Algorithms for the process management of sealed source brachytherapy.

Incidents and misadministrations suggest that brachytherapy may benefit from clarification of the quality management program and other mandates of the U.S. Nuclear Regulatory Commission. To that end, flowcharts of step by step subprocesses were developed and formatted with dedicated software. The overall process was similarly organized in a complex flowchart termed a general process map. Procedural and structural indicators associated with each flowchart and map were critiqued and pre-existing documentation was revised. "Step-regulation tables" were created to refer steps and subprocesses to Nuclear Regulatory Commission rules and recommendations in their sequences of applicability. Brachytherapy algorithms were specified as programmable, recursive processes, including therapeutic dose determination and monitoring doses to the public. These algorithms are embodied in flowcharts and step-regulation tables. A general algorithm is suggested as a template from which other facilities may derive tools to facilitate process management of sealed source brachytherapy.

Use of a 1 mm collimator to test the accuracy of stereotactic radiotherapy.

PURPOSE: To develop a method of measuring locations of the center of dose in stereotactic radiotherapy relative to the center of the target, and thereby obtain a test of the accuracy of stereotactic radiotherapy (SRT). METHODS AND MATERIALS: An insert was mounted in an SRT collimator on a 6 MV linear accelerator to provide a photon beam approximately 1 mm in diameter at isocenter, and a method of measuring radiation center coordinates of arced SRT beams. To simulate a small intracranial target, two halves of a Barium paste column were embedded in two adjacent slabs of a humanoid phantom. A film was placed between the slabs to image the radiation relative to the target center. A surgical head ring and computerized tomography (CT) localizer were attached to the phantom and CT scans were obtained. The scans were entered in a three-dimensional computerized treatment-planning system and radiation isocenter coordinates determined by iteratively moving the 90% isodose surface center of arced beam dose distributions to coincide with the target center. The phantom was bolted to an SRT floorstand with isocenter coordinates obtained from the treatment plan, and then irradiated in two sets of experiments. The first set applied five 1 mm noncoplanar arced beams with and without offsets of the planned coordinates in the transverse plane. The second set applied one large transverse arc coplanar to the film with and without offsets in the craniocaudal direction. Irradiations with coordinate offsets tested the sensitivity of the method. Films were developed and digitized with a high resolution film scanner to measure the location of the radiation relative to the target center. RESULTS AND CONCLUSION: The radiation center was found from 0.0 to 0.3 mm of the target center, within requirements of our clinical quality assurance program. The measurement and evaluation of coincidence of radiation and target centers are, thus, proposed as elements of radiosurgery facility acceptance and annual quality assurance.

The measurement of linear accelerator isocenter motion using a three-micrometer device and an adjustable pointer.

PURPOSE: The small motions of the major axes of a linear accelerator observed during gantry and treatment table rotation were measured to improve beam-target alignment during stereotactic radiosurgery (SRS). METHODS AND MATERIALS: Measurements of gantry isocenter motion and table rotational axis wobble were performed with an adjustable front pointer and a three-micrometer device. Nominal gantry and table isocenters were specified. The gantry motion path and table isocenter coordinates were then applied to offset simulated treatment target coordinates so as to compensate for gantry sag. Target simulation films were examined to document improvement of beam-target alignment. RESULTS: The overall precision of the measurement of gantry and table isocenter coordinates was 0.2 mm. Over gantry rotation of 0 to 360 degrees, the gantry isocenter was found to follow a pinched loop with a maximum point to point distance of 1 mm. Table axis motion was found to be negligible relative to the reproducibility of gantry isocenter motion. Thus, a table isocenter was defined that was invariant to table rotation. CONCLUSION: Results indicate that the three-micrometer device and adjustable front pointer are useful tools for three-dimensional (3D) mapping of gantry, collimator and table isocenters and their motions. It is suggested that such measurements may be useful in the quality assurance of linear accelerators, particularly to improve beam-target alignment during SRS and other high dose external beam therapy.

Demographic characteristics of physicists who evaluate mammographic units.

