Heavy-ion conformal irradiation in the shallow-seated tumor therapy terminal at HIRFL

Basic research related to heavy-ion cancer therapy has been done at the Institute of Modern Physics (IMP), Chinese Academy of Sciences since 1995. Now a plan of clinical trial with heavy ions has been launched at IMP. First, superficially placed tumor treatment with heavy ions is expected in the therapy terminal at the Heavy Ion Research Facility in Lanzhou (HIRFL), where carbon ion beams with energy up to 100 MeV/u can be supplied. The shallow-seated tumor therapy terminal at HIRFL is equipped with a passive beam delivery system including two orthogonal dipole magnets, which continuously scan pencil beams laterally and generate a broad and uniform irradiation field, a motor-driven energy degrader and a multi-leaf collimator. Two different types of range modulator, ripple filter and ridge filter with which Guassian-shaped physical dose and uniform biological effective dose Bragg peaks can be shaped for therapeutic ion beams respectively, have been designed and manufactured. Therefore, two-dimensional and three-dimensional conformal irradiations to tumors can be performed with the passive beam delivery system at the earlier therapy terminal. Both the conformal irradiation methods have been verified experimentally and carbon-ion conformal irradiations to patients with superficially placed tumors have been carried out at HIRFL since November 2006.

Secondary range shifting with range compensator for reduction of beam data library in heavy-ion radiotherapy

We report our experience with extended usage of range compensators in heavy-ion radiotherapy with broad beams to lighten the management task of the beam data library, which is a collection of the standard beams to be referenced in treatment planning. Partly due to interference between lateral spreading and range shifting, as many as hundreds of beam entries may be necessary to cover all the possible clinical situations. We have introduced downstream secondary range shifting with a range compensator to reduce the interference and consequently to simplify the library. In our case, 30% reduction in beam entries is achieved without significantly degrading the beam quality nor increasing the material consumption by more than 3%, which is experimentally verified with carbon-ion beams or statistically estimated from the clinical records.

On the discrepancies between Monte Carlo dose calculations and measurements for the 18 MV varian photon beam

Significant discrepancies between Monte Carlo dose calculations and measurements for the Varian 18 MV photon beam with a large field size (40 x 40 cm2) were reported by different investigators. In this work, we investigated these discrepancies based on a new geometry model ("New Model") of the Varian 21EX linac using the GEPTS Monte Carlo code. Some geometric parameters used in previous investigations (Old Model) were inaccurate, as suggested by Chibani in his AAPM presentation (2004) and later confirmed by the manufacturer. The entrance and exit radii of the primary collimator of the New Model are 2 mm larger than previously thought. In addition to the corrected dimensions of the primary collimator, the New Model includes approximate models for the lead shield and the mirror frame between the monitor chamber and the Y jaws. A detailed analysis of the phase space data shows the effects of these corrections on the beam characteristics. The individual contributions from the linac component to the photon and electron fluences are calculated. The main source of discrepancy between measurements and calculations based on the Old Model is the underestimated electron contamination. The photon and electron fluences at the isocenter are 5.3% and 36% larger in the New Model in comparison with the Old Model. The flattening filter and the lead shield (plus the mirror frame) contribute 48.7% and 13% of the total electron contamination at the isocenter, respectively. For both open and filtered (2 mm Pb) fields, the calculated (New Model) and measured dose distributions are within 1% for depths larger than 1 cm. To solve the residual problem of large differences at shallow depths (8% at 0.25 cm depth), the detailed geometry of an IC-10 ionization chamber was simulated and the dose in the air cavity was calculated for different positions on the central axis including at the surface, where half of the chamber is outside the phantom. The calculated and measured chamber responses are within 3% even at the zero depth.

