A simple system for manual image reconstruction from pairs of X ray films.

A simple mechanical back-projection system for X ray films is described which is easy to construct and implement. It enables mechanical simulation of the X ray geometry used when taking pairs of isocentric radiographs for reconstruction purposes. Such pairs may be conventional "AP and Lateral" sets but often it is preferable to take them in oblique directions on the order of 90 degrees apart. The device and the reconstruction method have proved to be very useful in determining target volumes for radiation treatment planning, especially if surgical clips and/or distinct anatomical structures are present. As an instructional tool it has advantages over an also locally developed computer-assisted method of reconstruction. The present system has proved to be highly useful especially in delineating the target volume for treatment planning of soft tissue sarcomas of the extremities and peripheral parts of the body, where detailed and accurate tailoring of shielding blocks is often of vital importance.

Clinical use of a match-line wedge for adjacent megavoltage radiation field matching.

The divergence and sharp penumbra of linear accelerator beams pose notorious problems when joining such beams side by side. One way of reducing the dose distribution nonuniformity in the matching region is to create a wide pseudo-penumbra with the use of a "match-line wedge." A single match-line wedge shape has been developed for 6 MV and 10 MV photon beams. The wide pseudo-penumbra created by the wedge drastically reduces the effect of random set-up errors. Special attention has been paid to ensure simple and reliable clinical use of the wedge. Details of the design, construction, dosimetry, and rules of practical application are presented. Comparisons of several matching methods are made.

The imaging revolution and radiation oncology: use of CT, ultrasound, and NMR for localization, treatment planning and treatment delivery.

The explosion of new imaging technologies such as X ray computed tomography (CT), ultrasound (US), positron emission tomography (PET), and nuclear magnetic resonance imaging (NMR) has forced a major change in radiation therapy treatment planning philosophy and procedures. Modern computer technology has been wedded to these new imaging modalities, making possible sophisticated radiation therapy treatment planning using both the detailed anatomical and density information that is made available by CT and the other imaging modalities. This has forced a revolution in the way treatments are planned, with the result that actual beam configurations are typically both more complex and more carefully tailored to the desired target volume. This increase in precision and accuracy will presumably improve the results of radiation therapy.

A technique for field matching in primary breast irradiation.

The intrinsic divergence of photon beams presents serious matching problems in three-field treatment of the breast and the adjoining supraclavicular area. A method is presented in which appropriate beam blocking combined with suitable isocentric rotation of the treatment couch neutralize the affects of divergence so that proper matching is achieved at all depths. The geometric principles and the set-up procedures are discussed and illustrated.

Dosimetric considerations in treating mediastinal disease with mantle fields: characterization of the dose under mantle blocks.

PURPOSE: While the rationale for using mantle fields is well understood and the prescription of these fields is straightforward, the underlying complexity of the dose distributions that result is not generally appreciated. This is especially true in the choice of lung block design, which affects the dose to both the target volume as well as to the normal lung tissue. The key to the design of optimal lung blocks is the physician's perception of the complex relationship between the geometric and dosimetric aspects of heavily modified fields, as well as how the physical and anatomical properties of the target volume and the shape of the patient's lungs relate to the images visualized on simulator films. METHODS AND MATERIALS: Depth doses and cross-beam profiles of blocks ranging in width from 1 cm to 10 cm were taken using an automated beam scanning system. These data were then converted to "shadow fields." The results were compared to open fields of the same size using standard methodology. RESULTS: Shadow fields behave quite similarly to small, open fields in terms of x-ray-light field congruence, flatness, symmetry, and penumbra. There is a 2-3 mm rim between the edge of the block and the point at which it becomes nominally effective. The dose at the center of a block, which gives the normalization of the shadow fields, is given by a block transmission factor (BTF), which produces results in excellent agreement with measurements over a wide variety of block sizes and tissue depths. CONCLUSION: The radiation dose under shielding blocks can be considerably higher than expected, and care must be exercised when drawing blocks close to critical structures. The effects of blocks can be described in terms of normalized shadow fields, which behave similar to narrow, open fields, but with a divergence characteristic of their position relative to the radiation source. The normalization value for these fields, which gives the relative dose under the block, can be obtained from a straightforward analytical expression, the BTF.

Is correction for lung density in radiotherapy treatment planning necessary?

From 1978-981 a series of 30 patients with cancer of the esophagus were treated at the National Cancer Institute. Each of these patients had a CT scan of the chest taken in the treatment position, but prior to any treatment being given. Using these scans a retrospective analysis of the effect of lung density on delivered dose was performed. This indicated that failure to correct for tissue inhomogeneity results in a much higher dose being delivered than is prescribed. This effect is dependent on the energy of the beam being used for treatment; it may exceed 30% for 60Co. It also showed that there is wide patient to patient variation in lung density and that this variation is non-randomly distributed. The average lung density in his group of patients was 0.21 compared to the standard estimate of 0.35 but some had densities substantially lower than this, these being the patients with the largest lung volumes. This variability acts to further increase the discrepancy between prescribed and delivered dose even in a very homogeneous group of patients being treated under identical conditions for the same malignancy. The implications of this for future clinical trials in thoracic malignancies are discussed.

The influence of lung density corrections on treatment planning for primary breast cancer.