PURPOSE: To determine whether there will be an adequate number of physicists to meet the 1996 American College of Radiology (ACR) Mammography Accreditation Program requirements and assess the qualifications of physicists available to evaluate mammography units as required by the federal Mammography Quality Standards Act of 1992. MATERIALS AND METHODS: A 21-question survey prepared by the ACR was sent to most medical physicists who perform evaluations of mammographic units. Replies were received from 1,011 individuals. RESULTS: Survey results are based on the replies of 824 individuals who indicated a willingness to perform evaluations in the future. In 1996, there will be approximately 510 certified physicists to evaluate an estimated 14,000 mammography units in the United States. CONCLUSION: A sufficient number of certified diagnostic medical physicists will be available to provide required annual performance evaluations of all mammography units in the United States only if the number of units evaluated per physicist increases substantially over current levels.

Fine tuning of linear accelerator accessories for stereotactic radiotherapy.

PURPOSE: Experience with the University of Wisconsin's stereotactic radiotherapy (SRT) accessory system was applied to build a new system, facilitate alignment of linac photon beams with a Brown-Roberts-Wells (BRW) stereotaxy, and increase the versatility and stability of the stereotaxy. METHODS AND MATERIALS: High tensile strength stainless steel was used in the floor stand to increase the range of gantry rotation relative to ranges allowed by truss-mounted stands. The collimator assembly and floor stand were each fitted with two-axis gimbal and translation adjustments in addition to the floor stand's three-axis adjustments. The head ring positioning assembly was fitted with two braces to prevent the head ring from deforming with patient motion. Six MV linac photon beam characteristics were measured with a computer-controlled scanning system and a diode in water, at source to surface distances (SSD) of 80 and 100 cm, and for 13 divergent collimators ranging in diameter from 1-4 cm at 100 cm SSD. Quality assurance software was applied to screen data for questionable consistency or symmetry. Integrity of the stereotaxy was evaluated with target simulation films and repeated measurements which were part of the quality assurance of clinical treatments. A method was developed using a glass etched contact reticle to obtain average simulated target to beam center distances (delta av) from target simulation films. RESULTS AND CONCLUSION: New aspects of the current system have improved the ability to fine tune and analyze stereotactic alignment. Beam characteristics met stringent output criteria and penumbral widths were the same or narrower than penumbral widths reported elsewhere. The precision of measuring delta av was 0.1 mm, and delta av averaged over 50 target simulation films was 0.7 +/- 0.1 mm. Results suggest that it may be useful to determine delta av from target simulation films with the method described here.

Comparison of dose calibration by ionometric and chemical dosimetry for 6- and 45-MV x-ray beams.

This work reports the results of an experiment carried out to investigate the compatibility of TG-21 ion chamber calibration with Fricke dosimetry near the extremes of the clinically available megavoltage photon energy range. Doses determined from ion chamber measurements and from chemical dosimetry are compared for a 6-MV linear accelerator x-ray beam and 45-MV betatron beam. The Fricke dosimetry was carried out by the National Institute of Standards and Technology. Agreement within 1% between the two techniques was obtained at both photon energies using a currently accepted G-value.

Spatial resolution measurements for passive microwave radiometry using a tissue-equivalent phantom.

A tissue-equivalent "hot" line source phantom is described for assessing spatial resolution in passive microwave radiometry systems. LSFs were measured for two rectangular waveguide antennas connected to a 4.7-GHz radiometer. The normalized LSFs and corresponding modulation transfer functions were found to be independent of line source temperature, but dependent upon antenna size, orientation, and line source depth.

An isodose shift technique for obliquely incident electron beams.

It is well known that when an electron beam is incident obliquely on the surface of a phantom, the depth dose curve measured normal to the surface is shifted toward the surface. Based on geometrical arguments alone, the depth of the nth isodose line for an electron beam incident at an angle theta should be equal to the product of cos theta and the depth of the nth isodose line at normal incidence. This method, however, ignores the effects of scatter and can lead to significant errors in isodose placement for beams at large angles of incidence. A semi-empirical functional relationship and a table of isodose shift factors have been developed with which one may easily calculate the depth of any isodose line for beams at incident angles of 0 degrees to 60 degrees. The isodose shift factors are tabulated in terms of beam energy (6-22 MeV) and isodose line (10%-90%) and are shown to be relatively independent of beam size and incident angle for angles less than 60 degrees. Extensive measurements have been made on a Varian Clinac 2500 linear accelerator with a parallel-plate chamber and polystyrene phantom. The dependence of the chamber response on beam angulation has been checked, and the scaling factor of the polystyrene phantom has been determined to be equal to 1.00.