Shielding design for a laser-accelerated proton therapy system

In this paper, we present the shielding analysis to determine the necessary neutron and photon shielding for a laser-accelerated proton therapy system. Laser-accelerated protons coming out of a solid high-density target have broad energy and angular spectra leading to dose distributions that cannot be directly used for therapeutic applications. A special particle selection and collimation device is needed to generate desired proton beams for energy- and intensity-modulated proton therapy. A great number of unwanted protons and even more electrons as a side-product of laser acceleration have to be stopped by collimation devices and shielding walls, posing a challenge in radiation shielding. Parameters of primary particles resulting from the laser-target interaction have been investigated by particle-in-cell simulations, which predicted energy spectra with 300 MeV maximum energy for protons and 270 MeV for electrons at a laser intensity of 2 x 10(21) W cm(-2). Monte Carlo simulations using FLUKA have been performed to design the collimators and shielding walls inside the treatment gantry, which consist of stainless steel, tungsten, polyethylene and lead. A composite primary collimator was designed to effectively reduce high-energy neutron production since their highly penetrating nature makes shielding very difficult. The necessary shielding for the treatment gantry was carefully studied to meet the criteria of head leakage <0.1% of therapeutic absorbed dose. A layer of polyethylene enclosing the whole particle selection and collimation device was used to shield neutrons and an outer layer of lead was used to reduce photon dose from neutron capture and electron bremsstrahlung. It is shown that the two-layer shielding design with 10-12 cm thick polyethylene and 4 cm thick lead can effectively absorb the unwanted particles to meet the shielding requirements.

Treatment-planning study of prostate cancer intensity-modulated radiotherapy with a Varian Clinac operated without a flattening filter

PURPOSE

To assess the feasibility of intensity-modulated radiotherapy for prostate cancer using photon beams from an accelerator operated without a flattening filter; and to determine potential benefits and drawbacks of using unflattened beams for this type of treatment.

METHODS AND MATERIALS

Intensity-modulated radiotherapy plans were generated for 10 patients with early-stage prostate cancer. For each patient, four plans were generated: with and without the flattening filter, at 6 and 18 MV. The prescription dose was 75.6 Gy to 98% of the planning target volume. The number of beams, their orientations, and optimization constraints were the same for all plans. Plans were generated with Eclipse 8.0 (Varian Medical Systems).

RESULTS

All the plans developed with unflattened beams were clinically acceptable. In terms of patient dose distributions, plans with unflattened beams were similar to the corresponding plans with flattened beams. Plans with unflattened beams required fewer monitor units (MUs) per plan: on average, by a factor of 2.0 at 6 MV and 2.6 at 18 MV, assuming that removal of the flattening filter was not followed by recalibration of MUs.

CONCLUSIONS

Clinically acceptable intensity-modulated radiotherapy plans for prostate cancer can be developed with unflattened beams at both 6 and 18 MV. Dosimetrically, flattened and unflattened beams generated similar treatment plans. The plans with unflattened beams required substantially fewer MUs. The reduction in the number of MUs indicates corresponding reduction in beam-on time and in the amount of radiation outside the target.

Dosimetric evaluation of a new collimator insert system for stereotactic radiotherapy

The prototype of a stereotactic collimator set developed in our department is evaluated for clinical use. This set consists of three cylindrical blocks mounted on a tray which slides in the wedge insert of a Siemens Primus accelerator. Each block has a collimating hole along its long axis to produce radiation fields of circular cross-section at the isocentre plane with diameters of 15 mm, 20 mm and 25 mm. Different geometric and dosimetric quality assurance tests were performed and results are found within the limits set for stereotactic radiotherapy. Dosimetry results measured using Kodak EDR-2 radiographic film and a pinpoint ion chamber also show good agreement with corresponding results calculated by Monte Carlo simulation of the linear accelerator head and the collimators. Measured dosimetry data were used to adapt a conventional PLATO treatment planning system for stereotactic radiotherapy using the prototype collimator set. Treatment planning system calculations and film measurements for treatment of an intracranial lesion in an anthropomorphic head phantom using coplanar 180 degrees arcs are compared and found to agree within 2 mm. This supports the accuracy of dose delivery using the prototype stereotactic collimators. Despite their increased penumbra (2.5-3.5 mm relative to 2-2.5 mm for commercially available collimators) the ease of construction makes the proposed stereotactic collimators an interesting alternative for accomplishing cost effective stereotactic treatments.

