2.1. SARRP overview
highlights the components of the latest working version of the SARRP. As has been described previously (Wong et al 2007
), the system comprises a constant-voltage, dual-focus (0.4 and 3 mm spot sizes), 225 kVp source (GE ISOVOLT 225 M2, Lewistown, PA) mounted on a rotating arm; a set of robotic stages providing four degrees of freedom (x
) used in positioning the animal; and a 512 × 512 amorphous Si flat panel detector (20 × 20 cm2
). Oriented orthogonal to the flat-panel imaging system is a digital fluoroscopic camera box used for taking anterio-posterior (or vice versa) radiographs, to be potentially used in conjunction with lateral radiographs from the flat panel detector. This latter imaging capability was developed for experiments not requiring CBCT-based image guidance.
Figure 1 (a) Design rendering and (b) photo of the latest working version of the SARRP. The system comprises a constant-voltage, dual-focus, 225 kVp source mounted on a rotating arm, a collimation system with interchangeable nozzles, a set of robotic stages for (more ...)
2.2. SARRP beam collimation and filtration
shows the high-precision beam collimation system that will be the focus of the present work. The collimating components, or those that the primary x-ray beam impinges upon, are made of brass. Mostly aluminum is used for all other components. Mechanical stability of this system was maximized by mounting the collimator directly to the exit flange, or intrinsic collimator, of the x-ray tube with a special pin-hole fitting at the interface for rigidity and reproducibility. It has been designed to be strongly self-shielding, as no primary x-rays can escape the system without first traversing at least 25 mm of brass. Generally, the constraint that no upstream components shadow the primary beam from downstream components was imposed. The downstream face of the last field-shaping ‘nozzle’ collimator is nominally located at 30 cm from the presumed source of the x-ray tube. With the nominal source-to-isocenter distance of 35 cm, this collimator design minimizes the geometric radiation penumbrae.
Figure 2 Design rendering of the high-precision beam collimation system for the SARRP. The collimating components are made of brass. Aluminum is used elsewhere. The downstream edge of the last field-shaping ‘nozzle’ collimator is located at 30 (more ...)
Multiple, interchangeable nozzles provide rectangular radiation fields of 10 × 5, 5 × 5 and 3 × 3 mm2 or circular fields of 1.16 and 0.58 mm in diameter as scaled to 35 cm from the source. (The latter two field sizes will be referred to as 1 and 0.5 mm in the remainder of this report.) A special design was required for the smallest collimator. In this case, a double-aperture design comprising lead was used, minimizing the thickness of material required to be accurately machined (see ).
Filtration is used to remove the low-energy contamination in the x-ray spectrum. Intrinsically, the x-ray tube incorporates 1 mm of beryllium filtration as an exit window. For the treatment of mice and rats, balancing beam hardening against dose rate, we have initially chosen to incorporate additional copper filtration of either 0.16 or 0.25 mm in thickness. Results for both will be presented.
2.3. Commissioning device
shows the commissioning jig, phantom assembly and related equipment that was designed and built for the SARRP. Using this system, exposed EBT films, which are sandwiched between layers of 5 mm thick kV-equivalent solid water (CIRS, Norfolk, VA), are rigidly indexed to the beam axis by way of bisecting aluminum rods. A custom film hole puncher, also shown, was constructed for this purpose. The phantom dimensions are 6 cm × 6 cm × 8 cm, the latter dimension being the maximum depth. The circular bottom plate of the phantom was constructed of solid water, assuring that the distribution of back-scattered photons is water-equivalent to the last depth sampled. The bottom aluminum plate of the jig can slide along the beam axis, its position being determined by the configuration of precisely machined spacers. This allows variable source-to-surface (SSD) distances; the system can sample SSDs between 32 and 38 cm, in increments of 5 mm.
Figure 3 The commissioning jig, 6 × 6 × 8 cm cubed plastic water phantom assembly and related equipment. Using this system, EBT films are sandwiched between layers of 5 mm thick, kV-equivalent plastic water and are rigidly indexed to the beam axis. (more ...)
