Characterization of a diode system for in vivo dosimetry with electron beams]

PURPOSE: Current quality assurance regulation stresses the basic role of in vivo dosimetry. Our study evaluates the usefulness and reliability of semiconductor diodes in determining the electron absorbed dose. MATERIAL AND METHODS: P-type EDE semiconductor detectors were irradiated with electron beams of different energies produced by a CGR Saturn Therac 20. The diode and ionization chamber response were compared, and effect of energy value, collimator opening, source skin distance and gantry angle on diode response was studied. RESULTS: Measurements show a maximum increment of about 20% in diode response increasing the beam energy (6-20 MeV). The response also increases with: collimator opening, reaching 5% with field sizes larger than 10x10 cm2 (with the exception of 20 MeV energy); SSD increase (with a maximum of 8% for 20 MeV); transversal gantry incidence, compared with the diode longitudinal axis; it does not affect the response in the interval of +/- 45 degrees. Absorbed dose attenuation at dmax, due to the presence of diode on the axis of the beam as a function of electron energy was also determined : the maximum attenuation value is 15% in 6 MeV electron beams. A dose calculation algorithm, taking into account diode response dependence was outlined. In vivo dosimetry was performed in 92 fields for 80 patients, with an agreement of +/-4 % (1 SD) between prescribed and measured dose. DISCUSSION AND CONCLUSIONS: It is possible to use the EDE semiconductor detectors on a quality control program of dose delivery for electron beam therapy, but particular attention should be paid to the beam incidence angle and diode dose attenuation.     

Electron beam effective source surface distances for a high energy linear accelerator

The design of the Varian Clinac 1800 linear accelerator electron applicator system does not allow clearance for all head and neck patients to be treated at the standard calibration distance of 100 cm. Discrepancies have been found between dose calculations using the inverse square law for extended distances and their measured data. A 4 X 4 cm2 applicator at an energy of 9 MeV, for example, had dose differences of 13 and 23% at distances of 105 and 110 cm SSD. Because of these discrepancies, effective source surface distances (SSDeff) were determined for all the standard electron energies and applicators of a Clinac 1800. These effective source surface distances ranged from 41.6 cm to 92.6 cm for the 4 X 4 cm2 cone/6 MeV electron beam through the 25 X 25 cm2 cone/20 MeV electron beam. A summary of these distances and an analysis of the clinical use of both a best fit SSDeff and a common SSDeff for patient dosimetry calculations is presented.     

A method of scaling the 3D electron pencil-beam dose calculation to obtain accurate monitor units for irregularly-shaped electron beams.

A method for determining monitor units (MU) for electron beams using a 3D pencil beam dose algorithm is described. A set of correction factors (OF(c)) to the pencil beam dose is generated that transforms the output into dose per MU. The OF(c)s depend on cone selection, field size, and source-to-surface distance (SSD) for a given electron energy. The dose per MU is determined by scaling the dose by the OF(c). The OF(c) value is determined using a measured set of relative output factors (OF(rel)) for various field sizes and SSD. The pencil-beam algorithm is used to compute the "raw" value (OF(p)) to each of the OF(rel) measurement points. The OF(c) is the ratio of the measured OF(rel) to the calculated OF(p). The OF(c) value for irregularly-shaped electron fields or for electron fields at extended SSD may be interpolated from the OF(c) table. The interpolation over SSD is performed linearly using the central axis SSD. The interpolation over field size is more complicated and uses the minimum area-circumscribing rectangle around the field shape. Due to the relative flatness of the OF(c), the interpolation is less error prone than the more common direct interpolation of the output factors. Computations were performed in the ADAC Pinnacle3 Planning System. Measurements were obtained using a Varian 2300c for 6 and 15 MeV. The results show that this method can predict the dose per MU within 1% to 2% for clinical fields and within 3% to 4% for extreme field shapes.     

