Fast 2D phantom dosimetry for scanning proton beams

Verification of patient-specific dose distributions in proton therapy using a commercial two-dimensional ion chamber array

PURPOSE

The purpose of this study was to determine whether a two-dimensional (2D) ion chamber array detector quickly and accurately measures patient-specific dose distributions in treatment with passively scattered and spot scanning proton beams.

METHODS

The 2D ion chamber array detector MatriXX was used to measure the dose distributions in plastic water phantom from passively scattered and spot scanning proton beam fields planned for patient treatment. Planar dose distributions were measured using MatriXX, and the distributions were compared to those calculated using a treatment-planning system. The dose distributions generated by the treatment-planning system and a film dosimetry system were similarly compared.

RESULTS

For passively scattered proton beams, the gamma index for the dose-distribution comparison for treatment fields for three patients with prostate cancer and for one patient with lung cancer was less than 1.0 for 99% and 100% of pixels for a 3% dose tolerance and 3 mm distance-to-dose agreement, respectively. For spot scanning beams, the mean (+/- standard deviation) percentages of pixels with gamma indices meeting the passing criteria were 97.1% +/- 1.4% and 98.8% +/- 1.4% for MatriXX and film dosimetry, respectively, for 20 fields used to treat patients with prostate cancer.

CONCLUSIONS

Unlike film dosimetry, MatriXX provides not only 2D dose-distribution information but also absolute dosimetry in fractions of minutes with acceptable accuracy. The results of this study indicate that MatriXX can be used to verify patient-field specific dose distributions in proton therapy.

Bath and shower effects in the rat parotid gland explain increased relative risk of parotid gland dysfunction after intensity-modulated radiotherapy

PURPOSE

To assess in a rat model whether adding a subtolerance dose in a region adjacent to a high-dose irradiated subvolume of the parotid gland influences its response (bath-and-shower effect).

METHODS AND MATERIALS

Irradiation of the whole, cranial 50%, and/or the caudal 50% of the parotid glands of Wistar rats was performed using 150-MeV protons. To determine suitable (i.e., subtolerance) dose levels for a bath-dose, both whole parotid glands were irradiated with 5 to 25 Gy. Subsequently groups of Wistar rats received 30 Gy to the caudal 50% (shower) and 0 to 10 Gy to the cranial 50% (bath) of both parotid glands. Stimulated saliva flow rate (function) was measured before and up to 240 days after irradiation.

RESULTS

Irradiation of both glands up to a dose of 10 Gy did not result in late loss of function and is thus regarded subtolerance. Addition of a dose bath of 1 to 10 Gy to a high-dose in the caudal 50% of the glands resulted in enhanced function loss.

CONCLUSION

Similar to the spinal cord, the parotid gland demonstrates a bath and shower effect, which may explain the less-than-expected sparing of function after IMRT.

Exploration of the potential of liquid scintillators for real-time 3D dosimetry of intensity modulated proton beams

In this study, the authors investigated the feasibility of using a 3D liquid scintillator (LS) detector system for the verification and characterization of proton beams in real time for intensity and energy-modulated proton therapy. A plastic tank filled with liquid scintillator was irradiated with pristine proton Bragg peaks. Scintillation light produced during the irradiation was measured with a CCD camera. Acquisition rates of 20 and 10 frames per second (fps) were used to image consecutive frame sequences. These measurements were then compared to ion chamber measurements and Monte Carlo simulations. The light distribution measured from the images acquired at rates of 20 and 10 fps have standard deviations of 1.1% and 0.7%, respectively, in the plateau region of the Bragg curve. Differences were seen between the raw LS signal and the ion chamber due to the quenching effects of the LS and due to the optical properties of the imaging system. The authors showed that this effect can be accounted for and corrected by Monte Carlo simulations. The liquid scintillator detector system has a good potential for performing fast proton beam verification and characterization.

Liquid scintillator for 2D dosimetry for high-energy photon beams

Complex radiation therapy techniques require dosimetric verification of treatment planning and delivery. The authors investigated a liquid scintillator (LS) system for application for real-time high-energy photon beam dosimetry. The system was comprised of a transparent acrylic tank filled with liquid scintillating material, an opaque outer tank, and a CCD camera. A series of images was acquired when the tank with liquid scintillator was irradiated with a 6 MV photon beam, and the light data measured with the CCD camera were filtered to correct for scattering of the optical light inside the liquid scintillator. Depth-dose and lateral profiles as well as two-dimensional (2D) dose distributions were found to agree with results from the treatment planning system. Further, the corrected light output was found to be linear with dose, dose rate independent, and is robust for single or multiple acquisitions. The short time needed for image acquisition and processing could make this system ideal for fast verification of the beam characteristics of the treatment machine. This new detector system shows a potential usefulness of the LS for 2D QA.

