Evaluating the peak-to-valley dose ratio of synchrotron microbeams using PRESAGE fluorescence.

Synchrotron-generated microbeam radiotherapy holds great promise for future treatment, but the high dose gradients present conventional dosimetry with a challenge. Measuring the important peak-to-valley dose ratio (PVDR) of a microbeam-collimated synchrotron source requires both a dosimeter and an analysis method capable of exceptional spatial resolution. The PVDR is of great interest since it is the limiting factor for potential application of the microbeam radiation therapy technique clinically for its tissue-sparing properties (i.e. the valley dose should be below the tolerance of normal tissue). In this work a new method of measuring the dose response of PRESAGE dosimeters is introduced using the fluorescence from a 638 nm laser on a confocal laser-scanning microscope. This fluorescent microscopy method produces dosimetry data at a pixel size as low as 78 nm, giving a much better spatial resolution than optical computed tomography, which is normally used for scanning PRESAGE dosimeters. Using this technique the PVDR of the BL28B2 microbeam at the SPring-8 synchrotron in Japan is estimated to be approximately 52:1 at a depth of 2.5 mm. The PVDR was also estimated with EBT2 GAFchromic films as 30.5:1 at the surface in order to compare the PRESAGE fluorescent results with a more established dosimetry system. This estimation is in good agreement with previously measured ratios using other dosimeters and Monte Carlo simulations. This means that it is possible to use PRESAGE dosimeters with confocal microscopy for the determination of PVDR.

Clinical accuracy of ExacTrac intracranial frameless stereotactic system.

PURPOSE: In this paper, the authors assess the accuracy of the Brainlab ExacTrac system for frameless intracranial stereotactic treatments in clinical practice. METHODS: They recorded couch angle and image fusion results (comprising lateral, longitudinal, and vertical shifts, and rotation corrections about these axes) for 109 stereotactic radiosurgery and 166 stereotactic radiotherapy patient treatments. Frameless stereotactic treatments involve iterative 6D image fusion corrections applied until the results conform to customizable pass criteria, theirs being 0.7 mm and 0.5° for each axis. The planning CT slice thickness was 1.25 mm. It has been reported in the literature that the CT slices' thickness impacts the accuracy of localization to bony anatomy. The principle of invariance with respect to patient orientation was used to determine spatial accuracy. RESULTS: The data for radiosurgery comprised 927 image pairs, of which 532 passed (pass ratio of 57.4%). The data for radiotherapy comprised 15983 image pairs, of which 10 050 passed (pass ratio of 62.9%). For stereotactic radiotherapy, the combined uncertainty of ExacTrac calibration, image fusion, and intrafraction motion was (95% confidence interval) 0.290-0.302 and 0.306-0.319 mm in the longitudinal and lateral axes, respectively. The combined uncertainty of image fusion and intrafraction motion in the anterior-posterior coordinates was 0.174-0.182 mm. For stereotactic radiosurgery, the equivalent ranges are 0.323-0.393, 0.337-0.409, and 0.231-0.281 mm. The overall spatial accuracy was 1.24 mm for stereotactic radiotherapy (SRT) and 1.35 mm for stereotactic radiosurgery (SRS). CONCLUSIONS: The ExacTrac intracranial frameless stereotactic system spatial accuracy is adequate for clinical practice, and with the same pass criteria, SRT is more accurate than SRS. They now use frameless stereotaxy exclusively at their center.

Stereotactic dose perturbation from an aneurysm clip measured by Gafchromic EBT film.

Aneurysm clips within stereotactic treatment volumes enhance spatial accuracy but perturb the dose distribution. The dose perturbations caused by a standard titanium alloy aneurysm clip (Ti6Al4V) have been measured with Gafchromic EBT film. The maximum dose perturbation was an increase of 6 % within 0.5 mm of the beam entry surface of the clip, and a decrease of 7 % within 0.5 mm of the beam exit surface of the clip. Results also showed perturbations to film readout due to the presence of micro dust particles on the film affecting optical properties at high spatial resolution (21um) scanning. Special procedures should be used when film is immersed in water, dried and then read at high spatial resolution. We recommend that films should be immersed only in distilled water and tools such as canned air puffs should be used to clean films without scratches.

