A holistic measurement model of movement competency in children.

Different countries have different methods for assessing movement competence in children; however, it is unclear whether the test batteries that are used measure the same aspects of movement competence. The aim of this paper was to (1) investigate whether the Test of Gross Motor Development (TGMD-2) and Körperkoordinations Test für Kinder (KTK) measure the same aspects of children's movement competence and (2) examine the factorial structure of the TGMD-2 and KTK in a sample of Australian children. A total of 158 children participated (M age = 9.5; SD = 2.2). First, confirmatory factor analysis examined the independent factorial structure of the KTK and TGMD-2. Second, it was investigated whether locomotor, object control and body coordination loaded on the latent variable Movement Competency. Confirmatory factor analysis indicated an adequate fit for both the KTK and TGMD-2. An adequate fit was also achieved for the final model. In this model, locomotor (r = .86), object control (r = .71) and body coordination (r = .52) loaded on movement competence. Findings support our hypothesis that the TGMD-2 and KTK measure discrete aspects of movement competence. Future researchers and practitioners should consider using a wider range of test batteries to assess movement competence.

Electron contamination modeling and skin dose in 6 MV longitudinal field MRIgRT: Impact of the MRI and MRI fringe field.

PURPOSE: In recent times, longitudinal field MRI-linac systems have been proposed for 6 MV MRI-guided radiotherapy (MRIgRT). The magnetic field is parallel with the beam axis and so will alter the transport properties of any electron contamination particles. The purpose of this work is to provide a first investigation into the potential effects of the MR and fringe magnetic fields on the electron contamination as it is transported toward a phantom, in turn, providing an estimate of the expected patient skin dose changes in such a modality. METHODS: Geant4 Monte Carlo simulations of a water phantom exposed to a 6 MV x-ray beam were performed. Longitudinal magnetic fields of strengths between 0 and 3 T were applied to a 30 × 30 × 20 cm(3) phantom. Surrounding the phantom there is a region where the magnetic field is at full MRI strength, consistent with clinical MRI systems. Beyond this the fringe magnetic field entering the collimation system is also modeled. The MRI-coil thickness, fringe field properties, and isocentric distance are varied and investigated. Beam field sizes of 5 × 5, 10 × 10, 15 × 15 and 20 × 20 cm(2) were simulated. Central axis dose, 2D virtual entry skin dose films, and 70 µm skin depth doses were calculated using high resolution scoring voxels. RESULTS: In the presence of a longitudinal magnetic field, electron contamination from the linear accelerator is encouraged to travel almost directly toward the patient surface with minimal lateral spread. This results in a concentration of electron contamination within the x-ray beam outline. This concentration is particularly encouraged if the fringe field encompasses the collimation system. Skin dose increases of up to 1000% were observed for certain configurations and increases above Dmax were common. In nonmagnetically shielded cases, electron contamination generated from the jaw faces and air column is trapped and propagated almost directly to the phantom entry region, giving rise to intense dose hot spots inside the x-ray treatment field. These range up to 1000% or more of Dmax at the CAX, depending on field size, isocenter, and coil thickness. In the case of a fully magnetically shielded collimation system and the lowest MRI field of 0.25 T, the entry skin dose is expected to increase to at least 40%, 50%, 65%, and 80% of Dmax for 5 × 5, 10 × 10, 15 × 15, and 20 × 20 cm(2), respectively. CONCLUSIONS: Electron contamination from the linac head and air column may cause considerable skin dose increases or hot spots at the beam central axis on the entry side of a phantom or patient in longitudinal field 6 MV MRIgRT. This depends heavily on the properties of the magnetic fringe field entering the linac beam collimation system. The skin dose increase is also related to the MRI-coil thickness, the fringe field, and the isocenter distance of the linac. The results of this work indicate that the properties of the MRI fringe field, electron contamination production, and transport must be considered carefully during the design stage of a longitudinal MRI-linac system.

Monte Carlo study of the energy response and depth dose water equivalence of the MOSkin radiation dosimeter at clinical kilovoltage photon energies.