Primary breast cancer is generally treated with opposed radiation beams oriented tangentially with respect to the breast. This technique attempts to minimize the dose to the lung and other normal tissues, while at the same time producing a uniform dose distribution throughout the irradiated breast. Although a part of the lung is always included in the tangential breast fields, the effect of this low density tissue on the dose distribution is rarely taken into account. In the present work, the effect of lung density correction on the dose distribution resulting from tangential breast fields is analyzed. Treatment plans for a series of 34 patients treated for breast cancer have been performed using CT data. To study the effect of density corrections on the tangential field treatment plans for these patients, eight separate treatment plans for each patient have been optimized. For each of four photon energies (60Co, and 4, 6, and 10 MV X rays), treatment plans have been optimized for each patient when density correction is employed, and when unit density is assumed. Four additional dose calculations have been obtained for each patient corresponding to use of the unit density plan, but with density corrections employed in the calculation. The effects that density correction has on the wedge angles used, on the maximum dose ("hot spot") for each of several cross-sectional cuts, on the prescription isodose level which is chosen for each plan, and on homogeneity of the dose distribution over the target volume are all analyzed for the above described plans.

Intraoperative radiation therapy at the National Cancer Institute: technical innovations and dosimetry.

The technical complexity of intraoperative radiotherapy (IORT) requires modification of the standard physical and dosimetric methods used in external electron beam therapy. At the National Cancer Institute, a number of technical innovations have been integrated into ongoing clinical studies of IORT. These include: (1) an electron beam applicator system that is significantly different from other IORT systems and includes customized "squircle" applicators; (2) peripheral dose shields; (3) a modified surgical table replacing the standard radiation treatment couch; and (4) routine use of multiple IORT fields that necessitates field matching. The IORT applicator system and related devices and techniques are dosimetrically characterized in detail both for use in the IORT program and in order to illustrate many useful facets of electron dosimetry.

An isocentric chair for the simulation and treatment of radiation therapy patients.

There are a variety of clinical situations in which patients undergoing radiation therapy can benefit from being treated in an upright position. The authors describe a new design for a treatment chair to assist in accomplishing this task. The present chair differs from previous designs in that it can be used with existing radiotherapy simulators as well as treatment units and that it permits isocentric setup and treatment of tumors either at the nominal source-to-axis distance (SAD) of a machine or at extended distance. This design permits treatment of mediastinal tumors as well as those of the head and neck using a variety of field arrangements including AP-PA, opposed laterals, and multiple obliques. The seat is designed on the "tool platform" principle. A wide variety of devices can be attached onto it to ensure accurate and reproducible, yet comfortable, patient positioning.

A simple CT aperture emulator for use with a radiotherapy simulator.

Tumor localization in radiation treatment planning often involves the generation of quantitative anatomical data from multiple imaging modalities. It is desirable to take all of the images in the selected treatment position, which is usually decided upon during the initial simulator session. The different scanning modalities are often operated by different staff, at different times and in different locations; thus, it is difficult to ensure consistency in the position of the patient's body, and its documentation, at various times and places. Also, devices such as CT and MR scanners frequently pose restrictions due to their limited apertures. Failure to consider the physical limitations of such scanning equipment at the time of simulation or localization may result in placing the patient in a treatment position which will not fit through the aperture of the CT (or MRI) scanner, or which will result in a clinically important portion of the anatomy being "cut off" in the resulting scans. This can lead to re-simulation of the patient or result in a lack of accurate coordination of simulator and CT scan data. To minimize problems such as these, we have developed a CT Aperture Emulator which can be used at the time of the initial simulation. This is a lightweight "halo" easily attached to the simulator, which mimics the size and shape of the CT aperture. It permits reproducible adjustment of the patient's position, while allowing technologists and physicians to set up the patient with respect to potential CT constraints, in particular with regard to the use of immobilization and support devices. The emulator device also facilitates reproducing a patient's treatment position on the CT scanner. The concept has been found to have additional clinical uses and can be extended to a variety of imaging equipment.

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.

Time-dose response of human tumors and normal tissues during and after fractionated radiation treatment. A new model.

This paper presents the background and some results of initial applications of a new model of time-dose response of tumors as well as fast-renewing normal tissues, to fractionated radiation therapy. Both the linear-quadratic and the single-hit/single-target, single-hit/multi-target model may be used for the single-dose survival of both the viable stem cells and the clonogenic tumor cells. Normal tissue tolerance is expressed as a minimum acceptable level of normal tissue functionality, due to insufficient production of replacement cells, which in turn is caused by radiation-induced depletion of the viable stem cell population. A logistic function describes the homeostatically controlled inter-fraction and post-treatment normal tissue stem cell repopulation. The onset of stem cell repopulation may be delayed, and the doubling rate of clonogenic tumor cells may increase, upon the onset of treatment. Criteria for the selection of acceptable parameter values for normal tissue as well as tumors are described. An interactive Fortran 77 program has been developed to assist in the search for acceptable parameter values, the simulation of the time-dose response of normal tissues and tumors to conventional clinical fractionation schemes and the exploration of alternative schedules, including hyperfractionation. Some provisional results are presented.

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.

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.

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.

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.

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)

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.

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.

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.