Transient noise characterization and filtration in CCD cameras exposed to stray radiation from a medical linear accelerator

Charge coupled devices (CCDs) are being increasingly used in radiation therapy for dosimetric purposes. However, CCDs are sensitive to stray radiation. This effect induces transient noise. Radiation-induced noise strongly alters the image and therefore limits its quantitative analysis. The purpose of this work is to characterize the radiation-induced noise and to develop filtration algorithms to restore image quality. Two models of CCD were used for measurements close to a medical linac. The structure of the transient noise was first characterized. Then, four methods of noise filtration were compared: median filtering of a time series of identical images, uniform median filtering of single images, an adaptive filter with switching mechanism, and a modified version of the adaptive switch filter. The intensity distribution of noisy pixels was similar in both cameras. However, the spatial distribution of the noise was different: The average noise cluster size was 1.2 +/- 0.6 and 3.2 +/- 2.7 pixels for the U2000 and the Luca, respectively. The median of a time series of images resulted in the best filtration and minimal image distortion. For applications where time series is impractical, the adaptive switch filter must be used to reduce image distortion. Our modified version of the switch filter can be used in order to handle nonisolated groups of noisy pixels.

Delivery verification using 3D dose reconstruction based on fluorescence measurement in a carbon beam scanning irradiation system

The authors have developed a method to reconstruct the 3D dose distribution in a particle beam scanning irradiation system. In this scheme, 3D dose distribution is reconstructed by using the measured images of fluence distribution, which are taken for each iso-energy slice (i.e., the unit of the depth scanning). A fluorescent screen with a CCD camera is used to measure the fluence distribution. This system was installed at the HIMAC experimental port and tested by using carbon ion beams. Since a maximum difference between the reconstructed dose and the ionization chamber measurement was around 5% in the target volume, this system can be useful for quick verification of 3D dose distribution.

The DosiMap, a new 2D scintillating dosimeter for IMRT quality assurance: characterization of two Cerenkov discrimination methods

New radiation therapy techniques such as IMRT present significant efficiency due to their highly conformal dose distributions. A consequence of the complexity of their dose distributions (high gradients, small irradiation fields, low dose distribution, ...) is the requirement for better precision quality assurance than in classical radiotherapy in order to compare the conformation of the delivered dose with the planned dose distribution and to guarantee the quality of the treatment. Currently this control is mostly performed by matrices of ionization chambers, diode detectors, dosimetric films, portal imaging, or dosimetric gels. Another approach is scintillation dosimetry, which has been developed in the last 15 years mainly through scintillating fiber devices. Despite having many advantages over other methods it is still at an experimental level for routine dosimetry because the Cerenkov radiation produced under irradiation represents an important stem effect. A new 2D water equivalent scintillating dosimeter, the DosiMap, and two different Cerenkov discrimination methods were developed with the collaboration of the Laboratoire de Physique Corpusculaire of Caen, the Comprehensive Cancer Center François Baclesse, and the ELDIM Co., in the frame of the MAESTRO European project. The DosiMap consists of a plastic scintillating sheet placed inside a transparent polystyrene phantom. The light distribution produced under irradiation is recorded by a CCD camera. Our first Cerenkov discrimination technique is subtractive. It uses a chessboard pattern placed in front of the scintillator, which provides a background signal containing only Cerenkov light. Our second discrimination technique is colorimetric. It performs a spectral analysis of the light signal, which allows the unfolding of the Cerenkov radiation and the scintillation. Tests were carried out with our DosiMap prototype and the performances of the two discrimination methods were assessed. The comparison of the dose measurements performed with the DosiMap and with dosimetric films for three different irradiation configurations showed discrepancies smaller than 3.5% for a 2 mm spatial resolution. Two innovative discrimination solutions were demonstrated to separate the scintillation from the Cerenkov radiation. It was also shown that the DosiMap, which is water equivalent, fast, and user friendly, is a very promising tool for radiotherapy quality assurance.