Skin dose is often the quantity of interest for radiological protection, as the skin is the organ that receives maximum dose during kilovoltage X-ray irradiations. The purpose of this study was to simulate the energy response and the depth dose water equivalence of the MOSkin radiation detector (Centre for Medical Radiation Physics (CMRP), University of Wollongong, Australia), a MOSFET-based radiation sensor with a novel packaging design, at clinical kilovoltage photon energies typically used for superficial/orthovoltage therapy and X-ray CT imaging. Monte Carlo simulations by means of the Geant4 toolkit were employed to investigate the energy response of the CMRP MOSkin dosimeter on the surface of the phantom, and at various depths ranging from 0 to 6 cm in a 30 × 30 × 20 cm water phantom. By varying the thickness of the tissue-equivalent packaging, and by adding thin metallic foils to the existing design, the dose enhancement effect of the MOSkin dosimeter at low photon energies was successfully quantified. For a 5 mm diameter photon source, it was found that the MOSkin was water equivalent to within 3% at shallow depths less than 15 mm. It is recommended that for depths larger than 15 mm, the appropriate depth dose water equivalent correction factors be applied to the MOSkin at the relevant depths if this detector is to be used for depth dose assessments. This study has shown that the Geant4 Monte Carlo toolkit is useful for characterising the surface energy response and depth dose behaviour of the MOSkin.

Monte Carlo characterization of skin doses in 6 MV transverse field MRI-linac systems: effect of field size, surface orientation, magnetic field strength, and exit bolus.

PURPOSE: The main focus of this work is to continue investigations into the Monte Carlo predicted skin doses seen in MRI-guided radiotherapy. In particular, the authors aim to characterize the 70 microm skin doses over a larger range of magnetic field strength and x-ray field size than in the current literature. The effect of surface orientation on both the entry and exit sides is also studied. Finally, the use of exit bolus is also investigated for minimizing the negative effects of the electron return effect (ERE) on the exit skin dose. METHODS: High resolution GEANT4 Monte Carlo simulations of a water phantom exposed to a 6 MV x-ray beam (Varian 2100C) have been performed. Transverse magnetic fields of strengths between 0 and 3 T have been applied to a 30 x 30 x 20 cm3 phantom. This phantom is also altered to have variable entry and exit surfaces with respect to the beam central axis and they range from -75 degrees to +75 degrees. The exit bolus simulated is a 1 cm thick (water equivalent) slab located on the beam exit side. RESULTS: On the entry side, significant skin doses at the beam central axis are reported for large positive surface angles and strong magnetic fields. However, over the entry surface angle range of -30 degrees to -60 degrees, the entry skin dose is comparable to or less than the zero magnetic field skin dose, regardless of magnetic field strength and field size. On the exit side, moderate to high central axis skin dose increases are expected except at large positive surface angles. For exit bolus of 1 cm thickness, the central axis exit skin dose becomes an almost consistent value regardless of magnetic field strength or exit surface angle. This is due to the almost complete absorption of the ERE electrons by the bolus. CONCLUSIONS: There is an ideal entry angle range of -30 degrees to -60 degrees where entry skin dose is comparable to or less than the zero magnetic field skin dose. Other than this, the entry skin dose increases are significant, especially at higher magnetic fields. On the exit side there is mostly moderate to high skin dose increases for 0.2-3 T with the only exception being large positive angles. Exit bolus of 1 cm thickness will have a significant impact on lowering such exit skin dose increases that occur as a result of the ERE.

High resolution entry and exit Monte Carlo dose calculations from a linear accelerator 6 MV beam under the influence of transverse magnetic fields.

A current concern with 6 MV transverse field MRI-linac hybrid systems is the predicted increases in skin dose (both the entry and exit sides) caused by the effects of the magnetic field on secondary electrons. In this work high resolution GEANT4 Monte Carlo simulations have been performed at the beam central axis in the entry and exit regions of a water phantom to predict surface (0 microm depth) and skin (70 microm depth) doses when placed in such a hybrid system. A 30 x 30 x 20 cm3 water phantom with 10 microm thick voxels has been simulated by being irradiated perpendicularly with a 6 MV photon beam (Varian 2100C) of sizes of 5 x 5, 10 x 10, 15 x 15, and 20 x 20 cm2. Uniform transverse magnetic fields of 0.2, 0.75, 1.5, and 3 T with varying thickness above the phantom have been investigated. Simulations with and without lepton contamination have been performed. In the entry region the high resolution scoring has yielded unexpected surface and skin doses. There is a small amount of nonpurged air-generated lepton contamination that originates immediately above the phantom surface and delivers its dose over very short longitudinal distances in the entry region. At 0.2 T the surface and skin doses are not accurately predicted using lepton-contamination-free simulations and extrapolated lower resolution scoring. Lepton-free simulations are up to 7% of Dmax lower than simulations with leptons. However, compared to 0 T, entry skin dose is reduced at 0.2 and 0.75 T but increases to 28%-31% of Dmax at 3 T. For skin doses at the central axis in the exit region, high resolution scoring shows relative increases of 38%-106%, depending on the magnetic field strength and field size. These values are also up to 20% higher than lower resolution results. The shape of the exit dose profiles varies unpredictably and so extrapolation of low resolution data is insufficient. In order to achieve accurate Monte Carlo skin dosimetry in a transverse field MRI-linac system, the authors recommend using high resolution scoring. In systems of 0.2 T the inclusion of air-generated lepton contamination is also recommended.

