J. van de Geijn

Dose to lung in primary breast irradiation.

Using anatomic data derived from computerized tomography (CT) scans of the torso, the volume of lung irradiated during primary breast treatment has been measured for a variety of irradiation techniques. Two-field tangential plans which are angled into lung to treat also the internal mammary nodes have been compared to three-field plans which include a separate internal mammary field (IMF). The volume of lung achieving high dose (greater than 3000 rad) is similar in both techniques when photons only are used. Electron beam treatment of the IMF is successful in lowering the lung dose. Additional treatment plans that angle the IMF parallel to the tangential fields may offer some theoretical advantage.

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.

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 6 0Co and 10‐MV x rays, and to 15‐MV data measured at the NCI.

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.

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.

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.

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.