Central Axis Depth Dose Research Articles

Benchmark measurements for lung dose corrections for X-ray beams.

A well defined set of clinically relevant reference measurements for photon dose calculations in the presence of the lung have been provided. These benchmark data were mainly obtained in low-density (rho = 0.31 gcm-3) lunglike material as well as in waterlike plastic for 4 and 15 MV X-ray beams. Some additional measurements were performed with materials having a density of 0.015 gcm-3 and 0.18 gcm-3. Phantom geometries included simple layered geometries, finite lung cross section geometries, simulated mediastinum geometries, and simulated tumor in lung geometries. The data are reported as central axis depth doses. A number of parameters were varied, including the field size, the lung geometry, and the distance in and behind the lung.

A Monte Carlo investigation of surface doses for broad electron beams

High surface doses and a trend for surface dose to increase with increasing field size were observed with broad electron beams from a Philips SL75-20 linear accelerator. This indicates substantial contamination of the beam by the beam defining system. The EGS4 Monte Carlo code has been used to simulate the transport of the 10 MeV electron beam through the beam defining system. The trend for surface dose to increase with field size has been reproduced and the agreement between simulated and experimental central axis depth dose data is good. Two reasons for the unusual behaviour of surface dose with field size have been found. The adjustable X-ray collimators shield the phantom from secondary electrons. The electron applicators filter the beam as they get narrower, raising the mean energy of the electrons. The variations in photon dose and electron angular distribution with field size were not found to have a significant effect on surface dose. The electron energy spectra are broader than conventional formulae imply, using the half-value-layer depth and practical range. High photon fluence was observed for all field sizes.

A model of heat production and transport in a water calorimeter.

The heat diffusion equation for a water calorimeter under isothermal and adiabatic boundary conditions has been solved analytically for a general source function. Based upon this model an interactive computer program has been developed and employed to predict the temporal variation of the temperature distribution in a water calorimeter irradiated with 250 kV, 4-, 6-, and 25-MV x rays and 7-, 13-, and 32-MeV electrons. The model requires central-axis depth dose curves as inputs and allows for any number of alternate heating and cooling periods. It is found that the shape of the cooling curve following an irradiation period depends upon the location of the point of measurement, the choice of boundary conditions, and the thermal history of the calorimeter.

Dose computations for asymmetric fields defined by independent jaws.

Asymmetric fields defined by independent jaws can be used to split a beam or to match adjacent fields. We have extended a method originally developed for symmetric fields to calculate the dose for asymmetric fields. The dose to a point is computed as the product of the tissue maximum ratio (TMR), the off center ratio (OCR), and the inverse square factor. The TMR is computed from the measured central axis depth doses for symmetric fields. The OCR is obtained by multiplying the primary OCR (POCR) and the boundary factors (BF's) for the four jaws. The POCR's and BF's were derived from measured beam profiles, which include the effect of off-axis beam quality variations. Using this method, the beam profiles and isodose distributions for asymmetric fields of a 6-MV accelerator were calculated and compared with the measured data. The agreement is within experimental errors both in the penumbra region and along the central ray of the asymmetric field.

Dosimetric Evaluation in Heterogeneous Tissue of Anterior Electron Beam Irradiation for Treatment of Retinoblastoma: Medical Physics, Volume 14, No. 5, September/October 1987, 772–779

A dosimetric study of anterior electron beam irradiation for treatment of retinoblastoma was performed to evaluate the influence of tissue heterogeneities on the dose distribution within the eye and the accuracy of the dose calculated by a pencil beam algorithm. Film measurements were made in a variety of polystyrene phantoms and in a removable polystyrene eye incorporated into a tissue substitute phantom constructed from a human skull. Measurements in polystyrene phantoms were used to demonstrate the algorithm's ability to predict the effect of a lens block placed in the beam, as well as the eye's irregular surface shape. The eye phantom was used to measure dose distributions within the eye in both the sagittal and transverse planes in order to test the algorithm's ability to predict the dose distribution when bony heterogeneities are present. Results show (1) that previous treatment planning conclusions based on flat, uniform phantoms for central-axis depth dose are adequate; (2) that a three-dimensional heterogeneity correction is required for accurate dose calculations; and (3) that if only a two-dimensional heterogeneity correction is used in calculating the dose, it is more accurate for the sagittal than the transverse plane.