Energy dependence corrections to MOSFET dosimetric sensitivity.

Metal Oxide Semiconductor Field Effect Transistors (MOSFET's) are dosimeters which are now frequently utilized in radiotherapy treatment applications. An improved MOSFET, clinical semiconductor dosimetry system (CSDS) which utilizes improved packaging for the MOSFET device has been studied for energy dependence of sensitivity to x-ray radiation measurement. Energy dependence from 50 kVp to 10 MV x-rays has been studied and found to vary by up to a factor of 3.2 with 75 kVp producing the highest sensitivity response. The detectors average life span in high sensitivity mode is energy related and ranges from approximately 100 Gy for 75 kVp x-rays to approximately 300 Gy at 6 MV x-ray energy. The MOSFET detector has also been studied for sensitivity variations with integrated dose history. It was found to become less sensitive to radiation with age and the magnitude of this effect is dependant on radiation energy with lower energies producing a larger sensitivity reduction with integrated dose. The reduction in sensitivity is however approximated reproducibly by a slightly non linear, second order polynomial function allowing corrections to be made to readings to account for this effect to provide more accurate dose assessments both in phantom and in-vivo.

Evaluation of the magnitude of EBT Gafchromic film polarization effects.

Gafchromic EBT film, has become a main dosimetric tools for quantitative evaluation of radiation doses in radiation therapy application. One aspect of variability using EBT Gafchromic film is the magnitude of the orientation effect when analysing the film in landscape or portrait mode. This work has utilized a > 99% plane polarized light source and a non-polarized diffuse light source to investigate the absolute magnitude of EBT Gafchromic films polarization or orientation effects. Results have shown that using a non-polarized light source produces a negligible orientation effect for EBT Gafchromic film and thus the angle of orientation is not important. However, the film exhibits a significant variation in transmitted optical density with angle of orientation to polarized light producing more than 100% increase, or over a doubling of measured OD for films irradiated with x-rays up to dose levels of 5 Gy. The maximum optical density was found to be in a plane at an angle of 14 degrees +/- 7 degrees (2 SD) when the polarizing sheet is turned clockwise with respect to the film. As the magnitude of the orientation effect follows a sinusoidal shape it becomes more critical for alignment accuracy of the film with respect to the polarizing direction in the anticlockwise direction as this will place the alignment of the polarizing axes on the steeper gradient section of the sinusoidal pattern. An average change of 4.5% per 5 degrees is seen for an anticlockwise polarizer rotation where as the effect is 1.2% per 5 degrees for an clockwise polarizer rotation. This may have consequences to the positional accuracy of placement of the EBT Gafchromic film on a scanner as even a 1 degree alignment error can cause an approximate 1% error in analysis. The magnitude of the orientation effect is therefore dependant on the degree of polarization of the scanning light source and can range from negligible (diffuse LED light source) through to more than 100% or doubling of OD variation with a fully linear polarized light source.

Dose and absorption spectra response of EBT2 Gafchromic film to high energy x-rays.