Multichannel detectors for profile measurements in clinical proton fields

Two beam profile measurement detectors have been developed at Indiana University Cyclotron Facility to address the need for a tool to efficiently verify dose distributions created with active methods of clinical proton beam delivery. The multipad ionization chamber (MPIC) has 128 ionization chambers arranged in one plane and is designed to measure lateral profiles in fields up to 38 cm in diameter. The MPIC pads have a 5 mm pitch for fields up to 20 cm in diameter and a 7 mm pitch for larger fields, providing the accuracy of field size determination about 0.5 mm. The multilayer ionization chamber (MLIC) detector contains 122 small-volume ionization chambers stacked at a 1.82 mm step (water-equivalent) for depth-dose profile measurements. The MLIC detector can measure profiles up to 20 cm in depth, and determine the 80% distal dose fall-off with about 0.1 mm precision. Both detectors can be connected to the same set of electronics modules, which comprise the detectors' data acquisition system. The detectors have been tested in clinical proton fields produced with active methods of beam delivery such as uniform scanning and energy stacking. This article describes detector performance tests and discusses their results. The test results indicate that the MPIC and MLIC detectors can be used for dosimetric characterization of clinical proton fields. The detectors offer significant time savings during measurements in actively delivered beams compared with traditional measurements using a water phantom.

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.

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.

Development of an easy-to-handle range measurement tool using a plastic scintillator for proton beam therapy

Proton beam therapy can concentrate the dose on a tumour. In order to offer high-precision proton beam therapy to the patient, it is important to confirm the range every day. At the National Cancer Center Hospital East (NCC), the range measurement tool consists of a water phantom and an ionization chamber. These large and heavy tools take a long time to set up. Therefore, we developed a simple and easy-to-handle range measurement tool for proton beam therapy. This tool consists of a plastic scintillator block and a CCD camera. We recorded visible scintillation light generated by proton irradiation on the scintillator, and could measure the range from the shape of light distribution by using a computer with automatic analysis software installed. We carried out proton irradiation experiments with this tool to examine its performance as a tool of daily range measurements. The precision of the range measurement is within 0.3 mm (sd). The tool can measure possible short-term range variation with 1 s sampling time during the time interval of a typical treatment in a few minutes. We conclude that this tool can measure the range with sufficient resolution in a short time, and is useful for range control in a clinical setting.

Influence of adjacent low-dose fields on tolerance to high doses of protons in rat cervical spinal cord

PURPOSE

The dose-response relationship for a relatively short length (4 mm) of rat spinal cord has been shown to be significantly modified by adjacent low-dose fields. In an additional series of experiments, we have now established the dose-volume dependence of this effect.

METHODS AND MATERIALS

Wistar rats were irradiated on the cervical spinal cord with single doses of unmodulated protons (150 MeV) to obtain sharp lateral penumbras, by use of the shoot-through technique, which employs the plateau of the depth-dose profile rather than the Bragg peak. Three types of inhomogeneous dose distributions were administered: Twenty millimeters of cervical spinal cord were irradiated with variable subthreshold (= bath) doses (4 and 18 Gy). At the center of the 20-mm segment, a short segment of 2 mm or 8 mm (= shower) was irradiated with variable single doses. These inhomogeneous dose distributions are referred to as symmetrical bath-and-shower experiments. An asymmetrical dose distribution was arranged by irradiation of 12 mm (= bath) of spinal cord with a dose of 4 Gy. The caudal 2 mm (= shower) of the 12-mm bath was additionally irradiated with variable single doses. This arrangement of inhomogeneous dose distribution is referred to as asymmetrical bath-and-shower experiment. The endpoint for estimation of the dose-response relationships was paralysis of the fore limbs or hind limbs and confirmation by histology.

RESULTS

The 2-mm bath-and-shower experiments with a 4-Gy bath dose showed a large shift of the dose-response curves compared with the 2-mm single field, which give lower ED50 values of 61.2 Gy and 68.6 Gy for the symmetrical and asymmetrical arrangement, respectively, compared with an ED50 of 87.8 Gy after irradiation of a 2-mm field only. If the bath dose is increased to 18 Gy, the ED50 value is decreased further to 30.9 Gy. For an 8-mm field, addition of a 4-Gy bath dose did not modify the ED50 obtained for an 8-mm field only (23.2 and 23.1 Gy).