TG-69: Radiographic film for megavoltage beam dosimetry

TG-69 is a task group report of the AAPM on the use of radiographic film for dosimetry. Radiographic films have been used for radiation dosimetry since the discovery of x-rays and have become an integral part of dose verification for both routine quality assurance and for complex treatments such as soft wedges (dynamic and virtual), intensity modulated radiation therapy (IMRT), image guided radiation therapy (IGRT), and small field dosimetry like stereotactic radiosurgery. Film is convenient to use, spatially accurate, and provides a permanent record of the integrated two dimensional dose distributions. However, there are several challenges to obtaining high quality dosimetric results with film, namely, the dependence of optical density on photon energy, field size, depth, film batch sensitivity differences, film orientation, processing conditions, and scanner performance. Prior to the clinical implementation of a film dosimetry program, the film, processor, and scanner need to be tested to characterize them with respect to these variables. Also, the physicist must understand the basic characteristics of all components of film dosimetry systems. The primary mission of this task group report is to provide guidelines for film selection, irradiation, processing, scanning, and interpretation to allow the physicist to accurately and precisely measure dose with film. Additionally, we present the basic principles and characteristics of film, processors, and scanners. Procedural recommendations are made for each of the steps required for film dosimetry and guidance is given regarding expected levels of accuracy. Finally, some clinical applications of film dosimetry are discussed.

A beam source model for scanned proton beams

A beam source model, i.e. a model for the initial phase space of the beam, for scanned proton beams has been developed. The beam source model is based on parameterized particle sources with characteristics found by fitting towards measured data per individual beam line. A specific aim for this beam source model is to make it applicable to the majority of the various proton beam systems currently available or under development, with the overall purpose to drive dose calculations in proton beam treatment planning. The proton beam phase space is characterized by an energy spectrum, radial and angular distributions and deflections for the non-modulated elementary pencil beam. The beam propagation through the scanning magnets is modelled by applying experimentally determined focal points for each scanning dimension. The radial and angular distribution parameters are deduced from measured two-dimensional fluence distributions of the elementary beam in air. The energy spectrum is extracted from a depth dose distribution for a fixed broad beam scan pattern measured in water. The impact of a multi-slab range shifter for energy modulation is calculated with an own Monte Carlo code taking multiple scattering, energy loss and straggling, non-elastic and elastic nuclear interactions in the slab assembly into account. Measurements for characterization and verification have been performed with the scanning proton beam system at The Svedberg Laboratory in Uppsala. Both in-air fluence patterns and dose points located in a water phantom were used. For verification, dose-in-water was calculated with the Monte Carlo code GEANT 3.21 instead of using a clinical dose engine with approximations of its own. For a set of four individual pencil beams, both with the full energy and range shifted, 96.5% (99.8%) of the tested dose points satisfied the 1%/1 mm (2%/2 mm) gamma criterion.

Calibration of a proton beam energy monitora)

Delivery of therapeutic proton beams requires an absolute energy accuracy of +/-0.64 to 0.27 MeV for patch fields and a relative energy accuracy of +/-0.10 to 0.25 MeV for tailoring the depth dose distribution using the energy stacking technique. Achromatic switchyard tunes, which lead to better stability of the beam incident onto the patient, unfortunately limit the ability of switchyard magnet tesla meters to verify the correct beam energy within the tolerances listed above. A new monitor to measure the proton energy before each pulse is transported through the switchyard has been installed into a proton synchrotron. The purpose of this monitor is to correct and/or inhibit beam delivery when the measured beam energy is outside of the tolerances for treatment. The monitor calculates the beam energy using data from two frequency and eight beam position monitors that measure the revolution frequency of the proton bunches and the effective offset of the orbit from the nominal radius of the synchrotron. The new energy monitor has been calibrated by measuring the range of the beam through water and comparing with published range-energy tables for various energies. A relationship between depth dose curves and range-energy tables was first determined using Monte Carlo simulations of particle transport and energy deposition. To reduce the uncertainties associated with typical scanning water phantoms, a new technique was devised in which the beam energy was scanned while fixed thickness water tanks were sandwiched between two fixed parallel plate ionization chambers. Using a multitude of tank sizes, several energies were tested to determine the nominal accelerator orbit radius. After calibration, the energy reported by the control system matched the energy derived by range measurements to better than 0.72 MeV for all nine energies tested between 40 and 255 MeV with an average difference of -0.33 MeV. A study of different combinations of revolution frequency and radial offsets to test the envelope of algorithm accuracy demonstrated a relative accuracy of +/-0.11 MeV for small energy changes between 126 and 250 MeV. These new measurements may serve as a data set for benchmarking range-energy relationships.