Calculation of electron beam depth-dose curves and output factors for arbitrary field shapes

A previously presented method to calculate depth-dose curves and output factors for arbitrarily shaped electron beams [6] is evaluated. The method employs a Gaussian pencil model for direct incident and applicator scattered electrons; the parameter values are derived from measured central axis depth-dose distributions. In addition, an empirical model is used to compute the dose due to electrons scattered by field-defining frames. In this way, the properties of the clinical electron beams are taken into account. In this paper, calculations and measurements for electron beams with energies between 6 and 20 MeV, treatment field dimensions between 3 and 14 cm, and various applicator sizes are compared. The results demonstrate the importance of irregular field dose calculations and the scope of the present method. Agreement better than 3% in dose and 0.2 cm in depth is achieved. For electron beams without applicators, the calculations show the same accuracy. Another method in electron treatment planning that derives values for the radial width parameter of the pencil beam from measured broad beam profiles is also investigated. This method gives good results for dose calculations in beams without applicator scatter. It should be used with care, however, for beams that contain such a scatter component. When electrons scattered by the applicator walls and field-defining frames are neglected, differences between measured and calculated dose up to 8% are found.

Properties of the 18-MV photon beam from a dual energy linear accelerator.

The beam characteristics of the 18-MV photon beam of a Varian Clinac 1800 are presented. The clinically relevant parameters of central axis depth dose, tissue-phantom ratios, peak-scatter factors, and relative output factors are discussed and compared to 18-MV beam data previously reported. The nominal beam energy was found to be 18.3 +/- 0.8 MV on the central axis. The beam symmetry and uniformity meet the manufacturer's specifications. The inverse-square law is applicable within 1.4% over the clinically useful range of distances and field sizes. An empirical fit equation for the central axis depth dose is presented.

Performance characteristics of an orthovoltage x-ray therapy machine.

Performance characteristics sufficient to provide physical data base specific to the Siemen's Stabilipan 2 orthovoltage x-ray therapy machine are presented. Operating conditions covering the working range of the unit from 100 to 300 kVp are selected. Beam quality, output, the central axis depth dose, relative output factors, field flatness, uniformity index, and filtration characteristics of the beams are studied. Selected results are reported.

Dosimetric evaluation in heterogeneous tissue of anterior electron beam irradiation for treatment of retinoblastoma.

A dosimetric study of anterior electron beam irradiation for treatment of retinoblastoma was performed to evaluate the influence of tissue heterogeneities on the dose distribution within the eye and the accuracy of the dose calculated by a pencil beam algorithm. Film measurements were made in a variety of polystyrene phantoms and in a removable polystyrene eye incorporated into a tissue substitute phantom constructed from a human skull. Measurements in polystyrene phantoms were used to demonstrate the algorithm's ability to predict the effect of a lens block placed in the beam, as well as the eye's irregular surface shape. The eye phantom was used to measure dose distributions within the eye in both the sagittal and transverse planes in order to test the algorithm's ability to predict the dose distribution when bony heterogeneities are present. Results show (1) that previous treatment planning conclusions based on flat, uniform phantoms for central-axis depth dose are adequate; (2) that a three-dimensional heterogeneity correction is required for accurate dose calculations; and (3) that if only a two-dimensional heterogeneity correction is used in calculating the dose, it is more accurate for the sagittal than the transverse plane.

Parametrisation of depth dose for electron beams

Knowledge of the central axis depth dose curve is essential for the development of an accurate electron beam treatment planning system. The equation introduced by Kawachi (1975) and modified by Steben et al. (1976) gave general agreement with experimental data but it required a complex set of equations to extract the central axis depth dose. Even though interpolation of experimental data is one of the most common methods used in the available commercial treatment planning systems, an analytical expression might have the advantage of reducing both computer time and memory space. Unfortunately the electron beam central axis depth dose distribution has not yet been expressed accurately in an analytical form. Monte Carlo calculations are reasonably accurate, so they can be used as the standard for verifying complex experimental and theoretical work. However, they are impractical in day-to-day clinical work because of their demanding computational time. The authors have tried to reproduce the results of Monte Carlo calculations and experiments with an analytical expression. The analytical form may prove useful to centres which are now working on three-dimensional treatment planning codes for electron beam treatment planning.

