Dosimetric aspects of the therapeutic photon beams from a dual-energy linear accelerator

Parameters of the photon beams (6 and 20 MV) from a dual-energy linear accelerator (Mevatron-KD, Siemens Medical Laboratories, CA) are presented. The depth dose characteristics of the photon beams are dmax of 1.8 and 3.8 cm and percentage depth dose of 68% and 80% at 10-cm depth and 100-cm source-surface distance for a field size of 10 X 10 cm2 for 6 and 20 MV, respectively. The 6 and 20 MV beams were found to correspond to nominal accelerating potentials of 4.7 and 17 MV, respectively. The stability of output is within +/- 1% and flatness and symmetry are within +/- 3%. These figures compare favorably with the manufacturer's specifications.     

Dosimetric properties of photon beams from a flattening filter free clinical accelerator

Basic dosimetric properties of 6 MV and 18 MV photon beams from a Varian Clinac 21EX accelerator operating without the flattening filter have been measured. These include dose rate data, depth dose dependencies and lateral profiles in a water phantom, total scatter factors and transmission factors of a multileaf collimator. The data are reviewed and compared with measurements for the flattened beams. The unflattened beams have the following: a higher dose rate by factors of 2.3 (6 MV) and 5.5 (18 MV) on the central axis; lower out-of-field dose due to reduced head scatter and softer spectra; less variation of the total scatter factor with field size; and less variation of the shape of lateral dose profiles with depth. The findings suggest that with a flattening filter free accelerator better radiation treatments can be developed, with shorter delivery times and lower doses to normal tissues and organs.     

Corner transmission in several linear accelerator photon beams

Maximum x-ray field sizes on many linear accelerators are obtained only with truncated corners. Transmissions through the corners of such fields has been measured utilizing film and ion chamber dosimetry for a number of accelerators. Transmissions are found to be significantly larger than for the movable jaws.     

The characterization of unflattened photon beams from a 6 MV linear accelerator

Commissioning data have been measured for an Elekta Precise linear accelerator running at 6 MV without a flattening filter with the aim of studying the effects of flattening filter removal on machine operation and beam characterization. Modern radiotherapy practice now routinely relies on the use of fluence modifying techniques such as IMRT, i.e. the active production of non-flat beams. For these techniques the flattening filter should not be necessary. It is also possible that the increased intensity around the central axis associated with unflattened beams may be useful for conventional treatment planning by acting as a field-in-field or integrated boost technique. For this reason open and wedged field data are presented. Whilst problems exist in running the machine filter free clinically, this paper shows that in many ways the beam is actually more stable, exhibiting almost half the variation in field symmetry for changes in steering and bending currents. Dosimetric benefits are reported here which include a reduction in head scatter by approx. 70%, decreased penumbra (0.5 mm), lower dose outside of the field edge (11%) and a doubling in dose rate (2.3 times for open and 1.9 times for wedged fields). Measurements also show that reduced scatter also reduces leakage radiation by approx. 60%, significantly lowering whole body doses. The greatest benefit of filter-free use is perceived to be for IMRT where increased dose rate combined with reduced head scatter and leakage radiation should lead to improved dose calculation, giving simpler, faster and more accurate dose delivery with reduced dose to normal tissues.     

Measurements of Gamma-Knife helmet output factors using a radiophotoluminescent glass rod dosimeter and a diode detector