Spatial resolution of 2D ionization chamber arrays for IMRT dose verification: single-detector size and sampling step width

The spatial resolution of 2D detector arrays equipped with ionization chambers or diodes, used for the dose verification of IMRT treatment plans, is limited by the size of the single detector and the centre-to-centre distance between the detectors. Optimization criteria with regard to these parameters have been developed by combining concepts of dosimetry and pattern analysis. The 2D-ARRAY Type 10024 (PTW-Freiburg, Germany), single-chamber cross section 5 x 5 mm(2), centre-to-centre distance between chambers in each row and column 10 mm, served as an example. Additional frames of given dose distributions can be taken by shifting the whole array parallel or perpendicular to the MLC leaves by, e.g., 5 mm. The size of the single detector is characterized by its lateral response function, a trapezoid with 5 mm top width and 9 mm base width. Therefore, values measured with the 2D array are regarded as sample values from the convolution product of the accelerator generated dose distribution and this lateral response function. Consequently, the dose verification, e.g., by means of the gamma index, is performed by comparing the measured values of the 2D array with the values of the convolution product of the treatment planning system (TPS) calculated dose distribution and the single-detector lateral response function. Sufficiently small misalignments of the measured dose distributions in comparison with the calculated ones can be detected since the lateral response function is symmetric with respect to the centre of the chamber, and the change of dose gradients due to the convolution is sufficiently small. The sampling step width of the 2D array should provide a set of sample values representative of the sampled distribution, which is achieved if the highest spatial frequency contained in this function does not exceed the 'Nyquist frequency', one half of the sampling frequency. Since the convolution products of IMRT-typical dose distributions and the single-detector lateral response function have no or very small frequency contributions beyond 0.1 mm(-1), the mathematical approach introduced by Nyquist and Shannon shows that the sampling frequency of 0.2 mm(-1) is appropriate. Overall it is shown that the spatial resolution of the 2D-ARRAY Type 10024 is appropriate for the dose verification of IMRT plans. The insights obtained are also applied in the discussion of other available two-dimensional detector arrays.

Development of an ultrasmall C-band linear accelerator guide for a four-dimensional image-guided radiotherapy system with a gimbaled x-ray head

We are developing a four-dimensional image-guided radiotherapy system with a gimbaled x-ray head. It is capable of pursuing irradiation and delivering irradiation precisely with the help of an agile moving x-ray head on the gimbals. Requirements for the accelerator guide were established, system design was developed, and detailed design was conducted. An accelerator guide was manufactured and basic beam performance and leakage radiation from the accelerator guide were evaluated at a low pulse repetition rate. The accelerator guide including the electron gun is 38 cm long and weighs about 10 kg. The length of the accelerating structure is 24.4 cm. The accelerating structure is a standing wave type and is composed of the axial-coupled injector section and the side-coupled acceleration cavity section. The injector section is composed of one prebuncher cavity, one buncher cavity, one side-coupled half cavity, and two axial coupling cavities. The acceleration cavity section is composed of eight side-coupled nose reentrant cavities and eight coupling cavities. The electron gun is a diode-type gun with a cerium hexaboride (CeB6) direct heating cathode. The accelerator guide can be operated without any magnetic focusing device. Output beam current was 75 mA with a transmission efficiency of 58%, and the average energy was 5.24 MeV. Beam energy was distributed from 4.95 to 5.6 MeV. The beam profile, measured 88 mm from the beam output hole on the axis of the accelerator guide, was 0.7 mm X 0.9 mm full width at half maximum (FWHM) width. The beam loading line was 5.925 (MeV)-Ib (mA) X 0.00808 (MeV/mA), where Ib is output beam current. The maximum radiation leakage of the accelerator guide at 100 cm from the axis of the accelerator guide was calculated as 0.33 cGy/min at the rated x-ray output of 500 cGy/min from the measured value. This leakage requires no radiation shielding for the accelerator guide itself per IEC 60601-2-1.