With new advancements in radiochromic film designs and sensitivity to suit different niche applications, EBT2 is the latest offering for the megavoltage radiotherapy market. New construction specifications including different physical construction and the use of a yellow coloured dye has provided the next generation radiochromic film for therapy applications. The film utilises the same active chemical for radiation measurement as its predecessor, EBT Gafchromic. Measurements have been performed using photo spectrometers to analyse the absorption spectra properties of this new EBT2 Gafchromic, radiochromic film. Results have shown that whilst the physical coloration or absorption spectra of the film, which turns yellow to green as compared to EBT film, (clear to blue) is significantly different due to the added yellow dye, the net change in absorption spectra properties for EBT2 are similar to the original EBT film. Absorption peaks are still located at 636nm and 585nm positions. A net optical density change of 0.590 +/- 0.020 (2SD) for a 1 Gy radiation absorbed dose using 6 MV x-rays when measured at the 636nm absorption peak was found. This is compared to 0.602 +/- 0.025 (2SD) for the original EBT film (2005 Batch) and 0.557 +/- 0.027 (2009 Batch) at the same absorption peak. The yellow dye and the new coating material produce a significantly different visible absorption spectra results for the EBT2 film compared to EBT at wavelengths especially below approximately 550nm. At wavelengths above 550nm differences in absolute OD are seen however, when dose analysis is performed at wavelengths above 550nm using net optical density changes, no significant variations are seen. If comparing results of the late production EBT to new production EBT2 film, net optical density variations of approximately 10 % to 15 % are seen. As all new film batches should be calibrated for sensitivity upon arrival this should not be of concern.

Measuring energy response for RTQA radiochromic film to improve quality assurance procedures.

RTQA Gafchromic, radiochromic film is assessed for its radiation energy dependence in photon beams ranging from superficial to megavoltage energies. RTQA radiochromic film has uses in radiation quality assurance procedures due to its auto development and visualisation properties. These properties allow for immediate comparison of x-ray alignment and coincidence not available with radiographic films. Results show that the RTQA film produces an energy dependant darkening to x-rays which results in x-ray energies of 69 keV photon equivalent (150 kVp) to produce 2.14 times the optical density to dose ratio of a 6MV x-ray beam. The following dose ratio's (normalized to 1 at 150 kVp) provide the same net optical density change for RTQA film. 1.47-50 kVp : 1.21-75 kVp : 1.09-100 kVp : 1.01-125 kVp: 1.00-150 kVp : 1.03-200 kVp : 1.07-250 kVp : 2.14-6 MVp : 2.14 10 MVp. Although the film is not designed to be used as a quantitative measure of radiation it is still useful to know its energy response at differing x-ray energies to expose the film to the appropriate dose to provide optimal darkening characteristics for a given QA test at the appropriate energy. Our results have shown that a 0.3 optical density change with RTQA film provides a colour change level useable for accurate alignment procedures

Evaluating TOPAS for the calculation of backscatter factors for low energy x-ray beams.

For low x-ray energies, backscatter is an important parameter for determining the absorbed dose, making accurate knowledge of backscatter factors (BSFs) essential. BSFs can be difficult to measure experimentally so published values are often derived using Monte Carlo methods. This study evaluated the Monte Carlo code TOPAS as a tool for dose calculations for kilovoltage x-rays, and for calculating BSFs for energies ranging from 50 to 280 kVp. BSFs were also measured experimentally for comparison using Gafchromic EBT3 film and optically stimulated luminescence dosimeters (OSLs). The BSFs calculated using TOPAS were found to be consistent within 2% of the values published in the AAPM TG-61 protocol. The TOPAS BSF calculations were also found to be consistent with film measurements, typically within 2%. The largest discrepancy measured was 5% for the 3 cm field size and 180 kVp beam. OSLs were found to overestimate BSFs for large field sizes and high energies so were found to be unsuitable for BSF measurements with differences of up to 18%.

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.

Verification of nose irradiation using orthovoltage x-ray beams.

Determination of dose distributions from superficial and orthovoltage irradiations of basal cell carcinoma of the nose has been performed using a nose shaped phantom constructed from paraffin wax. EBT type radiochromic film was used for dose measurements. A 2 cm diameter 50 kVp anterior field was used to irradiate the nose phantom, with sheets of film placed at 7 mm, 14 mm and 23 mm physical depth. The percentage depth doses at these points were measured to be 84% +/- 1.6%, 66% +/- 2.7% and 50% +/- 1.2% respectively, compared to the expected percentage depth doses of 72%, 52% and 34%, measured in full scatter conditions. This discrepancy is believed to be due to the steep drop off at the sides of the nose phantom, resulting in reduced attenuation at the edges of the beam, which in turn results in an increase in the scatter contribution to the dose at depth on the central axis. Measured dose profiles from this technique showed a reasonably uniform distribution. A second technique using a 250 kVp tangent-like field to irradiate the tip of the nose was also tested. Radiochromic film was placed against the edges of the phantom for dose measurement. The dose at the surface was measured to be 27% +/- 1.5% less than the expected dose. It is believed that this discrepancy is due to a combination of the lack of backscatter from the phantom, and a small offset between the phantom and the treatment cone. Dose measurements and profiles showed that this technique results in a variation in dose across the treated volume of 7%. However, the difficulty in predicting the delivered dose prohibited it from clinical use.