The use of intensity-modulated radiation therapy photon beams for improving the dose uniformity of electron beams shaped with MLC

Electrons are ideal for treating shallow tumors and sparing adjacent normal tissue. Conventionally, electron beams are collimated by cut-outs that are time-consuming to make and difficult to adapt to tumor shape throughout the course of treatment. We propose that electron cut-outs can be replaced using photon multileaf collimator (MLC). Two major problems of this approach are that the scattering of electrons causes penumbra widening because of a large air gap, and available commercial treatment planning systems (TPSs) do not support MLC-collimated electron beams. In this study, these difficulties were overcome by (1) modeling electron beams collimated by photon MLC for a commercial TPS, and (2) developing a technique to reduce electron beam penumbra by adding low-energy intensity-modulated radiation therapy (IMRT) photons (4 MV). We used blocks to simulate MLC shielding in the TPS. Inverse planning was used to optimize boost photon beams. This technique was applied to a parotid and a central nervous system (CNS) clinical case. Combined photon and electron plans were compared with conventional plans and verified using ion chamber, film, and a 2D diode array. Our studies showed that the beam penumbra for mixed beams with 90 cm source to surface distance (SSD) is comparable with electron applicators and cut-outs at 100 cm SSD. Our mixed-beam technique yielded more uniform dose to the planning target volume and lower doses to various organs at risk for both parotid and CNS clinical cases. The plans were verified with measurements, with more than 95% points passing the gamma criteria of 5% in dose difference and 5 mm for distance to agreement. In conclusion, the study has demonstrated the feasibility and potential advantage of using photon MLC to collimate electron beams with boost photon IMRT fields.     

Depth dose and profile analysis using Library Least-Squares method

The Library Least-Squares method has been applied to detect changes in the depth dose and in the profiles of 6 and 20 MeV electron beams, and in 6 MeV photon beams from medical linear accelerators. The changes in the beam energy of 0.78% and 0.32% at low and high electron energies, respectively, induced by the insertion of 0.1 mm A1 absorber were clearly visible in the residual distributions. The changes in the photon beam were introduced by rotation of the gantry or the collimator. The applicability of the method to quantitatively monitor any changes in the treatment beam characteristics is presented.     

Dosimetric effects of bolus and lens shielding in treating ocular lymphomas with low-energy electrons

Radiation therapy is an effective treatment for primary orbital lymphomas. Lens shielding with electrons can reduce the risk of high-grade cataracts in patients undergoing treatment for superficial tumors. This work evaluates the dosimetric effects of a suspended eye shield, placement of bolus, and varying electron energies. Film (GafChromic EBT3) dosimetry and relative output factors were measured for 6, 8, and 10?MeV electron energies. A customized 5-cm diameter circle electron orbital cutout was constructed for a 6?×?6-cm applicator with a suspended lens shield (8-mm diameter Cerrobend cylinder, 2.2-cm length). Point doses were measured using a scanning electron diode in a solid water phantom at depths representative of the anterior and posterior lens. Depth dose profiles were compared for 0-mm, 3-mm, and 5-mm bolus thicknesses. At 5?mm (the approximate distance of the anterior lens from the surface of the cornea), the percent depth dose under the suspended lens shield was reduced to 15%, 15%, and 14% for electron energies 6, 8, and 10?MeV, respectively. Applying bolus reduced the benefit of lens shielding by increasing the estimated doses under the block to 27% for 3-mm and 44% for 5-mm bolus for a 6?MeV incident electron beam. This effect is minimized with 8?MeV electron beams where the corresponding values were 15.5% and 18% for 3-mm and 5-mm bolus. Introduction of a 7-mm hole in 5-mm bolus to stabilize eye motion during treatment altered lens doses by about 1%. Careful selection of electron energy and consideration of bolus effects are needed to account for electron scatter under a lens shield.     

Diode verification of routine electron-beam treatments.