Integral test phantom for dosimetric quality assurance of image guided and intensity modulated stereotactic radiotherapy

The objective of this work is to develop a dosimetric phantom quality assurance (QA) of linear accelerators capable of cone-beam CT (CBCT) image guided and intensity-modulated radiotherapy (IG-IMRT). This phantom is to be used in an integral test to quantify in real-time both the performance of the image guidance and the dose delivery systems in terms of dose localization. The prototype IG-IMRT QA phantom consisted of a cylindrical imaging phantom (CatPhan) combined with an array of 11 radiation diodes mounted on a 10 cm diameter disk, oriented perpendicular to the phantom axis. Basic diode response characterization was performed for 6 and 18 MV photons. The diode response was compared to planning system calculations in the open and penumbrae regions of simple and complex beam arrangements. The clinical use of the QA phantom was illustrated in an integral test of an IG-IMRT treatment designed for a clinical spinal radiosurgery case. The sensitivity of the phantom to multileaf collimator (MLC) calibration and setup errors in the clinical setting was assessed by introducing errors in the IMRT plan or by displacing the phantom. The diodes offered good response linearity and long-term reproducibility for both 6 and 18 MV. Axial dosimetry of coplanar beams (in a plane containing the beam axes) was made possible with the nearly isoplanatic response of the diodes over 360 degrees of gantry (usually within +/-1%). For single beam geometry, errors in phantom placement as small as 0.5 mm could be accurately detected (in gradient > or = 1% /mm). In clinical setting, MLC systematic errors of 1 mm on a single MLC bank introduced in the IMRT plan were easily detectable with the QA phantom. The QA phantom demonstrated also sufficient sensitivity for the detection of setup errors as small as 1 mm for the IMRT delivery. These results demonstrated that the prototype can accurately and efficiently verify the entire IG-IMRT process. This tool, in conjunction with image guidance capabilities has the potential to streamline this QA process and improve the level of performance of image guided and intensity modulated radiotherapy.

Development and verification of the pulsed scanned proton beam at The Svedberg Laboratory in Uppsala

In this paper we present the recent developments made for the scanning system for proton beams at TSL in Uppsala, showing that this system is now fully functional being able to produce conformal intensity modulated scan patterns with sufficient accuracy. A new control and supervising system handling the beam delivery including the control of the synchrocyclotron and the scanning system is developed and described in detail. A complete dosimetry system with transmission ionization chambers and a multi-wire ionization chamber for monitoring of the beam during scanning has been constructed. The details of the dose monitors and the position sensitive multi-wire ionization chamber are presented in this work. Furthermore, we have established procedures for verification measurements to ensure the quality of the beam and also methods for calibration of the beam monitors and relative and absolute dosimetry for complex scanned beams.

Extrapolated surface dose measurements using a NdFeB magnetic deflector for 6 MV x-ray beams

Extrapolated surface dose measurements have been performed using radiographic film to measure 2-Dimensional maps of skin and surface dose with and without a magnetic deflector device aimed at reducing surface dose. Experiments are also performed using an Attix parallel plate ionisation chamber for comparison to radiographic film extrapolation surface dose analysis. Extrapolated percentage surface dose assessments from radiographic film at the central axis of a 6 MV x-ray beam with magnetic deflector for field size 10 x 10 cm2, 15 x 15 cm2 and 20 x 20 cm2 are 9 +/- 3%, 13 +/- 3% and 16 +/- 3%, these compared to 14 +/- 3%, 19 +/- 3%, and 27 +/- 3% for open fields, respectively. Results from Attix chamber for the same field size are 12 +/- 1%, 15 +/- 1% and 18 +/- 1%, these compared to 16 +/- 1%, 21 +/- 1% and 27 +/- 1% for open fields, respectively. Results are also shown for profiles measured in-plane and cross-plane to the magnetic deflector and compared to open field data. Results have shown that the surface dose is reduced at all sites within the treatment field with larger reductions seen on one side of the field due to the sweeping nature of the designed magnetic field. Radiographic film extrapolation provides an advanced surface dose assessment and has matched well with Attix chamber results. Film measurement allows for easy 2 dimensional dose assessments.

