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 for calculating 10-MV x-ray primary and scatter dose including electron contamination dose.

This paper has improved some of the weak points appearing in a previous article [A. Iwasaki, Med. Phys. 17, 203-211 (1990)] dealing with the calculation of 10-MV x-ray primary and scatter dose. The main improved points are as follows: (i) A pair of new functional equations expressing the primary dose spread array has been yielded. Consequently, the accuracy of the primary dose calculation both in the aluminum layer and in the soft tissue layer beyond the aluminum has been improved. (ii) A new functional equation expressing the backscatter factor has been developed. It has been utilized in the differential backscatter factor equation. Consequently, the calculated scatter dose spread array has been improved. (iii) A method of calculating the dose due to electron contamination has been introduced. With respect to the primary dose, the primary plus scatter dose, and the primary plus scatter plus electron contamination dose, it has been shown how the depth of maximum dose (dmax) varies with field size.     

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.     

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.     

Dosimetry of 24-MV x rays from a linear accelerator

Dosimetric characteristics of a 24-MV photon beam produced by a Varian Clinac 2500 linear accelerator are presented. Particular attention is paid to measurements and applications relating to depth of maximum dose and scatter correction factors. New experimental methods were adopted to investigate scatter factors for field sizes ranging from 5 X 5 to 40 X 40 cm. For the largest field investigated, a 3% phantom scatter factor relative to a 10 X 10 cm field was determined. From the study, clinically useful scatter-phantom ratios were generated. The study also demonstrated that the estimated correction factors for scatter in a medium caused negligible changes (approximately equal to 0.1%) on the percent depth-dose values derived from the measured tissue-phantom ratio (TPR) data. The paper also quantitatively compares results obtained for dosimetric parameters like scatter output, tissue-phantom ratio, and percent depth-dose with those obtained from similar machines used for radiotherapy. Measured data relating to these parameters are expressed by taking a reference depth of 8 g/cm2 in a medium for normalization purposes and its importance discussed. Additional measurements presented in this work include beam quality and collimator effect on dose rate. Phantoms of various sizes and materials and a variety of detectors were used throughout the investigation.     

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.     

Dosimetry characteristics of large wedges for 4- and 6-MV x rays

Two sets of newly designed large wedge filters for field sizes up to 20 X 20 cm2 have become commercially available for use with 4- and 6-MV linear accelerators. Such field sizes are sometimes required to ensure adequate coverage in certain treatment techniques. This work reports base line data resulting from an investigation of the dosimetric properties of these wedges. Measurements of wedge angles, transmission factors, and beam hardening effects are described, and comparisons are made with the smaller standard wedges.     

The effects of a universal wedge and beam obliquity upon the central axis dose buildup for 6-MV x rays

In a number of clinical situations, the dose to the skin and the superficial tissues is of concern. Both beam obliquity and a beam modifier will modify the dose delivered to these regions due to changes in the scattering geometry, scattered photon and secondary electron production, and changes in the energy spectrum of a polyenergetic beam. Some linear accelerators use a single universal wedge mounted within the treatment head. Because such a wedge is at an extended distance from the patient, its contribution to the beam contaminants incident to the skin will be limited. Measurements of the ionization in the buildup region have been performed in a polystyrene phantom irradiated with a 6-MV x-ray beam from a linear accelerator equipped with a universal wedge. The variation of the buildup dose with obliquity, universal wedging, and distance from the source has been measured for angles of incidence between 0 degrees and 60 degrees and for effective wedge angles between 0 degrees and 45 degrees. The results indicate that the percentage buildup has a much stronger dependence upon the angle of incidence than upon the effective wedge angle. For distances approaching the treatment head, it is shown that the universal wedge generates secondary electrons that elevate the surface dose, but that this contribution decreases with distance.     

Tissue compensators with use of vinyl lead sheets for head and neck portals on 4-MV x rays

Tissue compensators made with a high atomic number material are compact and retain the skin-sparing effect of megavoltage beams. In this note, we have described the use of a commercially available vinyl lead as a material for tissue compensators for use with 4-MV beams on a Clinac-4/80. The dosimetric data obtained included the sheet's lead equivalence, thickness ratios, surface dose and buildup region, and transmission factors. The presence of a compensator did not alter the beam's skin-sparing effects. It was concluded that the vinyl lead sheets allowed an easy and rapid fabrication of a tissue compensator for head and neck portals of up to 15 X 15 cm.     