CONCLUSIONS

The spinal cord tolerance of relatively small volumes (shower) is strongly affected by low-dose irradiation (= bath) of adjacent tissue. The results of all bath-and-shower experiments show the effect of a low bath dose to be highest for a field of 2 mm, less for 4 mm, and absent for 8 mm. Adding a 4-Gy bath to only 1 side of a 2-mm field still showed a large effect. Because glial progenitor cells are known to migrate over at least 2 to 3 mm, this observation indicates that interference with stem cell migration is not the most likely mechanism of a bath effect.

Regional differences in radiosensitivity across the rat cervical spinal cord

PURPOSE

To study regional differences in radiosensitivity within the rat cervical spinal cord.

METHODS AND MATERIALS

Three types of inhomogeneous dose distributions were applied to compare the radiosensitivity of the lateral and central parts of the rat cervical spinal cord. The left lateral half of the spinal cord was irradiated with two grazing proton beams, each with a different penumbra (20-80% isodoses): lateral wide (penumbra = 1.1 mm) and lateral tight (penumbra = 0.8 mm). In the third experiment, the midline of the cord was irradiated with a narrow proton beam with a penumbra of 0.8 mm. The irradiated spinal cord length (C1-T2) was 20 mm in all experiments. The animals were irradiated with variable single doses of unmodulated protons (150 MeV) with the shoot-through method, whereby the plateau of the depth-dose profile is used rather than the Bragg peak. The endpoint for estimating isoeffective dose (ED(50)) values was paralysis of fore and/or hind limbs within 210 days after irradiation. Histology of the spinal cords was performed to assess the radiation-induced tissue damage.

RESULTS

High-precision proton irradiation of the lateral or the central part of the spinal cord resulted in a shift of dose-response curves to higher dose values compared with the homogeneously irradiated cervical cord to the same 20-mm length. The ED(50) values were 28.9 Gy and 33.4 Gy for the lateral wide and lateral tight irradiations, respectively, and as high as 71.9 Gy for the central beam experiment, compared with 20.4 Gy for the homogeneously irradiated 20-mm length of cervical cord. Histologic analysis of the spinal cords showed that the paralysis was due to white matter necrosis. The radiosensitivity was inhomogeneously distributed across the spinal cord, with a much more radioresistant central white matter (ED(50) = 71.9 Gy) compared with lateral white matter (ED(50) values = 28.9 Gy and 33.4 Gy). The gray matter did not show any noticeable lesions, such as necrosis or hemorrhage, up to 80 Gy. All lesions induced were restricted to white matter structures.

CONCLUSIONS

The observed large regional differences in radiosensitivity within the rat cervical spinal cord indicate that the lateral white matter is more radiosensitive than the central part of the white matter. The gray matter is highly resistant to radiation: no lesions observable by light microscopy were induced, even after a single dose as high as 80 Gy.

Verification of step-and-shoot IMRT delivery using a fast video-based electronic portal imaging device

We present an investigation into the use of a fast video-based electronic portal-imaging device (EPID) to study intensity modulated radiation therapy (IMRT) delivery. The aim of this study is to test the feasibility of using an EPID system to independently measure the orchestration of collimator leaf motion and beam fluence; simultaneously measuring both the delivered field fluence and shape as it exits the accelerator head during IMRT delivery. A fast EPID that consists of a terbium-doped gadolinium oxysulphide (GdO2S:Tb) scintillator coupled with an inexpensive commercial 30 frames-per-second (FPS) CCD-video recorder (16.7 ms shutter time) was employed for imaging IMRT delivery. The measurements were performed on a Varian 2100 C/D linear accelerator equipped with a 120-leaf multileaf-collimator (MLC). A characterization of the EPID was performed that included measurements of spatial resolution, linac pulse-rate dependence, linear output response, signal uniformity, and imaging artifacts. The average pixel intensity for fields imaged with the EPID was found to be linear in the delivered monitor units of static non-IMRT fields between 3x3 and 15x15 cm2. A systematic increase of the average pixel intensity was observed with increasing field size, leading to a maximum variation of 8%. Deliveries of a clinical step-and-shoot mode leaf sequence were imaged at 600 MU/min. Measurements from this IMRT delivery were compared with experimentally validated MLC controller log files and were found to agree to within 5%. An analysis of the EPID image data allowed identification of three types of errors: (1) 5 out of 35 segments were undelivered; (2) redistributing all of the delivered segment MUs; and (3) leaf movement during segment delivery. Measurements with the EPID at lower dose rates showed poor agreement with log files due to an aliasing artifact. The study was extended to use a high-speed camera (1-1000 FPS and 10 micros shutter time) with our EPID to image the same delivery to demonstrate the feasibility of imaging without aliasing artifacts. High-speed imaging was shown to be a promising direction toward validating IMRT deliveries with reasonable image resolution and noise.