Catching the proton wave

Proton therapy is hailed as a huge leap in treating certain cancers. But hospitals that want proton centers need deep pockets, lots of land and the right staff.

Integrating a MRI scanner with a 6 MV radiotherapy accelerator: impact of the surface orientation on the entrance and exit dose due to the transverse magnetic field

At the UMC Utrecht, in collaboration with Elekta and Philips Research Hamburg, we are developing a radiotherapy accelerator with integrated MRI functionality. The radiation dose will be delivered in the presence of a lateral 1.5 T field. Although the photon beam is not affected by the magnetic field, the actual dose deposition is done by a cascade of secondary electrons and these electrons are affected by the Lorentz force. The magnetic field causes a reduced build-up distance: because the trajectory of the electrons between collisions is curved, the entrance depth in tissue decreases. Also, at tissue-air interfaces an increased dose occurs due to the so-called electron return effect (ERE): electrons leaving tissue will describe a circular path in air and re-enter the tissue yielding a local dose increase. In this paper the impact of a 1.5 T magnetic field on both the build-up distance and the dose increase due to the ERE will be investigated as a function of the angle between the surface and the incident beam. Monte Carlo simulations demonstrate that in the presence of a 1.5 T magnetic field, the surface dose, the build-up distance and the exit dose depend more heavily on the surface orientation than in the case without magnetic field. This is caused by the asymmetrical pointspread kernel in the presence of 1.5 T and the directional behaviour of the re-entering electrons. Simulations on geometrical phantoms show that ERE dose increase at air cavities can be avoided using opposing beams, also when the air-tissue boundary is not perpendicular to the beam. For the more general case in patient anatomies, more problems may arise. Future work will address the possibilities and limitations of opposing beams in combination with IMRT in a magnetic field.

Comparison of 4 MV photon surface dose among Varian, Siemens, and Elekta linear accelerators for tangential breast treatment: a phantom study

PURPOSE

We have compared the differences in a 4-MV photon surface dose among Varian, Siemens, and Elekta linear accelerators (linacs) with wedges for tangential breast treatment.

MATERIALS AND METHODS

The wedge factor and the surface dose were measured using a solid water phantom and an ion chamber for each linear accelerator with various field sizes and wedge angles. A tangential treatment plan was applied to an elliptical hollow cylinder water phantom with a radiochromic film placed thereon. A dose was delivered to a simulated target in the phantom, and the resulting dose distribution was analyzed using a film scanner.

RESULTS

Varian's wedges resulted in the highest wedge factors, ranging from 0.37 to 0.75 depending on the wedge angles. Varian's wedges led to the highest normalized skin doses, ranging between 0.40 and 0.73 depending on the wedge angles and field sizes. In the cylinder phantom test with two tangential beams, the Varian linac provided a nearly 20% higher maximum dose than the Siemens and Elekta linacs.

CONCLUSION

The Varian linac resulted in the highest surface doses, and the Elekta linac led to the lowest for nearly all the measurement conditions we employed, including open beams.

A Monte Carlo model for calculating out-of-field dose from a varian 6 MV beam

Dose to the patient outside of the treatment field is important when evaluating the outcome of radiotherapy treatments. However, determining out-of-field doses for any particular treatment plan currently requires either time-consuming measurements or calculated estimations that may be highly uncertain. A Monte Carlo model may allow these doses to be determined quickly, accurately, and with a great degree of flexibility. MCNPX was used to create a Monte Carlo model of a Varian Clinac 2100 accelerator head operated at 6 MV. Simulations of the dose out-of-field were made and measurements were taken with thermoluminescent dosimeters in an acrylic phantom and with an ion chamber in a water tank to validate the Monte Carlo model. Although local differences between the out-of-field doses calculated by the model and those measured did exceed 50% at some points far from the treatment field, the average local difference was only 16%. This included a range of doses as low as 0.01% of the central axis dose, and at distances in excess of 50 cm from the central axis of the treatment field. The out-of-field dose was found to vary with field size and distance from the central axis, but was almost independent of the depth in the phantom except where the dose increased substantially at depths less than dmax. The relationship between dose and kerma was also investigated, and kerma was found to be a good estimate of dose (within 3% on average) except near the surface and in the field penumbra. Our Monte Carlo model was found to well represent typical Varian 2100 accelerators operated at 6 MV.

