Evaluation of a beam-spot camera for megavoltage x rays

A beam-spot camera for measurements of x-ray focal spots of accelerators has been evaluated. The device consists of closely packed 0.25-mm-thick lead and 0.25-mm-thick cardboard strips. It is placed on radiographic film with the lamellae parallel to the beam axis and an exposure made. The images were scanned with a microdensitometer. The results indicate that the broadening of the source intensity profile at half maximum is of the order 1 mm, which permits the use of the beam-spot camera for acceptance testing and quality control. Longer tails in the density profile limit the quantitative information that can be extracted from the images.     

Collimator-produced energy degradation of megavoltage x rays

Monte Carlo computer calculations have been made to assess the effects of small collimators (i.e., output field sizes less than or equal to 1 cm) on megavoltage x-ray beams. Such collimators are often used in experiments to measure beam energy, half value layers, and other beam parameters. Our calculations demonstrate, however, that such procedures can introduce significant changes in these same parameters. Under certain conditions, more than 40% of the transmitted photons exiting the collimator do so with degraded energy. Optimum collimator design, however, can greatly reduce these effects. In particular, it is shown that the thickness of material required to effect a good collimator is not only dependent upon the photon energy, but also upon the size of the collimator hole.     

Correction of scatter in megavoltage cone-beam CT

The role of scatter in a cone-beam computed tomography system using the therapeutic beam of a medical linear accelerator and a commercial electronic portal imaging device (EPID) is investigated. A scatter correction method is presented which is based on a superposition of Monte Carlo generated scatter kernels. The kernels are adapted to both the spectral response of the EPID and the dimensions of the phantom being scanned. The method is part of a calibration procedure which converts the measured transmission data acquired for each projection angle into water-equivalent thicknesses. Tomographic reconstruction of the projections then yields an estimate of the electron density distribution of the phantom. It is found that scatter produces cupping artefacts in the reconstructed tomograms. Furthermore, reconstructed electron densities deviate greatly (by about 30%) from their expected values. The scatter correction method removes the cupping artefacts and decreases the deviations from 30% down to about 8%.     

A megavoltage scatter correction technique for cone-beam CT images acquired during VMAT delivery

Kilovoltage cone-beam CT (kV CBCT) can be acquired during the delivery of volumetric modulated arc therapy (VMAT), in order to obtain an image of the patient during treatment. However, the quality of such CBCTs is degraded by megavoltage (MV) scatter from the treatment beam onto the imaging panel. The objective of this paper is to introduce a novel MV scatter correction method for simultaneous CBCT during VMAT, and to investigate its effectiveness when compared to other techniques. The correction requires the acquisition of a separate set of images taken during VMAT delivery, while the kV beam is off. These images--which contain only the MV scatter contribution on the imaging panel--are then used to correct the corresponding kV/MV projections. To test this method, CBCTs were taken of an image quality phantom during VMAT delivery and measurements of contrast to noise ratio were made. Additionally, the correction was applied to the datasets of three VMAT prostate patients, who also received simultaneous CBCTs. The clinical image quality was assessed using a validated scoring system, comparing standard CBCTs to the uncorrected simultaneous CBCTs and a variety of correction methods. Results show that the correction is able to recover some of the low and high-contrast signal to noise ratio lost due to MV scatter. From the patient study, the corrected CBCT scored significantly higher than the uncorrected images in terms of the ability to identify the boundary between the prostate and surrounding soft tissue. In summary, a simple MV scatter correction method has been developed and, using both phantom and patient data, is shown to improve the image quality of simultaneous CBCTs taken during VMAT delivery.     

Dependence of scatter on atomic number for x rays from tungsten and molybdenum anodes in the mammographic energy range.

A study was done to determine the relative amounts of scatter for the following materials with atomic numbers ranging from Z=6 to Z=82: C, Al, Ti, Fe, Cu, Zn, Zr, Y, Mo, Ta, and Pb. Measurements were performed for each material on two constant potential x-ray units--one fitted with a molybdenum (Mo) anode-Mo filter and the other with a tungsten (W) anode-aluminum (Al) filter (medium filtration) at 30 kVp. Theoretical calculations were also performed for each anode to explain the scatter behavior and to aid in predicting the behavior for materials where measurements were not made. There was good agreement between the theoretical calculations and the experimental data.     

Penumbral measurements in water for high-energy x rays

Ionization chambers of varying inside diameter have been used to investigate the penumbral region of 60Co, 6-MV, and 31-MV x-ray beams. Measurements were made in water at varying depths up to 25 cm for a square field of side length 10 cm. The dependence of the penumbral widths on both the inside diameter of the ionization chamber and the depth in water is established along with the asymmetry of the penumbral distributions about the 50% level. A standard correction is indicated to eliminate the dependence of the measured penumbral widths on the inside diameter of the ionization chamber.     