A platinum doped n type commercial silicon electron diode/electrometer system was evaluated for use in our comprehensive external beam radiotherapy quality assurance program. Directional dependence of diode response was investigated by positioning the diode at the radiation isocenter and recording the response versus beam incident direction. Dose rate response was examined by testing for nonidealities in inverse square law behavior. Energy dependence of response was explored using 6-21 MeV beams. Dose response linearity was investigated from 50-800 cGy. Radiation perturbation was measured using localization film. Finally, the system was evaluated for initial treatment quality assurance in a number of clinical cases (N = 34). The diode response exhibited +/- 5% variation within +/- 20 degrees of the detector's marked "preferred" direction for the 6 MeV beam. Higher energy electron beams exhibited much less directional dependence. Dose rate response was found constant from 178-326 cGy/min (or 96-130 cm SSD). Energy dependence of response was constant within +/- 5.5% from 6-21 MeV. Dose response was linear (r2 = 0.999). Radiation field perturbation was significant at the depth of dose maximum (dmax). The maximum perturbation measured was -12.5% at dmax using the 6 MeV beam. Clinical trials of the system demonstrated that the diodes could be utilized for initial treatment quality assurance. The system proved effective for routine initial treatment electron beam in vivo dosimetry when the directional and energy dependent limitations of the system were respected.     

Build-up and depth-dose characteristics of different fast neutron beams relevant for radiotherapy.

Build-up and central axis depth-dose curves have been obtained for d(50) + Be and d + T neutron beams. Measurements carried out with the collimator opening covered with a layer of lead showed that for all three neutron beams the entrance dose is approximately 60% of the dose at the maximum. Consequently the skin-sparing properties of these neutron beams will be approximately equal and comparable to those for electron beam therapy. Central axis depth-dose curves have been established for d(50) + Be neutrons at 129 cm SSD, for p(42) + Be neutrons at 125 cm SSD and d + T neurtons and 60Co gamma rays at 80 cm SSD. The 50% dose values in a water phantom are at depths of 12.7 cm, 12.0 cm, 9.7 cm and 12.7 cm respectively, for field sizes of approximately 15 cm x 20 cm. Insertion of a 6 cm thick nylon filter in the p(42)+Be beam increases this value from 12.0 cm to 13.5 cm. The gamma component for the d+T neutron beam is higher than for the cyclotron beams.     

Dosimetric characteristics with spatial fractionation using electron grid therapy.

Recently, promising clinical results have been shown in the delivery of palliative treatments using megavoltage photon grid therapy. However, the use of megavoltage photon grid therapy is limited in the treatment of bulky superficial lesions where critical radiosensitive anatomical structures are present beyond tumor volumes. As a result, spatially fractionated electron grid therapy was investigated in this project. Dose distributions of 1.4-cm-thick cerrobend grid blocks were experimentally determined for electron beams ranging from 6 to 20 MeV. These blocks were designed and fabricated at out institution to fit into a 20 x 20-cm(2) electron cone of a commercially available linear accelerator. Beam profiles and percentage depth dose (PDD) curves were measured in Solid Water phantom material using radiographic film, LiF TLD, and ionometric techniques. Open-field PDD curves were compared with those of single holes grid with diameters of 1.5, 2.0, 2.5, 3.0, and 3.5 cm to find the optimum diameter. A 2.5-cm hole diameter was found to be the optimal size for all electron energies between 6 and 20 MeV. The results indicate peak-to-valley ratios decrease with depth and the largest ratio is found at Dmax. Also, the TLD measurements show that the dose under the blocked regions of the grid ranged from 9.7% to 39% of the dose beneath the grid holes, depending on the measurement location and beam energy.     

High energy fast neutrons from the Harwell variable energy cyclotron. I. Physical characteristics.