A radiophotoluminescent (RPL) glass rod dosimeter (GRD) and a small active volume p-type silicon diode detector are used for the measurement of the output factors from Gamma-Knife fields. The GRD system consists of small rod-shaped glass chip detectors and an automatic readout device. The output factors measured with the GRD from the 14, 8 and 4 mm helmets relative to the 18 mm helmet are 0.981, 0.942 and 0.877, respectively. Similarly, the corresponding output factors measured with the p-type silicon diode detector are 0.980, 0.949 and 0.867, respectively. The output factors are corrected for the end effect for each helmet. The output factors obtained from both detectors are in good agreement with the values in a recent publication and the values recommended by Elekta, the manufacturer. The directional dependence of these detectors is also measured. For the Gamma-Knife angle ranging from 6 to 36 degrees in the y-z plane of the stereotactic space, the measured angular dependence of the GRD is approximately 1.0% at a 4 MV x-ray beam. The response of the silicon diode detector indicates approximately 3-4% directional dependence for the same angular range for a 6 MV x-ray beam. The Gamma-Knife helmet output factors measured with the silicon diode detector are corrected for angular dependence.     

Determination of the source position for the electron beams from a high-energy linear accelerator

We have investigated the energy and field-size dependence of the source position of the electron beams from a Varian Clinac-2,500 accelerator. Three independent experimental methods were used: (1) multipinhole camera (MPC), (2) back projection of the full width at half maximum (FWHM), and (3) the inverse square law (ISL). The positions of the virtual and effective sources were calculated using the multiple Coulomb scattering (MCS) formalism. The results obtained from the MPC agree, within the experimental uncertainties, with the calculated values for the virtual source position. Similarly, the results from the FWHM method agree with the calculations with the exception of those for small field sizes at the lower energies. This is consistent with the fact that both kinds of measurements are not very sensitive to scattering in the photon and electron collimators. In contrast, the source position determined by the ISL method shows strong dependence on field size and energy, and does not agree with the values predicted by the MCS formalism. This is due to contamination from electrons scattered in the x ray and electron collimation system. The techniques and results reported here should be generally applicable to other scatter foil linear accelerators.     

A diamond detector in the dosimetry of high-energy electron and photon beams.

A diamond detector type 60003 (PTW Freiburg) was examined for the purpose of dosimetry with 4-20 MeV electron beams and 4-25 MV photon beams. Results were compared with those obtained by using a Markus chamber for electron beams and an ionization chamber for photon beams. Dose distributions were measured in a water phantom with the detector connected to a Unidos electrometer (PTW Freiburg). After a pre-irradiation of about 5 Gy the diamond detector shows a stability in response which is better than that of an ionization chamber. The current of the diamond detector was measured under variation of photon beam dose rate between 0.1 and 7 Gy min(-1). Different FSDs were chosen. Furthermore the pulse repetition frequency and the depth of the detector were changed. The electron beam dose rate was varied between 0.23 and 4.6 Gy min(-1) by changing the pulse-repetition frequency. The response shows no energy dependence within the covered photon-beam energy range. Between 4 MeV and 18 MeV electron beam energy it shows only a small energy dependence of about 2%, as expected from theory. For smaller electron energies the response increases significantly and an influence of the contact material used for the diamond detector can be surmised. A slight sublinearity of the current and dose rate was found. Detector current and dose rate are related by the expression i alpha Ddelta, where i is the detector current, D is the dose rate and delta is a correction factor of approximately 0.963. Depth-dose curves of photon beams, measured with the diamond detector, show a slight overestimation compared with measurements with the ionization chamber. This overestimation is compensated for by the above correction term. The superior spatial resolution of the diamond detector leads to minor deviations between depth-dose curves of electron beams measured with a Markus chamber and a diamond detector.     

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.     

Dosimetric aspects of a 3.3-MV linear accelerator

Both the design considerations and the dosimetric properties of the Siemens Model 5800 linear accelerator are discussed. This unit is of such an energy (3.3 MV) as to imitate Cobalt-60 teletherapy depth doses. A linear relation of dmax to depth dose at low energies was found for various wave guides and targets. The energy of the unit can be characterized by its nominal accelerating potential of 2.70 MV, its d80 of 5.3 cm, its first half-value layer of 0.8 cm lead and the measured energy of the electron beam at 3.3 MeV. The following selected commissioning aspects are reported: central axis depth dose, relative output factors, beam profiles, wedge factors, virtual source position, back scatter factors, penumbra and build-up region.     