Unexpected changes of rat cervical spinal cord tolerance caused by inhomogeneous dose distributions

PURPOSE

The effects of dose distribution on dose-effect relationships have been evaluated and, from this, iso-effective doses (ED(50)) established.

METHODS AND MATERIALS

Wistar rats were irradiated on the cervical spinal cord with single doses of unmodulated protons (150 MeV) to obtain sharp lateral penumbras, using the shoot-through technique, which employs the plateau of the depth-dose profile rather than the Bragg peak. Two types of inhomogeneous dose distributions have been administered: (1) 2 4-mm fields with 8- or 12-mm spacing between the center of the fields (referred to as split-field) were irradiated with variable single doses and (2) cervical spinal cord was irradiated with various combinations of relatively low doses to a large volume (20 mm) combined with high doses to a small volume (4 mm) (referred to as bath and shower). The endpoint for estimating the dose-response relationships was paralysis of the fore or hind limbs.

RESULTS

The split-field experiments (2 x 4 mm) showed a shift in the dose-response curves, giving significant higher ED(50) values of 45.4 Gy and 41.6 Gy for 8- and 12-mm spacing, respectively, compared with the ED(50) of 24.9 Gy for the single 8 mm (same total tissue volume irradiated). These values were closer to the ED(50) for a single 4-mm field of 53.7 Gy. The bath and shower experiments showed a large decrease of the ED(50) values from 15-22 Gy when compared with the 4-mm single field, even with a bath dose as low as 4 Gy. There were no histologic changes found in the low dose bath regions of the spinal cord at postmortem.

CONCLUSIONS

Not only the integral irradiated volume is a determining factor for the ED(50) of rat cervical spinal cord, but also the shape of the dose distribution is of great importance. The high ED(50) values of a small region or shower (4 mm) decreases significantly when the adjacent tissue is irradiated with a subthreshold dose (bath), even as low as 4 Gy. The significant shift to lower ED(50) values for induction of paralysis of the limbs by adding a low-dose bath was not accompanied by changes in histologic lesions. These observations may have implications for the interpretation of complex treatment plans and normal tissue complication probability in intensity-modulated radiotherapy.

Dose-volume effects in the rat cervical spinal cord after proton irradiation

PURPOSE

To estimate dose-volume effects in the rat cervical spinal cord with protons.

METHODS AND MATERIALS

Wistar rats were irradiated on the cervical spinal cord with a single fraction of unmodulated protons (150-190 MeV) using the shoot through method, which employs the plateau of the depth-dose profile rather than the Bragg peak. Four different lengths of the spinal cord (2, 4, 8, and 20 mm) were irradiated with variable doses. The endpoint for estimating dose-volume effects was paralysis of fore or hind limbs.

RESULTS

The results obtained with a high-precision proton beam showed a marginal increase of ED50 when decreasing the irradiated cord length from 20 mm (ED50 = 20.4 Gy) to 8 mm (ED50 = 24.9 Gy), but a steep increase in ED50 when further decreasing the length to 4 mm (ED50 = 53.7 Gy) and 2 mm (ED50 = 87.8 Gy). These results generally confirm data obtained previously in a limited series with 4-6-MV photons, and for the first time it was possible to construct complete dose-response curves down to lengths of 2 mm. At higher ED50 values and shorter lengths irradiated, the latent period to paralysis decreased from 125 to 60 days.

CONCLUSIONS

Irradiation of variable lengths of rat cervical spinal cord with protons showed steeply increasing ED50 values for lengths of less than 8 mm. These results suggest the presence of a critical migration distance of 2-3 mm for cells involved in regeneration processes.