Effect of gold shielding on the dosimetry of an 125I seed at close range.

Radioactive 125I plaques with gold shields have been used for the treatment of choroidal melanoma. The effect of the gold shield on the dosimetry of a single 125I seed (model 6711) has been investigated in this paper. Increases in dose at close range due to the presence of the shield are observed. Such enhancement is in agreement with Williamson's Monte Carlo calculations.

A dose homogeneity index for evaluating 192Ir interstitial breast implants.

To evaluate and optimize dose homogeneity of 192Ir interstitial breast implants, we define a quantity, the dose homogeneity index (DHI), as follows: DHI = [V(TDR)--V(HDR)]/V(TDR), where V(TDR) denotes the total treatment volume enclosed by the prescribed treatment dose rate (TDR) and V(HDR) denotes the volume enclosed by high-dose rate (HDR), which is 1.5 X TDR or greater. We have used the DHI to examine and compare 192Ir double-plane implants of various sizes planned by the Memorial system or the Tufts system. Criteria have been suggested for the number of planes required for implants in a given treatment volume. Anderson's volume-dose histogram with inverse square suppression is adopted for illustration.

Performance characteristics of a helical microwave interstitial antenna for local hyperthermia.

A helical microwave antenna has been designed to improve heat deposition by interstitial applicators used for clinical hyperthermia. Iso-specific-absorption-rate (SAR) curves of the helical antenna as well as a conventional monopole antenna were measured and compared in both muscle and brain tissue phantoms. The heating pattern of the helical antenna is more uniform along the length of the antenna which has important implications for multiarray implant configurations.

A beam-edge modifier for abutting electron fields.

Abutment of unmodified electron fields to irradiate large areas can lead to significant dose inhomogeneities in the region of junction of the fields. In this paper we describe the design and dosimetric characteristics of a device developed to broaden the electron beam penumbra and thereby to improve the dose uniformity in the overlap region. The device is a high-density triangular-toothed comb capable of reducing the beam intensity without seriously degrading the beam energy. The effect of the comb is such that a single device will generate a beam penumbra which is broad and very nearly linear at all depths for all clinically used beam energies. Results are shown for various field configurations and energies. With a gap of 5.0 cm between the treatment cone and phantom surface the dose "ripple" in the region beneath the teeth was found not to exceed +/- 5% at 0.5-cm depth.

Effective wedge angles for 6-MV wedges.

Measurements were undertaken with 30 degrees and 45 degrees large wedges on a 6-MV linac to determine the effective wedge angle for various combinations of open and wedged fields. The validity of Tatcher's equation, relating effective wedge angle to maximum dose weightings, was examined over a range of field sizes from 5 X 5 to 20 X 20 cm. An alternative equation involving only central axis quantities was also investigated. The results obtained from an analysis of point dose measurements indicate that, for these wedges, either equation yields sufficient accuracy for clinical purposes.

Tumor dose specification of I-125 seed implants.

A new calculation procedure for interstitial implants with I-125 seeds is presented. The total activity of I-125 seeds required to deliver a net minimum dose of 1 Gy is also given as a function of implant volume for various seed spacings. This will permit clinicians to prescribe a specific dose for patients as a primary or boost treatment. Elongation and shape corrections are studied. Discussions and comparisons with other dosimetric calculations such as "matched" peripheral dose, average minimum dose, and minimum peripheral dose are made. The results indicate that for a fixed total activity the net minimum dose is relatively insensitive to variations in seed spacing and individual seed activity.

Reduction of the "horns" observed on the beam profiles of a 6-MV linear accelerator.

The presence of large "horns" was found while plotting beam profiles during acceptance testing of a 6-MV linear accelerator. The in-phantom off-axis ratio (OAR), measured at 22 cm off the central axis along the diagonal of a 40 X 40 cm field at dmax was found to be 1.19, while beam uniformity was within specifications at 10-cm depth. A change in the gun injection voltage and the replacement of the magnet surrounding the magnetron with one of greater strength resulted in a reduction of the OAR to 1.085. The beam uniformity at depth was maintained within specifications. An alternative solution of adding a modifying filter in the primary beam was considered undesirable because of the 20%-25% reduction in dose rate caused by such filters. The relationship between the energy, the intensity distribution of the beam, and the magnitude of the horns is discussed, and the beam profiles, isodoses, and central axis depth doses before and after the changes are compared.