Optimization of parameters for fitting linear accelerator photon beams using a modified CBEAM model

Measured beam profiles and central-axis depth-dose data for 6- and 25-MV photon beams are used to generate a dose matrix which represents the full beam. A corresponding dose matrix is also calculated using the modified CBEAM model. The calculational model uses the usual set of three parameters to define the intensity at beam edges and the parameter that accounts for collimator transmission. An additional set of three parameters is used for the primary profile factor, expressed as a function of distance from the central axis. An optimization program has been adapted to automatically adjust these parameters to minimize the chi 2 between the measured and calculated data. The average values of the parameters for small (6 X 6 cm2), medium (10 X 10 cm2), and large (20 X 20 cm2) field sizes are found to represent the beam adequately for all field sizes. The calculated and the measured doses at any point agree to within 2% for any field size in the range 4 X 4 to 40 X 40 cm2.     

VMC++ versus BEAMnrc: a comparison of simulated linear accelerator heads for photon beams

BEAMnrc, a code for simulating medical linear accelerators based on EGSnrc, has been bench-marked and used extensively in the scientific literature and is therefore often considered to be the gold standard for Monte Carlo simulations for radiotherapy applications. However, its long computation times make it too slow for the clinical routine and often even for research purposes without a large investment in computing resources. VMC++ is a much faster code thanks to the intensive use of variance reduction techniques and a much faster implementation of the condensed history technique for charged particle transport. A research version of this code is also capable of simulating the full head of linear accelerators operated in photon mode (excluding multileaf collimators, hard and dynamic wedges). In this work, a validation of the full head simulation at 6 and 18 MV is performed, simulating with VMC++ and BEAMnrc the addition of one head component at a time and comparing the resulting phase space files. For the comparison, photon and electron fluence, photon energy fluence, mean energy, and photon spectra are considered. The largest absolute differences are found in the energy fluences. For all the simulations of the different head components, a very good agreement (differences in energy fluences between VMC++ and BEAMnrc <1%) is obtained. Only a particular case at 6 MV shows a somewhat larger energy fluence difference of 1.4%. Dosimetrically, these phase space differences imply an agreement between both codes at the <1% level, making VMC++ head module suitable for full head simulations with considerable gain in efficiency and without loss of accuracy.     

Comparative dosimetry in narrow high-energy photon beams

A comparison of the response of different dosimeters in narrow photon beams (phi > or = 4 mm) of 6 and 18 MV bremsstrahlung has been performed. The detectors used were a natural diamond detector, a liquid ionization chamber, a plastic scintillator and two dedicated silicon diodes. The diodes had a very small detection volume and one was a specially designed double diode using two parallel opposed active volumes with compensating interface perturbations. The characteristics of the detectors were investigated both for dose distribution measurements, such as depth-dose curves and lateral beam profiles, and for output factors. The dose rate and angular dependence of the diamond and the two diodes were also studied separately. The depth-dose distributions for small fields agree well for the diamond, the scintillator and the single diode, while the measured dose maximum for the double diode is about 1% higher and for the liquid chamber about 1% lower than the mean of the others when normalized at a depth of 10 cm. The plastic scintillator and the liquid ionization chamber detect a penumbra width that is slightly broadened due to the influence of their finite size, while the double diode may even underestimate the penumbra width due to its small size and high density. When corrected for the extension of the detector volume a good agreement with Monte Carlo calculated beam profiles was obtained for the plastic scintillator and the liquid ionization chamber. Profiles measured with the diamond show an asymmetry when positioned with the smallest dimension facing the beam, while the double diode, the scintillator and the liquid chamber measure symmetric profiles irrespective of positioning. Significant differences in the output factors were obtained with the different detectors. The natural diamond detector measures output factors close to those with an ionization chamber (less than 1% difference) for field sizes between 3 x 3 and 15 x 15 cm2, but overestimates the output factors for large fields and underestimates the output factors for the smallest field sizes. The single and double diodes overestimated the output factor for large field sizes by up to 7 and 12% respectively due to the high content of low-energy photons. The double diode, and to some extent the single diode, also showed a relative increase in response compared with the more water equivalent liquid chamber and plastic scintillator at the smallest fields where there is a lack of lateral electron equilibrium. Both the plastic scintillator and the liquid chamber also show responses that deviate from the ionization chamber for larger field sizes. The major deviations can be explained based on the characteristics of the sensitive materials and the construction of the detectors.     

