Many radiotherapy linear accelerators use electron beam applicators which extended close to the patient's surface when treating at the regular distance. The relatively large size of these applicators often necessitates the use of a larger SSD than that designed by the manufacturer. In such cases, the addition of shielding blocks to the applicator can significantly alter the factors which should be used to calculate the dose rate at the new SSD as compared with the unshielded beam condition. In some typical clinical situations, errors of greater than 60% may result from failure to account for this perturbation. This paper presents the proper correction of the dose vs SSD function for the presence of such shielding blocks for the Clinac-18 linear accelerator.

Electrons↗

PubMedDate not supplied

Peripheral dose from megavolt beams.

The peripheral dose (PD), defined as the dose outside of therapeutic radiation beams, has been investigated for 60Co, 4-, 6-, and 10-MV x-ray machines. The measurements have been carried out down to dose levels of about 0.1% of the peak dose in the beam, since that dose level may be of clinical importance in some situations. The PD measurements for the various machines are qualitatively similar, which allows the identification of a simple basic data set which can characterize the PD for any particular machine. The PD has been separated into two components: in-phantom scatter dose and transmission (leakage) dose. Knowledge of the two components is important clinically when shielding is considered.

Cobalt Radioisotopes↗

PubMedDate not supplied

The net fractional depth dose: a basis for a unified analytical description of FDD, TAR, TMR, and TPR.

The net fractional depth dose (NFD) is defined as the fractional depth dose (FDD) corrected for inverse square law. Analysis of its behavior as a function of depth, field size, and source-surface distance has led to an analytical description with only seven model parameters related to straightforward physical properties. The determination of the characteristic parameter values requires only seven experimentally determined FDDs. The validity of the description has been tested for beam qualities ranging from 60Co gamma rays to 18-MV x rays, using published data from several different sources as well as locally measured data sets. The small number of model parameters is attractive for computer or hand-held calculator applications. The small amount of required measured data is important in view of practical data acquisition for implementation of a computer-based dose calculation system. The generating function allows easy and accurate generation of FDD, tissue-air ratio, tissue-maximum ratio, and tissue-phantom ratio tables.

Cobalt Radioisotopes↗

PubMedDate not supplied

A new description of the photon beam peak-depth profile as a function of field size.

The dose profile at peak depth in water is described as the product of an apparatus function and a source function. In principle, the source function is the circularly symmetric profile which would be measured at peak depth without any collimation. In practice, the peak-depth profile in the diagonal plane, measured for the largest collimator setting, is used for this purpose. The apparatus function represents the collimator acting upon the source function, and is referred to as the collimator function. The collimator function for any field size can be developed from the ratio of the peak-depth profile for a single medium-sized field and the source function. The method has been tested for a set of irregularly flattened 4-MV x-ray beams as well as for practically flat 15-MV x-ray beams. The model requires as basic data only three peak-depth profiles: one in each principal plane of a medium-sized square field and the peak-depth profile in the diagonal plane for the largest field. It replaces the peak-depth transformation in the projective beam model.

Humans↗

PubMedDate not supplied

Modification of the fault logic circuit of a high-energy linear accelerator to accommodate selectively coded, large-field wedges.

A modification to the fault logic circuit that controls the collimator (COLL) fault is described. This modification permits the use of large-field wedges by adding an additional input into the reference voltage that determines the fault condition. The resistor controlling the amount of additional voltage is carried on board each wedge, within the wedge plug. This allows each wedge to determine its own, individual field size limit. Additionally, if no coding resistor is provided, the factory-supplied reference voltage is used, which sets the maximum allowable field size to 15 cm. This permits the use of factory-supplied wedges in conjunction with selected, large-field wedges, allowing proper sensing of the field size maximum in all conditions.

Equipment Safety↗

PubMedDate not supplied

The extended net fractional depth dose: correction for inhomogeneities, including effects of electron transport in photon beam dose calculation.

The extended net fractional depth dose (ENFD) is developed from the net fractional depth dose (NFD) previously described for unit-density media, basically by scaling the two geometric parameters, the side of the equivalent square field, and the depth along the ray by the relative electron density. Specifically, in the analytical description for the NFD, the geometric depth is replaced by the radiologic depth and, along the ray path, the geometric field side is scaled by the relative electron density. Interface effects on the electron and scattered-photon fluences are accounted for. In addition, a simple function is developed to correct for the effect of lateral as well as longitudinal electron transport at the central ray. In the present work the inhomogeneities are assumed to be of planar parallel shape and to extend across the entire beam. The treatment of smaller inhomogeneities is outlined but will be treated in detail separately. Calculated results are compared to measured and calculated data from the literature for 60Co and 10-MV x-rays, and to 15-MV data measured at the NCI.