Determination of dosimetric perturbations caused by aneurysm clip in stereotactic radiosurgery using gel phantoms and EBT-Gafchromic films.

Some radiotherapy patients are treated with titanium surgical aneurysm clips in the radiation field. This is of particular importance for stereotactic radiosurgery brain treatments, where the length of the blade of the clip may be comparable to the size of the radiation field. This study seeks to determine the extent of the dosimetric effects caused by surgical clips in stereotactic radiosurgery, using polyacrylamide gel phantoms and EBT type Gafchromic films. Using gel phantoms scanned with magnetic resonance imaging scanner, dose enhancement of around 20% was noted at distances less than 2 mm away from the clip surface. Gafchromic films showed about 6% variations in the dose up to few millimeters from the clip. These experimental results confirmed results predicted by Monte Carlo simulation techniques for higher density material surgical clips such as lead and platinum. Moreover, these experimental measurements clearly indicate dose reduction due to radiation attenuation behind the clip of about 4%.

High-resolution measurements of small field beams using polymer gels.

Small field sizes are increasingly becoming important in radiotherapy particularly since the introduction of intensity-modulated radiation therapy (IMRT) techniques. It is normally a challenging task to reliably measure the delivered dose and to determine its distribution in a medium for such small fields using conventional-type dosimeters such as gas ionisation chambers. Recently, attempts have been made to use films, but they are not tissue equivalent, they measure the dose only in two dimensions and they are not as responsive to radiations. In the present work, polyacrylamide gel (PAG) dosimeters are employed to measure the dose and its distribution in three dimensions for very small field sizes, such as those typically used in stereotactic radiosurgery. Field sizes of 6 x 6 and 18 x 18 mm in width are investigated. The results show an agreement with radiochromic film and ionisation diode measurements, with some variation in measured doses near the edge of the field, where the gel data decreases more rapidly than the other methods.

The effects of anaesthesia on cortical stimulation in rats: a functional MRI study.

The purpose of this study was to investigate the affects of equithesin and isoflurane on cortical activation in the rat using fMRI. Eight healthy male Sprauge-Dawley rats were anaesthetised separately with isoflurane and equithesin following a week in between. Functional EPI images were acquired in axial and sagittal orientations on a Bruker 47/30 Biospec system. Each experiment included repetitive air puffs over the right face region and was divided into 4 OFF (no stimulation) and 3 ON (repeated air puffs) periods. Changes in the BOLD-fMRI signal response were analysed using a box-car response function (SPM99) correlated against each voxel to determine regions of activation (p Corrected <0.0001, Z score>3.54). Neural activation was not detected when equithesin was used except in one rat compared to consistent activation with isoflurane in all 16 functional EPI scans. Equithesin appears to have effectively reduced brain activity in response to sensory stimuli. Isoflurane anaesthesia (1.6%) showed consistent, robust neural activations. It is therefore recommended that equithesin should be further investigated with other functional modalities or behavioural tests prior to consider it as an anaesthetic agent for future functional MRI studies.

Stereotactic radiosurgery planning with ictal SPECT images.

This paper is motivated by a clinical requirement to utilise ictal SPECT images for target localisation in stereotactic radiosurgery treatment planning using the xknife system which only supports CT and MRI images. To achieve this, the SPECT images were converted from raw (pixel data only) format into a part 10 compliant DICOM CT fileset. The minimum requirements for the recasting of a raw format image as DICOM CT or MRI data set are described in detail. The method can be applied to the importation of raw format images into any radiotherapy treatment planning system that supports CT or MRI import. It is demonstrated that the combination of the low spatial resolution SPECT images, depicting functional information, with high spatial resolution MRI images, which show the structural information, is suitable for stereotactic radiosurgery treatment planning.

Use of water-equivalent plastic scintillator for intravascular brachytherapy dosimetry.