2.4. EBT film dosimetry
Gafchromic EBT film (International Specialty Products, Wayne, NJ) is used for dosimetric measurements. It has been shown that EBT film is ideal for two-dimensional (2D) dose measurements for lower energies because of its energy independence and high sensitivity (see, e.g. Devic et al (2005
) and references therein). Our EBT dosimetry protocol is similar to that described by Devic et al (2005)
. EBT films were scanned as 48 bit RGB (16 bits per color) TIFF images at 400 or 600 dpi resolution with a professional-grade flatbed scanner (Epson Expression 10000 XL). The lower scanning resolution was employed for the larger beams, whereas 600 dpi was used exclusively for the 0.5 mm aperture. Only transmission data from the red channel was evaluated, as per standard EBT protocol. Using a modified version of the template (provided with the scanner), films were (re-)positioned for scanning with the scanning direction being consistent for all with respect to the orientation of the uncut film. The films were scanned prior to and at least 24 h post-irradiation. At both time points, an average was taken over three consecutive scans to reduce the random or sampling noise. All nine films (depths) for a given phantom setup could be scanned simultaneously with the aforementioned template.
To calibrate our batch of EBT, three films were exposed simultaneously to 60
Co gamma rays for each of 23 different exposure times spanning doses between 0 and 25 Gy using the Gamma Knife, Model C (Elekta AB, Stockholm, Sweden). In total 69 films were used. The films were positioned at the Gamma Knife focus using the standard (16 cm diameter) polystyrene calibration phantom. The exposure time of the films was related to dose, as monitored by the clinical calibration protocol in use at our institution for the Gamma Knife (AAPM Task Group 21, 1983
). The difference in scan-averaged pixel values, before and after exposure, was correlated to the radiation dose corresponding to each exposure time, averaging over the three films which had been exposed simultaneously. A 12th-order polynomial fit well described the data over its range, except in the extreme low-dose region (0–11 cGy), where a linear fit was smoothly matched. The scanning direction with respect to the uncut films was consistent with that used for the experimental measurements.
The result of this calibration process is shown in . An important feature is that the relationship between dose and detected transmission (scanner) value gives rise to noise amplification that is increasingly important for doses that are outside of the more ‘linear’ response range (approximately 0–3 Gy). For the peak doses sampled in this set of measurements (>10 Gy), the noise amplification factor was greater than 5 in some cases. Therefore, the measured results are more prone to statistical sampling error at the shallower depths sampled, especially for the larger apertures.
Figure 4 The result of the EBT film calibration process. 69 films were irradiated with doses (or exposure times) spanning from 0 to 25 Gy using a Gamma Knife (60Co) irradiator. Data points: the difference in scan-averaged transmission values, before and after (more ...)
All image processing was implemented in MATLAB (The MathWorks, Inc., Natick, MA). Functions were written to perform the following tasks: (1) apply a gain correction to the scan-averaged image for y-pixel location based on a polynomial fit to the flood-field (blank template) scans; (2) convert the pre- and post-exposure differences for each individual film to planar dose; (3) threshold post-exposure image and automatically segment the punched holes for each film to determine their locations in a reference coordinate system defined by the beam axis; (4) apply the indicated transformations to each of the measured planar dose images; (5) automatically tabulate results in terms of obtained radiation profiles, peak and region-averaged dose rates and penumbra widths—these results can be output in Microsoft Excel format for simple reference by SARRP users requiring simplified radiation descriptions for their laboratory studies.
EBT film sets, or stacks, were exposed for 4, 6 or 8 min, depending on the field size, under maximal tube current (13 mA) and potential (225 kVp). The larger focal-spot setting was used for all measurements. Two SSDs, 34 and 38 cm, were sampled for 0.16 mm Cu filtration—for the latter the 5 × 10 mm nozzle was not measured. One SSD (34 cm) and a subset of collimators were sampled for 0.25 mm Cu filtration, namely the 1 mm, 3 × 3 mm2 and 5 × 5 mm2 apertures. In total, 12 sets of exposures, each recording nine film depths (therefore 108 films in total) were included in the present analysis.
