Purpose: To study the attenuation and scattering effects of ocular implants, made from different materials, on the dose distributions of a 6 MV photon beam, and 6, 9, and 12 MeV electron beams used in orbital radiotherapy.

Methods And Materials: Central axis depth-dose measurements were performed in a polystyrene phantom with embedded spherical ocular implants using film dosimetry of a 6 MV photon beam and electron beams of 6, 9, and 12 MeV energy. The isodose distributions were also calculated by a computerized treatment planning system using computerized tomography (CT) scans of a polystyrene phantom that had silicone, acrylic, and hydroxyapatite ocular implants placed into it.

Effects of beam modifiers and immobilization devices on the dose in the build-up region.

Purpose: To analyze the effect that immobilization devices used in conjunction with beam modifiers may have on the dose to the skin and build-up region.

Methods And Materials: Central axis depth dose measurements were made in a polystyrene phantom in the build-up regions using the 6 and 15 MV photon beams, at two different source-to-phantom distances, and various field sizes. The effects of acrylic blocking trays, lead wedges, and cerrobend blocks were assessed in conjunction with the enhancement of dose in the build-up region due to immobilizing devices using plaster and thermoplastic casting materials of different thicknesses.

Diode dosimetry of 103Pd model 200 seed in water phantom.

The relative dose distribution around the 103Pd model 200 implant seed was measured with a computerized data acquisition system employing a p-n junction silicon diode immersed in a water phantom. Data are acquired in polar coordinates by computer control of (1) the diode distance from the seed center and (2) the rotation angle of seed about a transverse axis. Transverse axis data are compared with thermoluminescent dosimeter (TLD) measurements and a Monte Carlo calculation by others.

Three-dimensional conformal radiation therapy may improve the therapeutic ratio of high dose radiation therapy for lung cancer.

Purpose: The specific aim of 3-dimensional conformal radiation therapy is to improve the target dose distribution while concomitantly reducing normal tissue dose. Such an approach should permit dose escalation until the limits of acceptable normal tissue toxicity are reached. To evaluate the feasibility of tumor dose escalation for nine patients with lung cancer, we determined the dose distribution to the target and normal tissues with 3-dimensional conformal radiation therapy and conventional planning.

Diode dosimetry of models 6711 and 6712 125I seeds in a water phantom.

Two-dimensional relative dose distributions have been measured around 125I brachytherapy seeds. The two seed models studied, models 6711 and 6712, were manufactured by the 3M Company. Silicon detectors immersed in water phantoms were used to measure the dose.

Beam characteristics of a new model of 6-MV linear accelerator.

This paper describes the beam characteristics and dosimetry measurements performed on the 6-MV photon beam of a new model of linear accelerator, three of which were recently introduced and installed in our institution. Percent depth dose and tissue maximum ratio tables for a variety of field sizes and depths, as well as other parameters used for treatment planning are presented. These accelerators are the first of their kind using both hardware and software tools to control interlocks.

Recommendations on performance characteristics of diagnostic exposure meters: report of AAPM Diagnostic X-Ray Imaging Task Group No. 6.

Task Group 6 of the Diagnostic X-Ray Imaging Committee of the American Association of Physicists in Medicine (AAPM) was appointed to develop performance standards for diagnostic x-ray exposure meters. The recommendations as approved by the Diagnostic X-Ray Imaging Committee and the Science Council of the AAPM are delineated in this report and provide specifications on meter precision, calibration accuracy, calibration reference points, linearity, energy dependence, exposure rate dependence, leakage, amplification gain settings, directional dependence, the stem effect, constancy checks, and calibration intervals. The report summarizes recommendations for meters used in mammography, general purpose radiography including special procedures, computed tomography, and radiation safety surveys for x-ray radiography.

Simulating blocks in treatment planning calculations.

It is difficult to make an accurate calculation of dose distribution incorporating blocks using a ray model. One approach is to simulate the blocking in a treatment planning distribution by using negatively weighted beams. A second is to employ an external contour.

The effect of angular spread on the intensity distribution of arbitrarily shaped electron beams.

Knowledge of the relative intensity distribution at the patient's surface is essential for pencil beam calculations of three-dimensional dose distributions for arbitrarily shaped electron beams. To calculate the relative intensity distribution, the spatial spread resulting from angular spread is convolved with a two-dimensional step function whose shape corresponds to the applicator aperture. Two different approaches to obtain angular spread or the equivalent spatial spread are investigated.