Biophysical Phenomena↗

PubMedDate not supplied

A new model for computerized clinical electron beam dosimetry.

Clinical electron beams consist of primary electrons, primary bremsstrahlung generated in the regular photon and electron collimator system determining the composite beam, and some short-range contaminant photon and electron scatter arising from the lower parts of the standard or regular electron applicator. Any beam-shaping insert placed inside the applicator causes some extra ("contaminant") bremsstrahlung and electron scatter. The new dose calculation model is based on separate treatment of these components. For the calculation of the primary electron dose we use experimentally determined electron scatter functions and differential electron scatter functions. The primary bremsstrahlung is treated as an unflattened but otherwise regular x-ray beam. The contaminant components arising from the rim area of the regular electron collimator and from beam-shaping inserts are considered separately. The behavior of the in-air ionization profiles is described using the concepts of effective electron source position and effective electron source diameter. The model has been tested for several electron energies.

Computer Simulation↗

PubMedDate not supplied

The use of a bar code scanner to improve the utility and flexibility of record and verify systems used in radiation therapy.

Record and verify systems used in radiation therapy serve a useful purpose in verification of machine parameters for each radiation field and monitoring the treatment as it is administered. There are, however, limitations as to the completeness of this monitoring. These restrictions are primarily due to design limitations of accelerators, which provide only a limited number of hardwired signals for use by such systems. The extent of the signals provided varies among manufacturers. As a result, some commonly used treatment accessories, such as blocking trays, may not be recognized by these systems. Additionally, current commercial record and verify systems cannot be expanded to accommodate institution-specific, customized treatment accessories or devices for positioning or immobilization of patients. This paper describes a complementary approach to providing device detection using a bar code scanner to read coded labels mounted on treatment accessories and download the data into the record and verify system for processing. A microcomputer-based system employing a portable bar code scanner was developed to evaluate the potential of this concept. Implications of adding bar code scanners to record and verify systems are discussed.

Humans↗

PubMedDate not supplied

Improving precision and safety in the use of beam modifying devices in radiation therapy.

Reliable and safe implementation of beam modifying devices such as wedges and block trays requires careful design and construction. Inappropriate design may pose problems ranging from user-hostile operation to hard-to-track, but significant variations in actual position in a beam. This may cause variation in actual wedge output factors, or variation in the position of a block tray. In case of simple mechanical failure or personnel mistake, design related mechanical conditions may result in injury to either a patient or a staff member. This paper is based on experience with linear accelerators from one manufacturer, but similar conditions are likely to exist with other radiation machines. A simple technical modification is offered which improves both accuracy and reproducibility in the placement of wedge-type filters. For our machines the solution also provides improved safety in the use of both wedge trays and block trays.

Equipment Design↗

PubMedDate not supplied

Extra lethal damage due to residual incompletely repaired sublethal damage in hyperfractionated and continuous radiation treatment.

In the conventional linear-quadratic model of single-dose response, the alpha and beta terms reflect lethal damage created during the delivery of a dose, from two different presumed molecular processes, one linear with dose, the other quadratic. With the conventional one-fraction-per-day (or less) regimens, the sublethal damage (SLD), presumably repairing exponentially over time, is essentially completely fixed by the time of the next dose of radiation. If this assumption is true, the effects of subsequent fractions of radiation should be independent, that is, there should be little, if any, reversible damage left from previous fractions, at the time of the next dose. For multiple daily fractions, or for the limiting case, continuous radiation, this simplification may overlook damaged cells that have had insufficient time for repair. A generalized method is presented for accounting for extra lethal damage (ELD) arising from such residual SLD for hyperfractionation and continuous irradiation schemes. It may help to predict differences in toxicity and tumor control, if any, obtained with "unconventional" treatment regimens. A key element in the present model is the finite size and the dynamic character of the pool of sublethal damage. Besides creating the usual linear and quadratic components of lethal damage, each new fraction converts a certain fraction of the existing SLD into ELD, and creates some new SLD. The expressions developed by Thames [Int. J. Radiat. Biol. 47, 319-339 (1987)] for fractionated treatment (the IR model) and by Dale [Br. J. Radiol. 58, 515-528 (1985); 59, 919-927 (1986)] for protracted and fractionated treatment are found to be similar to our results in the limiting case where the pool of SLD is very large (infinite).(ABSTRACT TRUNCATED AT 250 WORDS)

Cell Survival↗

PubMedDate not supplied

[Dosimetry of large irregular fields].

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Radiation Dosage↗