Beta irradiation has recently been investigated as a possible technique for the prevention of restenosis in intravascular brachytherapy after balloon dilatation or stent implantation. Present methods of beta radiation dosimetry are primarily conducted using radiochromic film. These film dosimeters exhibit limited sensitivity and their characteristics differ from those of tissue, therefore the dose measurement readings require correction factors to be applied. In this work a novel, mini-size (2 mm diameter by 5 mm long) dosimeter element fabricated from Organic Plastic Scintillator (OPS) material was employed. Scintillation photon detection is accomplished using a precision photodiode and innovative signal amplification and processing techniques, rather than traditional photomultiplier tube methods. A significant improvement in signal to noise ratio, dynamic range and stability is achieved using this set-up. In addition, use of the non-saturating organic plastic scintillator material as the detector enables the dosimeter to measure beta radiation at very close distances to the source. In this work the plastic scintillators have been used to measure beta radiation dose at distances of less than 1 mm from an Sr-90 cardiovascular brachytherapy source having an activity of about 2.1 GBq beta radiation levels for both depth-distance and longitudinal profile of the source pellet chain, both in air and in liquid water, are measured using this system. The data obtained is compared with results from Monte Carlo simulation technique (MCNP 4B). Plastic scintillator dosimeter elements, when used in conjunction with photodiode detectors may prove to be useful dosimeters for cardiovascular brachytherapy beta sources, or other applications where precise near-source field dosimetry is required. The system described is particularly useful where measurement of actual dose rate in real time, a high level of stability and repeatability, portability, and immediate access to results are prime requirements.

Relationship between the area of cartilage shown on the magnetic resonance imaging middle-slice image of the medial and lateral tibial cartilages with cartilage volume and grade of osteoarthritis over time.

OBJECTIVE: Knee cartilage volume measurement requires significant time and training. Simplifying the measurement may improve feasibility. We investigated whether the area of cartilage shown on the middle slice of the medial and lateral tibial cartilages on sagittal MRI correlates with radiological features of osteoarthritis (OA), cartilage volume, and longitudinal change in cartilage volume. METHODS: One hundred and seventy-three subjects (normal and osteoarthritic), who had serial magnetic resonance imaging (cartilage volume measured) and baseline weight-bearing antero-posterior radiographs of the same knee were examined. RESULTS: In the lateral compartment, with increasing grade of OA there was a significant reduction in cartilage area. In the medial compartment, this was true for medial joint-space narrowing (after adjusting for gender). There was a moderate to strong association between cartilage area and volume, especially in those with early or no OA. However, when change over time was examined, the strength of these relationships was weak. CONCLUSION: Our data suggests that cartilage area may provide a simple surrogate measure of cartilage volume, in cross-sectional studies, after adjustment for gender: especially in subjects with early disease. However, before it can be widely used, further investigation will be required.

Dosimetry errors in endovascular high-dose-rate brachytherapy.

Monte Carlo data were used to demonstrate the dosimetry of the microSelectron high-dose-rate (HDR) iridium 192 (192Ir) stepping source. These data were used to assess the accuracy of the Nucletron brachytherapy planning system (BPS version 13) for peripheral vessel endovascular brachytherapy. Dose rates from the high-dose-rate (HDR) source are calculated using the Monte Carlo code MCNP4A. Calculations are made at 0.25-cm intervals in the longitudinal direction on sleeves of radii of 1 and 0.25 cm. The Monte Carlo data are summed and weighted to simulate the longitudinal dose distribution at a distance of 1 and 0.25 cm from an 192Ir source stepping through a straight pathway. A comparison is made between the simulated Monte Carlo dosimetry and the Nucletron brachytherapy planning system's dosimetry. This study illustrates and quantifies the dosimetric errors at small distances associated with a point source dose calculation algorithm. The effects of step size, dwell time optimization, and active length on the accuracy of BPS v.13 for HDR endovascular brachytherapy are demonstrated.

Verification of brachytherapy dosimetry with radiochromic film.

The aim of this work is to empirically validate the optimized dose distribution calculated by the Nucletron Brachytherapy Planning System (v. 13.3) at a distance of 1.0 cm from a stepping source of high-dose-rate-iridium 192 (192Ir). The longitudinal dose distribution at 1.0 cm from a straight pathway of multiple-source positions is measured using radiochromic film and compared with the planning system's calculated results. The optical density of the exposed films was determined with a modified Scanditronix film scanner, and the film was calibrated with 192Ir using manually calculated exposure times. A calibration equation was used to convert scanner output to dose. Our results illustrate the significance of exacting geometry in the experimental setup due to the inverse square law and the small distances involved. The dose distribution calculated by the Nucletron Brachytherapy Planning System (v. 13.3), at a distance of 1.0 cm, is validated to within +/-4% of the measured dose distribution. The advantages and limitations of radiochromic film as a dosimetry tool are also addressed in this work.