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.     

Broad beam attenuation of cobalt-60 gamma rays and 6-, 18-, and 25-MV x rays by lead

The measurement of photon attenuation by absorbing materials in radiation beams is usually performed using narrow beam geometry. However, radiotherapy treatments rarely simulate narrow beam conditions. The effect of broad beams on the attenuation of cobalt-60 gamma rays and 6-, 18-, and 25-MV x rays by lead is considered both theoretically and experimentally. The theoretical calculations are based on first-scatter considerations and indicate a dependence on treatment geometry (i.e., maximum scattering angle). Measured attenuation coefficients vary by as much as 16%, comparing narrow and broad beam data for typical treatment conditions. Broad beam attenuation coefficients, calculated from the measured zero-area values agree with the measured results to within 2% on average with a maximum deviation of 5%. A very simple empirical procedure is proposed to determine attenuation coefficients for a wide range of field sizes. This agrees with the measured data to within 0.5% on average and 4% at worst. With irregularly shaped absorbers, a sector integration technique is considered in combination with a first-scatter calculation. For a long rectangular field, the agreement between measured and calculated attenuation coefficients is better than 1.5% for all energies. The question of accuracy in attenuation coefficients needed to give an accuracy of 3% in relative transmission is addressed and shows a dependence on absorber thickness. For 75% transmission, the required accuracy in attenuation coefficient is 10%, whereas for 25% transmission, it should be improved to 2%.     

Buildup/surface dose and exit dose measurements for a 6-MV linear accelerator

The central axis dose distribution in the buildup region for the Varian Clinac 6/100 6-MV x-ray beam was measured in a polystyrene phantom using a fixed volume (0.5 cm3) parallel-plate ionization chamber (2.4-mm plate separation). Results for the surface dose measurements ranged from approximately 8% of the maximum dose for a 5 X 5 cm field, up to 36% for a 40 X 40 cm field, 100-cm source-skin distance. The effect of a 0.6-cm-thick polycarbonate blocking tray and metal filters on the surface and buildup dose is also reported. In addition, ionization measurements were made to document the dose perturbations caused by the absence of backscattering material at the exit surface of a polystyrene phantom. Exit dose measurements showed a 15% reduction in dose with essentially no scattering material beyond the measurement point. Near full scatter condition could be restored by placing 5-10 mm (depending on field size) of unit density material directly behind the ion chamber's distal surface.     

Characteristics of Mevatron 77 15-MV photon beam

The characteristics of 15-MV photon beam of Mevatron 77 have been examined with respect to clinical parameters. Some of the characteristics of this 15-MV photon beam are different from those of similar machines used in radiotherapy. The tissue-maximum ratio values have been determined experimentally for SAD 100 cm and percentage depth dose values have been determined from tissue-maximum ratio data. It is found that the experimentally determined PDD values for SSD 80, 100, and 120 cm are within +/- 1% of the calculated data. The scatter-maximum ratio data have been calculated from the experimentally determined zero area attenuation coefficient and tissue-maximum ratio data and are presented as function of depth and radii of circular fields. The relative dose factors have been determined both for square and rectangular fields. The relative dose factor for rectangular fields depends on the two pairs of jaws differently and the maximum difference is about 1.5%. The flatness and symmetry are within the specifications set by the manufacturer.     