Techniques for precision irradiation of the lateral half of the rat cervical spinal cord using 150 MeV protons [corrected]

Techniques for high precision irradiation experiments with protons, to investigate the volume dependence of the tolerance dose of the rat cervical spinal cord are described. In the present study, 50% of the lateral cross section of the spinal cord was irradiated. The diameter of the cross section of this part of the rat spinal cord is at maximum 3.5 mm. Therefore, a dedicated procedure was developed to comply with the needs for a very high positioning accuracy and high spatial resolution dosimetry. By using 150 MeV protons a steep dose gradient (20-80% = 1 mm) in the centre of the spinal cord was achieved. This yields a good dose contrast between the left and right halves of the cord. A home-made digital x-ray imager with a pixel resolution of 0.18 mm/pixel was used for position verification of the spinal cord. A positioning accuracy of 0.09 mm was obtained by using information of multiple pixels. The average position stability during the irradiation was found to be 0.08 mm (1 SD) without significant systematic deviations. Profiles of the dose distribution were measured with a 2D dosimetry system consisting of a scintillating screen and a CCD camera. Dose volume histograms of the whole spinal cord as well as separately of the white and grey matters were calculated using MRI imaging of the cross section of the rat cervical spinal cord. From the irradiation of 20 animals a dose-response curve has been established. MRI showed radiation-induced damage at the high dose side of the spinal cord. Analysis of the preliminary dose-response data shows a significant dose-volume effect. With the described procedure and equipment it is possible to perform high precision irradiations on selected parts of the spinal cord.

Collimator scatter and 2D dosimetry in small proton beams

Monte Carlo simulations have been performed to determine the influence of collimator-scattered protons from a 150 MeV proton beam on the dose distribution behind a collimator. Slit-shaped collimators with apertures between 2 and 20 mm have been simulated. The Monte Carlo code GEANT 3.21 has been validated against one-dimensional dose measurements with a scintillating screen, observed by a CCD camera. In order to account for the effects of the spatial response of the CCD/scintillator system, the line-spread function was determined by comparison with measurements made with a diamond detector. The line-spread function of the CCD/scintillator system is described by a Gaussian distribution with a standard deviation of 0.22 mm. The Monte Carlo simulations show that protons that hit the collimator on the entrance face and leave it through the wall of the aperture make the largest scatter contribution. Scatter on air is the major contribution to the extent of the penumbra. From the energy spectra it is derived that protons with a relative biological effectiveness greater than 1 cause at most 1% more damage in tissue than what would be expected from the physical dose.

Dosimetry of low-energy protons and light ions

For the vertical beam facility at the 14 MV Munich tandem accelerator, various techniques for dosimetry were tested for radiation fields of low-energy protons and light ions (4He, 12C and 16O). A reference dose was determined from the fluence of particles by counting individual particles. A parallel-plate Markus chamber with a small sensitive air volume was used for beam dosimetry applying the ICRU protocol. The doses measured with the ionization chamber were compared with doses evaluated from the fluence measurements. Alternative dose measurements were performed using MTS-N LiF:Mg, Ti thermoluminescence detectors (TLDs) and a photometrically evaluated Fricke chemical dosimeter. An uncertainty of 8% was found in the determination of the dose relative to the reference method. Effects of an inhomogeneous energy loss and a finite track length of the projectiles in the sensitive detector volume of the dosimeters had to be taken into account.

Measurement of the response of Gd2O2S:Tb phosphor to 6 MV x-rays

The phosphor GdO2S:Tb is widely used in camera-based electronic portal imaging devices (EPIDs). There is considerable interest in the application of EPIDs to dosimetry and the verification of intensity modulated radiation therapy produced by dynamic multileaf collimation (DMLC). This paper presents direct measurement of Gd2O2S:Tb phosphor luminescence under 6 MV x-ray irradiation from a linear accelerator using a photomultiplier tube. The luminescence following each radiation pulse (3 micros duration) was observed to decay with a dominant lifetime of 558 micros. Using a specialized electrometer, the temporal variation of the optical signal has been compared with the dose rate incident on the phosphor measured using a semiconductor diode detector. Under dose rates typical of those used in the clinic (1.2 Gy min(-1) to the phosphor), measurements at beam-start confirmed that the optical signal is linear with dose per radiation pulse. Measurements at beam termination following phosphor doses up to 4.4 Gy showed no residual signal associated with long-lived luminescence (afterglow) from the phosphor above the noise level of the optical signal (0.17% standard deviation). This measurement demonstrates that afterglow from Gd2O2S:Tb is not of significance for its application to DMLC verification. Additionally, it was confirmed that the accelerator pulse repetition frequency has no effect on the optical signal from the phosphor in the range 25-400 Hz.