The forward production of high-energy electrons from megavoltage photon beams

The forward production of high-energy electrons from materials with various atomic numbers from carbon to lead has been measured for megavoltage photon beams from 4- to 25-MV peak bremsstrahlung energy by placing a thin-window parallel-plate ionization chamber directly behind foils of the various materials. The relative forward production of electrons decreases with atomic number for energies less than or equal to 10 MV until about Z = 50, after which it rises. For photon energies greater than or equal to 15 MV, forward production increases with atomic number with a break point at Z approximately 50, beyond which the curve becomes steeper.     

Dosimetry of a dual photon energy linear accelerator

We describe the dosimetric characteristics of a newly introduced dual photon energy linear accelerator, the Varian Clinac 1800. Depth doses are compared with other accelerators of the same nominal accelerating potentials (6 and 10 MV). Field flatness at dmax and at 10 cm depth, depth of dmax, wedge characteristics, output factors, and doses in the buildup region are presented.     

Characteristics of the high-energy photon beam of a 25-MeV accelerator

The CGR Saturne 25 is an isocentrically mounted standing wave medical linear accelerator that produces dual-energy photon beams and a scanned electron beam with six selectable energies between 4 and 25 MeV. The highest energy photon beam is nominally referred to as 23 MV. For this beam the mean energy of the accelerated electron beam on the 1.3 radiation length (4 mm) tungsten x-ray target is found to be approximately 21 MeV, with the energy acceptance stated to be +/- 5%. The electron beam traverses a 270 degrees bending magnet upstream of the x-ray production target. The resulting bremsstrahlung beam passes through a combination steel and lead flattening filter, 4-cm maximum thickness. Dosimetric data for the 23-MV beam are presented with respect to rectangular field output factor, depth of maximum dose as a function of field size, surface and buildup dose, central axis percent depth dose, tissue-phantom ratios, beam profile, applicability of inverse square, and block transmission. Some data are also presented on the effect of different flattening filter designs on apparent beam energy.     

Radiation protection aspects of a new high-energy linear accelerator

The Therac-25 is a new 25-MeV linear accelerator manufactured by Atomic Energy of Canada, Ltd. The first two units have recently been installed in Toronto, Ontario and Halifax, Nova Scotia. Calculations and measurements of primary and secondary radiation levels were made. Neutron dose-equivalent rates were measured inside and outside the room. The maximum leakage rate at 1 m from the accelerator target was 0.4% Sv per peak photon Gy. The tenth value layer for neutrons from the Therac-25, at the entrance to a one-legged maze was found to be 5.5 cm of polyethylene. Measurements were done to estimate daily technologist exposure due to induced activity in the treatment room.     

Characteristics of electron beams from a new 25-MeV linear accelerator

Clinically useful electron fields are produced on the Atomic Energy of Canada, Limited Therac 25 linear accelerator by computer-controlled scanning of the electron beam. Measurements were made to determine the properties of these electron fields. Central axis percentage depth dose and bremsstrahlung background were compared for these fields and for the fields from selected machines that use scattering foils. Dose calibrations were made in both water and polystyrene using the American Association of Physicists in Medicine Task Group 21 protocol. Measurements were made to determine the relative output factors, virtual source position, and the attenuation of the electron fields by lead.