Rectangular Phantom Research Articles

Validation of Ultrafast Computed Tomographic Left Ventricular Volume Measurement

Left ventricular volume has been measured with ultrafast computed tomography. However, the accuracy with which this can be done is unknown. We therefore imaged with ultrafast computed tomography 11 rectangular phantoms, 20 to 225 ml, and 17 left ventricular casts, 15 to 112 ml. Two observers planimetered serial tomographic images and computed volume from sequential tomograms. There was no significant inter- or intraobserver difference in measurement of phantoms. Deviation of ultrafast computed tomographic volume from true phantom volume was -0.1 +/- 3.5% SD, range 9.0 to -7.6%. Correlation of true phantom volume with ultrafast computed tomographic volume was 0.99, SEE = 1.9 ml. No significant difference was observed between merged and single ultrafast computed tomographic scanning sequences. Left ventricular cast volume determined by ultrafast computed tomography deviated from true volume by 6% +/- 20%, range 54% to -45%. Correlation of true volume with ultrafast computed tomographic volume was 0.99, SEE = 5.1 ml. There was no interobserver significant difference in measurement of left ventricular cast volume. Correlation between ultrafast computed tomographic volume and cineradiographic volume of the same left ventricular casts was 0.99, SEE = 4.4 ml. Thus, phantom volumes can be measured accurately without significant intra- or interobserver variation. Merged scanning sequences did not influence volume determination. Left ventricular cast volume determination was comparable to that obtained with cineradiography.

Medulloblastoma: Treatment technique and radiation dosimetry

It is well established that elective cranio-spinal irradiation is beneficial in the treatment of medulloblastoma. However, because of the geometry of the brain and the spinal column, a simple treatment procedure which results in a uniform dose over the entire target volume while maintaining a low dose to other tissues is not easy to perform. This report describes a relatively simple treatment technique which can be administered with radiotherapy apparatus that is available in most treatment centers. Special consideration is given to the dosimetry in the junction regions of the contiguous 60Co fields. Computer dosage calculations were made to check the effectiveness of this technique. This calculated data was verified experimentally with thermoluminescent dosimetry (TLD) in both a rectangular polystyrene phantom as well as the human like Alderson Rando phantom. The dose to critical organs such as the eyes was found to be less than 15% of the tumor dose, while the male gonadal dose was less than 10%. Detailed consideration is given to the factors affecting the overall variation of dose in the entire target volume. It has been estimated that using this technique, the overall dosage variation to the cranio-spinal axis is within ±10% of the prescribed tumor dose for the average adult, while for the average child it is better than ±5k.

TMR values in the build-up region for a 6MV linear accelerator

Absorbed dose in the build-up region of high energy X- or gamma -ray beams varies with angle of incidence and scattering conditions; this region is not adequately treated by most presently used digital methods of dose computation. Contributions of dose from build-up regions are most important when dose distributions from beams of grazing angular incident and/or from a combination of fields are computed. In an effort to improve accuracy of clinical dosimetry the authors have developed a method of dose and TMR determination in the build-up region for any field size and angle and have applied it to the Varian 6 MV Linac. Dose distributions in an Alderson, rectangular or other appropriate phantom can be obtained by equidensitometry of individually calibrated radiographic film exposed in the phantom, using an electro-optical method of image density quantization. The inherently high spatial resolution of this method is verified by simultaneous exposure of Harshaw TLD rods with the film at selected points in the phantom. Dose distributions computed using these TMR values were compared with the experimental results.

Dosimetry of irregularly shaped radiation therapy fields. I. Some factors influencing inhomogeneity.

Large radiotherapyfields are being employed with increasing frequency in the treatment of Hodgkin's disease. The rationale for their use has been discussed extensively elsewhere (1–7). One such portal, commonly called the "mantle" field, encompasses all structures of thorax and neck excluding only the lungs (Fig. 1). This study was undertaken to determine the magnitude of the inhomogeneities of dose within such an irregular field and to devise means of making the dose more uniform. Before dealing with inhomogeneities in dose related to the patient's thorax, however, we wished to first examine separately the importance of the penumbra of the lung shields upon dose inhomogeneity. In Part I, therefore, a rectangular water phantom was used in the experiments to be described. Experiment I Methods and Materials The University of Minnesota's Canadian AEC “Eldorado 8” cobalt-60 unit (source diameter = 2.5 cm) and lead lung shields (7 cm in thickness) were used in the same manner as they are employed clinically. A 40 × 40 × 50-cm water tank was positioned 100 cm from the source to serve as a phantom. The lung shields were placed with their closest surfaces 40 cm from the front surface of the water tank, approximating the distance of the blocks from the patient, which is usually employed clinically. This distance is hereafter called the blocks-to-surface distance or “BSD.” The lung shields were separated from each other in a plane perpendicular to the central axis of the treatment beam such that an 8-cm-wide “mediastinal” field was projected onto the front surface of the water tank. This 8 cm width as defined was held constant for all experiments, as was the overall field size of 30 × 30 cm. Several lead disks were mounted on a Lucite strip and placed 10 cm deep to the front of the tank to simulate a mid-medi-astinal mass density which could be related to the image of the lung shields on the portal film. With use of the field-localizing light of the cobalt machine, the margin of the largest lead disk was aligned so that its silhouette was tangent to the shadow of the medial border of the left lung shield as the two were projected onto a film cassette placed 20 cm from the front of the tank. This tangent relationship of the two shadows produced by the light source was adopted primarily as a means of aligning these two key shadows as various other experimental conditions were varied. With this arrangement, all the important elements producing images on the portal film of a patient with a mediastinal mass density were duplicated (Fig. 2). The exposure time producing the best tonal values across the penumbra was then determined, and this exposure time was used for all portal films made. The dosimeter probe of an automatic isodose plotter (Toshiba model MRA-101-3B) was inserted into the water tank (Fig. 2). The position of this probe at different depth-dose readings at 10 cm depth was recorded on “portal” films.