Characteristic parameters of 6-22 MeV electron beams from a 25-MeV linear accelerator.

Depth-ionization measurements were performed using a thin wall parallel plate chamber in water at nominal electron energies of 6, 9, 12, 15, 18, and 22 MeV for the standard available square field sizes. The characteristic parameters of the central axis depth-dose distributions were derived and compared to corresponding values for other accelerators. Vacuum packed therapy-verification films were used in water to obtain isodose distributions in a plane containing the central axis of the beam. The uniformity index and penumbra of the beams were measured from isodose distributions obtained in planes perpendicular to the beam central axis, at depths of 1/2R85 in water.

Source-surface distance and source-axis distance beam data acquisition.

Radiation beam characteristics are usually obtained by scanning in a water phantom. Central axis depth dose data, beam profiles, and isodose curves are derived from these measurements. Certain treatment planning systems require that beam profiles be scanned isocentrically rather than at a constant source-surface distance. We have looked at profiles taken both ways, with appropriate field-size correction, and find no significant difference between them. This should simplify data acquisition in many cases.

Film dosimetry of small electron beams for routine radiotherapy planning.

The characteristics of very small fields, 1 X 1 and 2 X 2 cm, of electron beams of nominal energies, 5, 7, 10, 12, 15, and 18 MeV have been studied and compared to a 10 X 10 cm field. A parallel-plate ion chamber and film have been used to obtain various dose parameters. The central axis depth dose measurements, field flatness, uniformity index, and relative output factors are presented. It was found that satisfactory results for determining the relative output factor can be obtained from film data using a scanning densitometer. It is our conclusion that film dosimetry is acceptable in determining the necessary clinical parameters needed to treat patients with fields as small as 2 X 2 cm. For the 1 X 1 cm field size and for the electron energies greater than 10 MeV, there was substantial disagreement between the ion chamber and film data in the buildup region as well as the regions beyond the depth of maximum dose to the depth of 90% dose.

Calculated buildup curves for photons with energies up to 60Co.

This report investigates the buildup region of the depth-dose curve for broad parallel beams of 60Co and lower energy photons incident on slabs of tissue as defined by the International Commission on Radiation Units and Measurements (ICRU) and other materials. The EGS Monte Carlo system was used to simulate the transport of photons and electrons in the materials. Calculated buildup regions of the depth-dose curves are presented for photons incident on ICRU tissue. For 60Co beams, the primary and scatter dose components are presented, both for the buildup region and at depth in the phantom. Effects of beam radius were investigated and found to have only a small effect in the buildup region of the central axis depth-dose curve. The calculated effects of beam radius are larger at depth and in good agreement with experiment. Effects of scatter and attenuation in a small miniphantom of water were investigated as a function of photon energy and were found to cancel within 0.8%. The mean distance of electron transport was calculated for 60Co beams incident on carbon, water, polystyrene, aluminum, and lead.

Effective wedge angles with a universal wedge

Some recently designed X-ray producing accelerators are equipped with a single built-in wedge, and different 'effective' wedge angles are obtained by combining an open (unwedged) and a wedged field in the appropriate proportions. This paper describes a technique for determining these proportions from measured isodose distributions of the two component fields. The authors' data for the Philips SL/75 6 MV accelerator are compared with two existing theoretical models. One model, in which the beams are weighted by the ratio of the tangents of the effective and nominal wedge angles, agrees with the data to within 3 degrees over the range of effective wedge angles and square field sizes examined. The second and simpler model, in which the beams are weighted by the ratio of the wedge angles directly, results in errors of as much as 11 degrees . It is shown that both of these models are approximations to an exact theoretical solution which may be formulated in terms of one free parameter. This parameter may be interpreted physically as the ratio of the slopes of the central-axis depth-dose curves for the open and wedged fields.

Electron beam central axis depth dose measurements.