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.

Activation processes in a medical linear accelerator and spatial distribution of activation products

Activation products have been identified by in situ gamma spectroscopy at the isocentre of a medical linear accelerator shortly after termination of a high energy photon beam irradiation with 15 x 15 cm field size. Spectra have been recorded either with an open or with a closed collimator. Whilst some activation products disappear from the spectrum with closed collimator or exhibit reduced count rates, others remain with identical intensity. The former isotopes are neutron-deficient and mostly decay by positron emission or electron capture; the latter have neutron excess and decay by beta(-) emission. This new finding is consistent with the assumption that photons in the primary beam produce activation products by (gamma, n) reactions in the treatment head and subsequently the neutrons created in these processes undergo (n, gamma) reactions creating activation products in a much larger area. These findings are expected to be generally applicable to all medical high energy linear accelerators.

The use of megavoltage cone-beam CT to complement CT for target definition in pelvic radiotherapy in the presence of hip replacement

In Europe and the USA combined, over half a million people had a hip joint replaced in 2005, contributing to the increasing number of radiotherapy patients with metallic hip prostheses. The treatment plan for external beam radiation therapy is based on the delineation of the anatomy in the planning CT scan. When implanted objects of high atomic number (Z) material are present, however, severe image artefacts are generated in conventional CT, strongly hindering the ability to delineate some organs. This is particularly the case for the planning of prostate patients with hip prostheses. This short communication presents the use of a new imaging modality, megavoltage cone-beam CT, to complement the regular CT for target definition of prostate cancer treatment of patients with hip replacements.

Design of a computer-controlled multileaf collimator for advanced electron radiotherapy

A multileaf collimator for electrons (eMLC) has been designed that fulfils the technical requirements for providing advanced irradiation techniques with electrons. In the present work, the basic design parameters of leaf material, leaf height, leaf width and number of leaves as well as leaf overtravel and leaf shape were determined such that an eMLC with motorized leaves can be manufactured by a company specialized in MLC technology. For this purpose, a manually driven eMLC with variable source-to-collimator distance (SCD) was used to evaluate the chosen leaf specification and investigate the impact of the SCD on the off-axis dose distribution. In order to select the final SCD of the eMLC, a compromise had to be found between maximum field size, minimum beam penumbra and necessary distance between eMLC and isocentre to eliminate patient realignments during gantry rotation. As a result, the eMLC is placed according to the target position at 72 and 84 cm SCD, respectively. This feature will be achieved by interchangeable distance holders. At these SCDs, the corresponding maximum field sizes at 100 cm source-to-isocentre distance are 20 x 20 cm and 17 x 17 cm, respectively. Finally, the off-axis dose distribution at the maximum opening of the eMLC was improved by fine-tuning the settings of the accelerator jaws and introducing trimmer bars above the eMLC. Following this optimization, a prototype eMLC consisting of 2 x 24 computer-controlled brass leaves is manufactured by 3D Line Medical Systems.

Incorporation of the aperture thickness in proton pencil-beam dose calculations

Field-specific apertures, of sufficient range-absorbing thickness, are used in the majority of proton-therapy treatments today. In current practice, these apertures are modelled as objects of infinitesimal thickness. Such an approximation, however, is not accurate if the aperture edge is close to, or extends over, the beam axis. Practical situations in which this occurs include off-axis patch fields, small apertures, and fields shaped with a multileaf collimator. We develop an extension of the pencil-beam dose model to incorporate the aperture thickness. We derive an exact solution as well as a computationally simpler approximate implementation. The model is validated using measurements of the lateral penumbra. For a set-up with a source size of 2.76 cm, a source-to-axis distance of 227 cm, and a aperture-to-axis distance of 35 cm, the maximum increase in penumbra for a 6 cm thick aperture compared to the thin-aperture model is about 2 mm. The maximum shift in the 95% isodose contour line is larger. The overall effect depends on the aperture thickness, the position of the aperture edge and the intrinsic source size and SAD, but is fairly insensitive to aperture-to-skin distance and depth in patient.