A high energy fast neutron beam potentially suitable for radiotherapy was built at the Harwell variable energy cyclotron. The beam line is described and results are given of physical measurements on the fast neutron beams produced by 42 MeV deuterons on thick (4 mm) and thin (2 mm) beryllium targets. With 20 muA beam current the entrance dose rate in a phantom 150 cm from the target was about 130 rad min-1 with the thick target and about 60 rad min-1 with the thin target. Therefore, it is possible to use both the thin target and the relatively large target-skin distance of 150 cm to improve depth dose for radiotherapy or radiobiology. With this arrangement the dose rate decreased to 50% at depths in the phantom of 11.3-15.4 cm, depending on the field size. The use of primarily hydrogenous materials for shielding and collimation provided beam edge definition similar to that of 60Co teletherapy units, and off-axis radiation levels of approximately 1% which compare favorably with 14 MeV deuteron-tritium generators. The copper backing of the thin target became highly radioactive and an alterative material may be preferable. Biologic characteristics of the beam are described in a companion paper.     

Study of a radiation technic with an anterior mantle-shaped field and 10 MV photons

The authors present a preliminary dosimetry study of 10 MV X-rays for irradiation technique with only one anterior mantle-field. The results of the studied beam for SSD = 120 cm are: 40 X 40 cm2 fields, build-up at 2.5 cm, penumbra (85% divided by 15%) variable between 8 divided by 9 mm, 50% of the dose at depth of 18 cm, homogeneity +/- 4%. These dosimetry results are suitable for beginning the clinical research.     

Dosimetric characteristics of unflattened 6 MV photon beams of a clinical linear accelerator: a Monte Carlo study.

The purpose of this study is to evaluate the dosimetric properties of a flattening filter free 6 MV photon beam. The 6 MV photon beam of a Varian Clinac 21 EX linac was modeled using the MCNP4C Monte Carlo (MC) code. Dosimetric features including central axis absorbed doses, beam profiles and photon energy spectra were calculated for flattened and unflattened 6 MV photon beams. A substantial increase in the dose rate was seen for the unflattened beam, which was decreased with field size and depth. The penumbra width was decreased less than 0.2 mm (about 5%) and a 25% decrease in out-of-field dose was observed for the unflattened beam. The photon energy spectra were softer for the unflattened beam and the mean energies of spectra were higher for smaller field size. Our study showed that increase in the dose rate and lower out-of-field dose could be considered as practical advantages for unflattened 6 MV beams.     

The Effect of a Carbon-Fiber Couch on the Depth-Dose Curves and Transmission Properties for Megavoltage Photon Beams

PURPOSE: To investigate the attenuation of a carbon-fiber tabletop and a combiboard, alongside with the depth-dose profile in a solid-water phantom. MATERIAL AND METHODS: Depth-dose measurements were performed with a Roos chamber for 6- and 10-MV beams for a typical field size (15 cm x 15 cm, SSD [source-surface distance] 100 cm). A rigid-stem ionization chamber was used to measure transmission factors. RESULTS: Transmission factors varied between 93.6% and 97.3% for the 6-MV beam, and 95.1% and 97.7% for the 10-MV photon beam. The lowest transmission factors were observed for the oblique gantry angle of 150 degrees with the table-combiboard combination. The surface dose normalized to a depth of 5 cm increased from 59.4% (without table, 0 degrees gantry), to 108.6% (tabletop present, 180 degrees gantry), and further to 120% (table-combiboard combination) for 6-MV photon beam. For 10 MV, the increase was from 39.6% (without table), to 88.9% (with table), and to 105.6% (table-combiboard combination). For the 150 degrees angle (tablecombiboard combination), the dose increased from 59.4% to 120% (6 MV) and from 39% to 108.1% (10 MV). CONCLUSION: Transmission factors for tabletops and accessories directly interfering with the treatment beam should be measured and implemented into the treatment-planning process. The increased surface dose to the skin should be considered.     

A study of Rhizophora spp wood phantom for dosimetric purposes using high-energy photon and electron beams.