Central axis depth dose measurements were made by the Radiological Physics Center on over 70 electron-producing machines used in radiation therapy. These data were consistent for each machine model and nominal energy. However, the data show that depth dose relations can vary significantly among different machine models for electron beams having the same nominal energy. Analysis shows that both the method used to achieve beam flatness and the mean incident electron energy determine the central axis depth dose curve past the depth of maximum dose. A linear relation of depth dose versus mean incident electron energy is used to predict depth dose to within 2 mm for most electron beams used clinically at depths greater than d95.

Book Reviews

Book reviewed in this article:BJR Supplement 17: Central Axis Depth Dose Data for Use in Radiotherapy.Exercises in Diagnostic Ultrasonography of the Abdomen. By F.S. Weill and A. de Mouel.Atlas of Radiologic Anatomy, 2nd Edition. By Lothar Wicke.Notes on Radiological Diagnosis. The Trainee's Survival Kit. By Lawrence S.W. Lau and John F. de Campo.Introduction to Organ Imaging. A Guide to Use and Interpretation. By John A. Bloom‐field.M.C.Q.'s in Radiology. By A.H. Chapman and P.J. Robinson.Nuclear Magnetic Resonance and its Clinical Applications. Edited by R.E. Steiner and G.K. Radda.NMR Tomography of the Normal Brain. By Guenther Gademann.Report of Proceedings of a Study on the Protection of the Australian Public from Ionising Radiation

Second-order multiple-scattering theory for charged-particle teletherapy beams.

The Fermi-Eyges theory of multiple scattering, applicable to charged-particle teletherapy beams, has been generalized to second order in small quantities representing deviations from the particles' initial lines of travel and direction. The second-order multiple-scattering theory provides increased accuracy, and in particular it takes into account the skewness of the particles' paths, in calculating dose. Only the assumptions and certain results of the second-order theory are presented in this communication, for the immediate use of other investigators. An application to rectangular fields shows that the second-order theory predicts one component of the buildup of electron central axis depth dose, which the (first-order) Fermi-Eyges theory cannot do. (The other major component of buildup, which is not yet incorporated into this theoretical work, is due to high-energy secondary electrons). Various approximate calculations of electron central axis depth dose are compared with the second-order calculation.

Reduction of the "horns" observed on the beam profiles of a 6-MV linear accelerator.

The presence of large "horns" was found while plotting beam profiles during acceptance testing of a 6-MV linear accelerator. The in-phantom off-axis ratio (OAR), measured at 22 cm off the central axis along the diagonal of a 40 X 40 cm field at dmax was found to be 1.19, while beam uniformity was within specifications at 10-cm depth. A change in the gun injection voltage and the replacement of the magnet surrounding the magnetron with one of greater strength resulted in a reduction of the OAR to 1.085. The beam uniformity at depth was maintained within specifications. An alternative solution of adding a modifying filter in the primary beam was considered undesirable because of the 20%-25% reduction in dose rate caused by such filters. The relationship between the energy, the intensity distribution of the beam, and the magnitude of the horns is discussed, and the beam profiles, isodoses, and central axis depth doses before and after the changes are compared.

Dosimetric characterization of the 18-MV photon beam from the Siemens Mevatron 77 linear accelerator.

A comprehensive set of dosimetric measurements has been made on the Mevatron 77.80.67 18-MV photon beam. Percentage depth dose, dose in the buildup region, field size dependence of output, transmission through lead, tray attenuation, and isodose curves for the open and wedged fields were measured using an ionization chamber in water and polystyrene phantoms. These dosimetric measurements sufficiently characterized the beam to permit clinical use. The depth dose at 10-cm depth for a 10 X 10 cm2 field at 100-cm source-to-skin distance (SSD) is 80.9%, which meets design specifications. Central axis depth-dose data were fitted to within 0.5% by a set of polynomial equations utilizing a two-dimensional linear regression analysis. Tissue-maximum ratios calculated from depth-dose data agree with measured data to within 2%. Output differences as large as 2.5% were measured for rectangular fields depending on which collimator jaws defined the long dimension of the field. The field size dependence of output was fit to within +/- 0.1% by a linear regression. The half-value thickness of the beam was measured to be 13 mm of lead.