Performance of a fluorescent screen and CCD camera as a two-dimensional dosimetry system for dynamic treatment techniques

A two-dimensionally position sensitive dosimetry system has been tested for different dosimetric applications in a radiation therapy facility with a scanning proton beam. The system consists of a scintillating (fluorescent) screen, mounted at the beam-exit side of a phantom and it is observed by a charge coupled device (CCD) camera. The observed light distribution at the screen is equivalent to the two-dimensional (2D)-dose distribution at the screen position. It has been found that the dosimetric properties of the system, measured in a scanning proton beam, are equal to those measured in a proton beam broadened by a scattering system. Measurements of the transversal dose distribution of a single pencil beam are consistent with dose measurements as well as with dose calculations in clinically relevant fields made with multiple pencil beams. Measurements of inhomogeneous dose distributions have shown to be of sufficient accuracy to be suitable for the verification of dose calculation algorithms. The good sensitivity and sub-mm spatial resolution of the system allows for the detection of deviations of a few percent in dose from the expected (intended or calculated) dose distribution. Its dosimetric properties and the immediate availability of the data make this device a useful tool in the quality control of scanning proton beams.

Dosimetric properties of the Theraview fluoroscopic electronic portal imaging device

Electronic portal imaging devices (EPIDs) can be used for non-imaging applications in radiotherapy such as patient dosimetry. Of the systems available, the fluoroscopic camera-based EPID Theraview (InfiMed Inc.) has not been studied to date, and a review of the dosimetric properties of the system is presented here. In the "single set-up" mode of image acquisition, pixel intensity increases sublinearly with applied dose. The response was dependent on the system's video signal gain and showed a threshold dose to the detector in the range 0.05-0.35 cGy, and pixel saturation at detector doses in the range 1.2-1.6 cGy. Repeated exposures of the EPID were observed to be extremely reproducible (standard deviation 0.5%). The sensitivity of the system showed a linear decline of 0.04% day-1 over a 68-day period, during which time the relative off-axis response within 10 x 10 cm2 field was constant to within a standard deviation of 0.56%. The system shows spatial non-uniformity, which requires correction for application to dose measurements in two-dimensions. Warm-up of the camera control unit required a period of at least 40 min and was associated with an enhancement in pixel intensity of up to 12%. A radiation dose history effect was observed at doses as low as 0.2 Gy. Camera dark current was shown to be negligible at normal accelerator operation. No discernible image distortion was found. Mechanical stability on gantry rotation was also assessed and image displacement of up to 5 mm at the isocentre was observed. It was concluded that the device could be used for dosimetry provided necessary precautions were observed and corrections made.

Slit x-ray beam primary dose profiles determined by analytical transport of Compton recoil electrons

Accurate measurement of radiation beam penumbras is essential for conformal radiotherapy. For this purpose a detailed knowledge of the dosimeter's spatial response is required. However, experimental determination of detector spatial response is cumbersome and restricted to the specific detector type and beam spectrum used. A model has therefore been developed to calculate in slit beam geometry both dose profiles and detector response profiles. Summations over representative photon beam spectra yield profiles for polyenergetic beams. In the present study the model is described and resulting dose profiles verified. The model combines Compton scattering of incident photons, transport of resulting electrons by Fermi-Eyges small-angle multiple scattering theory, and functions to limit electron transport. This analytic model thus yields line spread kernels of primary dose in a water phantom. It is shown that the spatial response of an ideal point detector to a primary photon beam can be well described by the model; the calculations are verified by measurements with a diamond detector in a telescopic slit geometry in which all dose contributions except for the primary dose can be excluded. Effects of photon detector behavior, source size of the linear accelerator (linac) and detector size are studied. Measurements show that slit dose profiles calculated by means of the kernel are accurate within 0.1 mm of the full-width at half-maximum. For a theoretical point source and point detector combined with a 0.2 mm wide slit, the full-width half-maximum values of the slit beam dose profiles are calculated as 0.37 mm and 0.42 mm in a 6 MV and 25 MV x-ray beam, respectively. The present study shows that the model is adequate to calculate local dose effects that are dominated by approximately mono-directional, primary photon fluence. The analytic model further provides directional electron fluence information and is designed to be applied to various detectors and linac beam spectra.