Head scatter off-axis for megavoltage x rays

Our objective in this study has been to investigate how head scatter varies with the off-axis position in a 6 MV x-ray beam. We define the head-scatter off-axis ratio, HOA, as the ratio of the kerma due to head-scatter photons at the off-axis position x to the kerma from direct primary photons on the central axis. "Direct primary" are those photons that come from the source without interactions in the intervening structures. We determined HOA from measurements with an ionization chamber in a miniphantom. Head-scatter and direct primary photons contribute to a measurement of the ionization per mu Q(x) at the off-axis position x in the open field cx x cy. The ionization per mu QP(x), measured in the same position but with the field collimated to the smallest possible opening (cx x 3 cm), is intended to include only direct primary photons. Head-scatter photons cannot be completely eliminated, and the errors due to remaining head scatter and radiation back-scattered by the movable collimators into the monitor were estimated. For normalization of the final results, ionization due to direct primary photons was also measured on the central axis, QP(0). HOA was derived from these three measurements as HOA(cx,cy,x)=(Q(cx,cy,x) - QP(cx,cy,x))/QP(cx,cy,0). On the central axis (x=y=0), HOA represents the "scatter-to-primary ratio" between head scatter and the direct primary dose. Monte Carlo simulations were made to help with the interpretation and evaluation of the results. HOA could be fitted to a Gaussian model with two components corresponding to sources of widths 1.8 and 14 cm, projected on a plane 5 cm below the x-ray source. The narrow Gaussian component is interpreted as the source of photons scattered in the flattening filter and the primary collimator. The broad component is attributed to photons scattered in the secondary (variable) collimators. Conventional head-scatter models (e.g., a single Gaussian source model) do not fit the measured HOA data for large collimator settings (c>20 cm) or outside beam collimation. The full width at half-maximum (FWHM) of HOA(x) across the field increased with the field width (cx) in the direction of the measurements in a manner consistent with the field of view of the two sources. It was not sensitive to the field measure in the orthogonal direction (cy). Head scatter outside the field also increased with field size, reflecting an increased contribution of photons scattered at large angles. It exceeds the leakage through the collimator 2 cm outside the edge for square fields c>10 cm. Monte Carlo calculations showed considerably less head scatter outside the field than measurements.     

A photon dose distribution model employing convolution calculations

A three-dimensional photon beam calculation is described which models the primary, first-scatter, and multiple-scatter dose components from first principles. Three key features of the model are (1) a multiple-scatter calculation based on diffusion theory, (2) the demonstration of the modulation transfer function of the radiation dose transport process, and (3) the use of the finite fast Fourier transform to perform the required convolutions. The results of calculations for cobalt-60 in a homogeneous phantom are used to verify the accuracy of the model.     

A convolution algorithm for brachytherapy dose computations in heterogeneous geometries.

Currently-available brachytherapy dose computation algorithms ignore heterogeneities such as tissue-air interfaces, shielded gynecological colpostats, and tissue-composition variations in 125I implants despite dose computation errors as large as 40%. To calculate dose in the presence of tissue and applicator heterogeneities, a computer code has been developed that describes scatter dose as a 3-D spatial integral which convolves primary photon fluence with a dose-spread array. The dose-spread array describes the distribution of dose due to multiple scattering about a single primary interaction site and is precomputed by the Monte Carlo method. To correct for heterogeneities traversed by the primary photons, the dose-spread array is renormalized to reflect the density and composition of the element, and the distance to the point of interest is scaled by the path-length of the intervening medium. Convolution calculations for 125I and 137Cs point sources in the presence of finite phantoms, air voids and high-density shields have been compared to the corresponding Monte Carlo calculations. The convolution code absolute and relative dose rate predictions are shown to agree with Monte Carlo calculations within 3%. Direct evaluation of the 3-D spatial convolution integral using 1-D adaptive integration reveals efficiency gains of 20-50 relative to Monte Carlo photon-transport calculations.     

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.     

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.     

A method of calculating peripheral dose distributions of photon beams below 10 MV.

The radiation dose outside the radiotherapy treatment field can be of clinical concern and, therefore, a method of accurately predicting the peripheral doses received by tissues would be beneficial. This paper describes a semiempirical method developed for calculating the peripheral dose received at points outside the collimated field edge for incident photon beams with energies below neutron production thresholds (less than 10 MV). The dependence of the peripheral dose upon depth, distance, field shape and size, azimuthal angle about the central axis, external contour variations, and tissue heterogeneities are accounted for by this calculation. Predictions by this algorithm are compared with measurements and it is shown that the method is capable of reproducing the measured peripheral dose values usually to within the statistical uncertainties of the data.