Focused beam-stop array for the measurement of scatter in megavoltage portal and cone beam CT imaging

We describe a focused beam-stop array (BSA) for the measurement of object scatter in imaging systems that utilize x-ray beams in the megavoltage (MV) energy range. The BSA consists of 64 doubly truncated tungsten cone elements of 0.5 cm maximum diameter that are arranged in a regular array on an acrylic slab. The BSA is placed in the accessory tray of a medical linear accelerator at a distance of approximately 50 cm from the focal spot. We derive an expression that allows us to estimate the scatter in an image taken without the array present, given image values in a second image with the array in place. The presence of the array reduces fluence incident on the imaged object. This leads to an object-dependent underestimation bias in the scatter measurements. We apply corrections in order to address this issue. We compare estimates of the flat panel detector response to scatter obtained using the BSA to those derived from Monte Carlo simulations. We find that the two estimates agree to within 10% in terms of RMS error for 30 cm x 30 cm water slabs in the thickness range of 10-30 cm. Larger errors in the scatter estimates are encountered for thinner objects, probably owing to extrafocal radiation sources. However, RMS errors in the estimates of primary images are no more than 5% for water slab thicknesses in the range of 1-30 cm. The BSA scatter estimates are also used to correct cone beam tomographic projections. Maximum deviations of central profiles of uniform water phantoms are reduced from 193 to 19 HU after application of corrections for scatter, beam hardening, and lateral truncation that are based on the BSA-derived scatter estimate. The same corrections remove the typical cupping artifact from both phantom and patient images. The BSA proves to be a useful tool for quantifying and removing image scatter, as well as for validating models of MV imaging systems.     

Generation of portal film charts for 10-MV x rays.

An analytical method to generate portal film charts for 10-MV photon beams, which takes into account the presence of the cassette front screen is presented. The selection of the best film-screen combination was based on the new AAPM recommendations for radiotherapy portal imaging [AAPM Rep. No. 24, AAPM Task Group No. 28 (1987)]: 2-g/cm2-thick copper front screen and 0.4-g/cm2-thick copper rear screen, and X-OMAT TL Kodak unwrapped film. Doses at the film position were measured as a function of patient thickness, field size, air gap, and the results compared well with the doses derived from the analytical method: within +/- 15%. An optical density of 1.6 was selected for construction of the portal film charts. The application of the method for routine treatment planning quality assurance allows a quick and precise determination of the best exposures to the portal films.     

Lung dose corrections for 6- and 15-MV x rays

We have measured the radiation dose in simple heterogeneous phantoms and compared our results with those obtained by various methods of computation. Dose data were obtained both within and distal to simulated regions of lung in order to test the ratio of tissue-air ratios (TAR), Batho, and equivalent TAR methods. These procedures are used routinely in manual and computer-aided planning of radiation therapy, but have been validated primarily for cobalt-60 radiation. Tests performed with 6- and 15-MV x rays reveal that incorrect doses can be computed within or near to a low-density medium, particularly when the field size is small. In these cases, electronic equilibrium is not achieved in the lateral direction, thereby violating an implicit assumption of all the above calculation methods. We quantify the errors in dose calculation for simple slab phantoms, and support our interpretation with a Monte Carlo simulation in which the energy transported by charged particles away from sites of x-ray interactions is considered directly.     

A convolution method of calculating dose for 15-MV x rays

Arrays were generated using the Monte Carlo method representing the energy absorbed throughout waterlike phantoms from charged particles and scatter radiation set in motion by primary interactions at one location. The resulting "dose spread arrays" were normalized to the collision fraction of the kinetic energy released by the primary photons. These arrays are convolved with the relative primary fluence interacting in a phantom to obtain three-dimensional dose distributions. The method gives good agreement for the 15-MV x-ray dose in electronic disequilibrium situations, such as the buildup region, near beam boundaries, and near low-density heterogeneities.     

Head scatter data for several linear accelerators (4-18 MV)

Measurements were made in air on the central axis of radiation beams from linear accelerators operating in the energy range from 4 to 18 MV, to determine the magnitude and source of head scattered radiation. Machines of several manufacturers were studied. The data indicate that, except for one unique collimator design, head scatter originates primarily in the flattening filter and is relatively independent of energy and machine.     

A comparison of ionization-chamber and water-calorimeter dosimetry for high-energy x rays.