The measurement of proton stopping power using proton-cone-beam computed tomography

A cone-beam computed tomography (CT) system utilizing a proton beam has been developed and tested. The cone beam is produced by scattering a 160 MeV proton beam with a modifier that results in a signal in the detector system, which decreases monotonically with depth in the medium. The detector system consists of a Gd2O2S:Tb intensifying screen viewed by a cooled CCD camera. The Feldkamp-Davis-Kress cone-beam reconstruction algorithm is applied to the projection data to obtain the CT voxel data representing proton stopping power. The system described is capable of reconstructing data over a 16 x 16 x 16 cm3 volume into 512 x 512 x 512 voxels. A spatial and contrast resolution phantom was scanned to determine the performance of the system. Spatial resolution is significantly degraded by multiple Coulomb scattering effects. Comparison of the reconstructed proton CT values with x-ray CT derived proton stopping powers shows that there may be some advantage to obtaining stopping powers directly with proton CT. The system described suggests a possible practical method of obtaining this measurement in vivo.

Verification of the alignment of a therapeutic radiation beam relative to its patient positioner

An easily-used system has been developed for routine measurements of the alignment of beams used for radiation therapy. The position of a beam of circular cross section is measured with respect to a steel sphere fixed to the patient positioning table and which should coincide with the isocenter. Since measurements can be done at all gantry angles (if one is available) and with all possible orientations of the patient table, the system is particularly suited for rapid and accurate measurements of gantry and/or couch isocentricity. Because it directly measures beam-to-positioner offset, the system provides an inclusive alignment verification of the total treatment system. The system has been developed for use with proton beams, but it could equally be used for alignment checks of an x-ray beam from a linear accelerator or other source. The measuring instrument consists of a scintillation screen viewed by a CCD camera, mounted on the gantry downstream of the sphere. The steel sphere is not large enough to stop protons of all energies of interest; however, it will always modify the energy and direction of protons which intersect it, creating a region of lower intensity (a "shadow") in the light spot created by the proton beam hitting the screen. The position of the shadow with respect to the light spot is a measure of the alignment of the system. An image-analysis algorithm has been developed for an automatic determination of the position of the shadow with respect to the light spot. The specifications and theoretical analysis of the system have been derived from Monte Carlo simulations, which are validated by measurements. We have demonstrated that the device detects beam misalignments with an accuracy (1 s.d.) of 0.05 mm, which is in agreement with the expected performance. This accuracy is more than sufficient to detect the maximum allowed misalignment of +/-0.5 mm.

An application of GafChromic MD-55 film for 67.5 MeV clinical proton beam dosimetry

The purpose of this study is to explore the use of GafChromic MD-55 (RC) film for 67.5 MeV clinical proton beam dosimetry at the Crocker Nuclear Laboratory, University of California, Davis. Several strips of RC film 6 cm x 6 cm in dimension were irradiated at a depth of 18.2 mm corresponding to the middle of a 24 mm spread-out Bragg peak (SOBP). The films were irradiated to a proton dose in the range of 0.5 Gy to 100 Gy. The beam profiles were also measured at the middle of the 24 mm SOBP. The Bragg peak was measured by using a wedge shaped phantom made of Lucite. The Bragg peak measured with RC film was compared with diode and ionization chamber measurements. After background subtraction, the calibration of the dose response of RC film showed, to a maximum deviation of 10%, a linear increase of optical density (OD) with dose from 0.5 to 100 Gy. The uniformity of OD over a single sheet of film showed a variation of +/-6%. The distal-fall off between 90% and 20% measured with GafChromic film for the Bragg peak was 1.3 mm as compared to 1.1 mm for a diode measurement and 1.4 mm for an ionization chamber measurement. The FWHM of the Bragg peak was 7.5 mm when measured with GafChromic film, 5.3 mm when measured with a diode and 8.1 mm as measured by an ionization chamber. The peak/plateau ratio with GafChromic film was 3.3 as compared to 3.7 with a diode and 3.2 with an ionization chamber. In conclusion, GafChromic MD-55 film may be a useful and convenient detector for dose measurement and quality assurance programmes of proton beams.