Standard linear plans in single channel high dose rate brachytherapy: a dosimetric analysis.

The use of standard linear plans is proposed for single channel intraluminal High Dose Rate brachytherapy. This technique employs the optimized dwell times derived from a straight line within a curved geometry. Such standardization of the planning procedure ensures expedient delivery of treatment. The 3-D dose distribution resulting from the use of standard linear plans within various curved geometries is investigated. In this study a comparison is made between the dose delivered to the perimeter of the target volume from both standard linear plans and individually optimized plans. Our results demonstrate that the use of a standard linear plan is acceptable in curved geometries, given the current practice of dose and volume specification for high dose rate intraluminal brachytherapy.

A comparative study of the work involved in measuring profiles using an ion chamber, a linear diode array and film.

In this work a method of using Kodak X-Omat V film to measure beam profiles for dynamically wedged fields is presented. Also, the profiles determined by film measurement are compared with those measured with an ion chamber (0.12 cm3 Scanditronix RFA 300 RK) and an array of silicon diodes (11 channel Scanditronix linear diode array). The beam investigated is a 6 MV photon beam from a Varian 2100 C linear accelerator. The geometric method of positioning film and determining the central axis (CAX) position of the beam yielded results which agreed to within 1 mm with the software determined position of the CAX. The profiles measured by film agreed well with the ion chamber measured profiles in terms of overall field size, position, penumbral width, height and position of maximum and profile shape between the 20% dose levels. Film profiles deviated most from ion chamber profiles in the post-penumbra regions. Linear diode array (LDA) measured profiles matched ion chamber profiles in the post-penumbra regions, field size and general profile shape. In the region of maximum dose differences in dose of up to 4% were seen along with horizontal shifts of around 2 mm between LDA and ion chamber profiles.

Comparative evaluation of Wellhöfer ion chamber array and Scanditronix diode array for dynamic wedge dosimetry.

Normalised profiles have been measured using the Scanditronix diode array and the Wellhöfer ion chamber array for the Varian dynamic wedge. Agreement was of the order of 0.1% of central axis peak dose for an open beam at depth, 0.3% for a dynamic wedge field at depth, and up to 0.6% at the peak depth. The use of the arrays for data acquisition is discussed, including user interface limitations. Data reproducibility is determined to be of the order of 0.1% for both systems. The issue of beam hardening within dynamic wedges is discussed and resolved in terms of the dose-gradient effect. A method for interpolation between dynamic wedge profiles using open beam data is presented that allows construction of isodoses to an estimated accuracy of 0.7%. Finally a benchmark for comparison of different measuring systems based on quality assurance requirements for the enhanced dynamic wedge is suggested.

Dynamic wedge factors for a comprehensive range of fields.

Assessing the methods available to model wedge factors as applied to dynamic wedges over a comprehensive range of field elongations is the primary intention of this paper. Wedge factors for dynamically produced wedges were investigated by a series of measurements for square and rectangular fields. A Varian 600C linear accelerator was used for the experimental work. Dynamic wedge angles of 15, 30, 45 and 60 degrees were studied. (The wedge factor was defined as the ratio of the central axis dose reading at 10 cm deep of the wedged field to the reading at 10 cm depth for the same sized open field). The possibility of improving dynamic wedge factors for elongated fields by modelling the rectangular field with the equivalent square (Worthley) and also by the square field of equal area (Arthur) methods was evaluated. Dynamic wedge factors were found to depend on the field size in the wedged direction dominantly. The influence of field elongation in the non-wedged axis was found to be negligible. The experiments indicate that the derivation of a dynamic wedge factor for use with rectangular fields is best based on a square field with a side equal to that in the wedged axis. The application of two square field models (Worthley and Arthur) for elongated fields was found to produce significantly worsened wedge factors for the majority of the fields considered.