The temperature-regulated, flexible, water calorimeter developed in the authors' laboratory was shown previously to yield a dose-to-water from 4-MV x rays that is in very close agreement with ionization measurements made in accordance with the AAPM dosimetry protocol. The range of beam energies for this type of comparison has been increased to include 60Co, and 4-, 6- and 25-MV x rays. The grand mean of the ratios of doses obtained from the calorimeter and ionization chamber, the Cal/Ion ratio, for the four beam energies studied is 1.001 +/- 0.001. As no significant trend with beam energy was detected, it is concluded that the calorimeter and ionization chamber yield equally accurate results. Because the calibration of the calorimeter depends solely upon the accuracy with which water temperatures in the range 2-10 degrees C can be measured, and dose is given by the product of the specific heat of water and the temperature change produced by irradiation, the water calorimeter has the potential to place radiation dosimetry on a much firmer foundation than presently exists.     

Microdosimetry of 10-15 MeV bremsstrahlung x rays

Experimental techniques have been developed for obtaining microdosimetric spectra on a hospital-based linear accelerator. Teletherapy beams of 10 and 15 MeV bremsstrahlung x rays from a Varian Clinac-18 and Clinac-20, respectively, have been produced at ultralow dose rates (50-200 microGy/h) which enables direct measurements of lineal energy distributions with a conventional Rossi-type gas proportional counter. Extensive measurements have been made to insure that the dosimetric properties of these low dose rate beams are nearly identical to those produced under high dose rate clinical conditions. Analytical procedures have been developed to correct measured lineal energy spectra for pileup caused by the low duty factor of the linear accelerator. The lineal energy spectra of these megavoltage beams differ significantly from Co-60, with dose averaged lineal energies (yD) being 20%-30% lower than for Co-60. Although such differences may not be important at clinical doses, the theory of dual radiation action does predict a lower biological effectiveness for these beams at very low dose levels.     

Accuracy of lung dose calculations for large-field irradiation with 6-MV x rays

In large-field irradiations of the upper half-body, there is a potential for severe respiratory complications. The incidence of severe radiation damage to lung can be reduced by limiting the lung dose and the volume of lung irradiated. It is necessary to have an accurate method of calculating the dose actually delivered to lung, and this work deals with such calculations and measurements in large (half-body) fields of 6-MV x rays. The results of various lung dose calculations by the Batho, the modified Batho (Lulu and Bj?rngard), the simple scaled tissue-maximum ratio and the detailed equivalent tissue-air ratio methods are compared with doses measured in phantoms representing the mediastinum and adjacent lungs.     

Dose measurements and calculations of small radiation fields for 9-MV x rays

Measurements of dose distribution for square fields with sizes ranging from 1 X 1 to 30 X 30 cm for a 9-MV x-ray beam from a Neptune 10 linear accelerator, manufactured by CGR, are reported. Special attention was paid to field sizes smaller than 4 X 4 cm, used in radiosurgery techniques. To express the dose-monitor units relationship, total, collimator, and phantom scatter correction factors were obtained by experimental measurements. A strong dependence of these factors on the smallest field sizes (less than 4 X 4 cm) was shown. Measurements of the maximum depth dose dmax, plotted as a function of field size, showed a maximum at about 5 X 5 cm, in good agreement with previous results. dmax was also measured for the smallest fields, demonstrating that the contaminating electron component of the x-ray beam was not responsible for the dmax shift. Analysis of the penumbra width of cross dose distributions, as a function of field sizes, allowed us to postulate that the dmax shift could be due to the phantom scattered photons, which in turn were generated by the collimator scattered photons. Newly derived tissue-maximum ratio and scatter-maximum ratio data were used for dose profile calculations of 2 X 2, 4 X 4, and 10 X 10 cm field sizes. The agreement between experimental and calculated data was found to be +/- 2% within the geometrical edges of the fields and +/- 6% outside of them. A dose profile from the isocenter of a 2 X 2 cm square field moving through a 360 degree rotation arc was obtained and compared with that from the center of a 125I shielded source, as measured by Ling. Advantages and problems relating to the use of x-ray beams from linear accelerators in radiosurgery are discussed.     

A modified power-law formula for inhomogeneity corrections in beams of high-energy x rays

The Batho power-law formula is in common use in many treatment planning systems to correct for the presence of lungs and other inhomogeneities. While giving excellent agreement with measurement for Cobalt-60 radiation, it tends to underestimate the lung correction required for higher energy x rays and is undefined for distances beyond an interface less than the buildup distance. This paper suggests a simple modification that greatly improves the agreement with measured data and gives a continuously defined function at all depths. Measurements have been made in a polystyrene and cork phantom to simulate the effects of lung; data are presented for beams of 8-MV x rays, 16-MV x rays, and Cobalt-60 gamma rays.