Previous scattering and depth-dose investigations involving use of the Malaysian hardwood Rhizophora spp have shown this medium to produce good agreement with measurements made in water. Present study extends the comparison, now including measurements of percentage depth-dose made for photons at 6MV and 5 and 12MeV electron beams. For the 6 MV photon and 5 MeV electron beams, discrepancies between percentage depth-dose for Rhizophora spp and water, at all depths, are found to be within 2.6 and 2.4% respectively. At 12 MeV electron energies, measured percentage depth-doses in Rhizophora spp beyond 3.5cm depth are found to be in significant discord with those for water. The absorbed dose in water measured in Rhizophora spp at d(max) for all three beams produces discrepancies of no more than 1.1% when compared with measurements made in water.     

Glandular breast dose for monoenergetic and high-energy X-ray beams: Monte Carlo assessment.

PURPOSE: To extend the utility of normalized glandular dose (DgN) calculations to higher x-ray energies (up to 120 keV) and to provide the tools for investigators to calculate DgN values for arbitrary mammographic and x-ray spectra. MATERIALS AND METHODS: Validated Monte Carlo methods were used to assess DgN values. One million x-ray photons (1-120 keV, in 1-keV increments) were input to a semicircular breast geometry of thicknesses from 2 to 12 cm and breast compositions from 0% to 100% glandular. DgN values for monoenergetic (1-120 keV) x-ray beams, polyenergetic (40-120 kV, tungsten anode) x-ray spectra, and polyenergetic mammographic spectra were computed. Skin thicknesses of 4-5 mm were used. RESULTS: The calculated DgN values were in agreement within approximately 1%-6% with previously published data, depending on breast composition. DgN tables were constructed for a variety of x-ray tube anode-filter combinations, including molybdenum anode-molybdenum filter, molybdenum anode-rhodium filter, rhodium anode-rhodium filter, tungsten anode-rhodium filter, tungsten anode-palladium filter, and tungsten anode-silver filter. DgN values also were graphed for monoenergetic beams to 120 keV and for general diagnostic x-ray beams to 120 kV. CONCLUSION: The tables and graphs may be useful for optimizing mammographic procedures. The higher energy data may be useful for investigations of the potential of dual-energy mammography or for calculation of dose in general diagnostic or computed tomographic procedures.     

A comparison of X-ray and proton beam low energy secondary electron track structures using the low energy models of Geant4

Abstract

Purpose: Lethal cell damage by ionising radiation is generally initiated by the formation of complex strand breaks, resulting from ionisation clusters in the DNA molecule. A better understanding of the effect of the distribution of ionisation clusters within the cell and particularly in regard to DNA segments could be beneficial to radiation therapy treatment planning. Low energy X-rays generate an abundance of low energy electrons similar to that associated with MeV protons. The study and comparison of the track structure of photon and proton beams could permit the substitution of photon microbeams for single cell ion irradiations at proton facilities used to predict the relative biological effectiveness (RBE) of charged particle fields. Materials and methods: The track structure of X-ray photons is compared with proton pencil beams in voxels of approximate DNA strand size (2 × 2 × 5 nm). The Very Low Energy extension models of the Monte Carlo simulation toolkit GEometry ANd Tracking 4 (Geant4) is used. Simulations were performed in a water phantom for an X-ray and proton beam of energies 100 keV and 20 MeV, respectively. Results: The track structure of the photon and proton beams are evaluated using the ionisation cluster size distribution as well as the radial dose deposition of the beam. Conclusions: A comparative analysis of the ionisation cluster distribution and radial dose deposition obtained is presented, which suggest that low energy X-rays could produce similar ionisation cluster distributions to MeV protons on the DNA scale of size at depths greater than ～10 μm and at distances greater than ～1 μm from the beam centre. Here the ionisation cluster size for each beam is less than ～100. The radial dose deposition is also approximately equal at large depths and at distances greater than 10 μm from the beam centre.