2.5. Monte Carlo modeling
BEAM is the basis for our MC modeling (Rogers et al 1995
). We have installed the BEAMnrc multi-platform package (GUI version, revision 1.10) on a Linux server (64 bit, quad-core processor 4 × 1.86 GHz) running SUSE Enterprise Server Edition, v10 (Novell, Inc., Waltham, MA). BEAM is advantageous in that the user can ‘build’ an ‘accelerator’ using an arbitrary arrangement of pre-defined geometrical elements (collimators, targets, filters) or ‘component modules’ (CMs) (Verhaegen and Seuntjens 2003
, Verhaegen et al 1999
). Our MC model considers all physical processes which are important at these low energies, namely bremsstrahlung radiation (for photon production in the target as well as throughout the geometry), bound Compton scattering, Rayleigh scattering, electron impact ionization and fluorescent atomic relaxation following photo-electric absorption. Full electron transport was only carried out in the target, filter and a subset of the collimating apertures. Bremsstrahlung yields were taken from the NIST library (Seltzer 1989
); XCOM photon cross sections were used (Berger and Hubbell 1987
shows a cut view in the gun-target (G-T)–z plane of the CMs which together comprise the BEAM model for the SARRP source and collimators. With the set of available CMs, it was not possible to incorporate exact geometries for all components of the SARRP collimator design, nor was this necessary. Rather, the goal was to closely approximate the system geometrically and show that this model could adequately reproduce the set of measured data.
Figure 5 Right: the SARRP x-ray source and collimator design rendered in the A–B–z plane for the 5 × 5 nozzle. Note that collimator 3 is interchangeable, depending on the desired field size. Left: corresponding SARRP BEAM Monte Carlo model (more ...)
A large ‘phase-space’ file (PSF) containing energy, position and history information for simulated particles was first generated or sampled from the indicated z-location 1, at the exit of the x-ray tube (X-TUBE CM). This CM included the geometry for the SARRP source, namely the 20°, 1 mm tungsten target with a parallel, rectangular electron beam providing an effective 3 mm × 3 mm x-ray source. 30 × 109 electron histories were simulated. Uniform bremsstrahlung splitting with a splitting factor of 300 was used for efficiency in generating the photons of interest.
This primary PSF was then used as the generator for the second-stage simulations, which allowed for the different choices of Cu filtration, 0.16 or 0.25 mm. Finally, these secondary PSFs, sampled at z-location 2, were used as input in the tertiary simulations which sampled particle phase space at z-location 3, at the exit of the given nozzle, each of which comprises two CMs, collimators 3a and 3b. Note that the indicated PYRAMIDS CM for collimator 3b only accommodates square-field nozzles whereas a CONESTAK CM was required for the two circular apertures (0.5 and 1 mm).
Depth–dose information was extracted from the tertiary simulations by inserting a water-filled CHAMBER CM into the geometry at the requisite SSD. The cross-sectional diameter of the active central region was set to correspond to the size of the dose averaging regions on the EBT films in the measurements. Likewise, the active dose-scoring layers of the CHAMBER were defined at the same depths as the measurements. In the simulations, the ‘film’ layers had 1 mm thicknesses to augment statistics, compared with approximately 0.2 mm actual film thicknesses5
For dose-profile comparisons with measurement, simulations of 3D (voxelized) dose-deposition in water were carried out for a subset of the measured data using DOSXYZnrc (Walters et al 2006
). For these, final PSFs sampled from z
-location 3 within the tertiary BEAM simulations were used as input. The resolution of the calculation grid was either 0.0625 mm squared (0.5 and 1 mm nozzles) or 0.250 mm squared (all others) transverse to and 1 mm along the beam axis.
For the tertiary simulations, each particle from the PSF (at z
-location 2) was recycled 99 times to improve dose-computation statistics6
. Additionally, for the DOSXYZ simulations, the input parameter ISMOOTH was set to ‘1’, which allows that each particle in the phase-space file be used a total of four times, with its x
starting position and trajectory in the scoring plane being recursively mirrored over four quadrants7