Radiochromic film for verification of superficial x-ray backscatter factors.

Accurate calculation and knowledge of backscatter factors (BSF) in superficial x-ray radiotherapy is required to perform accurate absorbed dose determination. These measurements have been performed historically with small thin parallel plate ionisation chambers and Thermoluminescent Dosimeters (TLD's). This note investigates the use of a low energy dependence radiochromic thin film (GAFCHROMIC EBT) for measurement and verification of backscatter factors. A single layer film and an extrapolation method with multiple films have been investigated. 50kVp to 150kVp beams were analysed and results for BSF were measured and compared to IPEMB (Institution of Physics and Engineering in Medicine and Biology UK) derived results. Agreement within 2% (1 SD) was found using both the single layer and extrapolation techniques with IPEMB derived results at 30cm SSD and equivalent photon energies. A 150kVp beam was found to have BSF of 1.12 +/- 0.02 (2cm circle), 1.24 +/- 0.01 (5cm circle), 1.36 +/- 0.02 (10cm circle) respectively compared to 1.11, 1.23 and 1.36 for IPEMB derived results. In summary a single layer film provided an accurate measurement and verification of BSF and was found to be within 2% of derived IPEMB results in all cases. The extrapolation method in general provided a slightly closer match to IPEMB results (<1%) but with no extra discernable accuracy than the single layer film most likely due to the already small thickness (0.3mm) of one film piece. GAFCHROMIC EBT, Radiochromic film provides a very simple and easy method for measurement and verification of BSF for x-ray energies commonly used for superficial x-ray therapy.

Measurement and production of electron deflection using a sweeping magnetic device in radiotherapy.

The deflection and removal of high energy electrons produced by a medical linear accelerator has been attained by a Neodymium Iron Boron (NdFeB) permanent magnetic deflector device. This work was performed in an attempt to confirm the theoretical amount of electron deflection which could be produced by a magnetic field for removal of electrons from a clinical x-ray beam. This was performed by monitoring the paths of mostly monoenergetic clinical electron beams (6 MeV to 20 MeV) swept by the magnetic fields using radiographic film and comparing to first order deflection models. Results show that the measured deflection distance for 6 MeV electrons was 18 +/- 6 cm and the calculated deflection distance was 21.3 cm. For 20 MeV electrons, this value was 5 +/- 2 cm for measurement and 5.1 cm for calculation. The magnetic fields produced can thus reduce surface dose in treatment regions of a patient under irradiation by photon beams and we can predict the removal of all electron contaminations up to 6 MeV from a 6 MV photon beam with the radiation field size up to 10 x 10 cm2. The model can also estimate electron contamination still present in the treatment beam at larger field sizes.

Comparison of skin dose between conventional radiotherapy and IMRT.

The purpose of this study was to measure skin dose using radiochromic film for two step-and-shoot IMRT fields and compare the results to the skin dose for a conventional open field. All exposures were made using a 6 MV photon beam produced by a Varian 21EX linear accelerator (Varian Medical Systems, CA, USA) equipped with a Millennium 120 leaf MLC. Three different field configurations were used, these were an open field, a step-and-shoot IMRT field and a clinical IMRT field. The mean ratio of the skin dose to dose at d(max) for an open 10 x 10 cm2 field at 100 cm SSD was 0.178 +/- 0.003. The step-and-shoot IMRT field consisted of 1 cm wide strips of decreasing intensity that were delivered using a step-and-shoot technique across a 10 x 10 cm2 field. The ratio of skin dose to dose at d(max) ranged from 0.180 to 0.257, with the low intensity steps having a higher relative skin dose compared to the high intensity steps. A model was derived that attributed these variations to the electron contamination from both the adjacent and more distant high intensity steps. The clinical field consisted of a 25 segment 9.8 x 10.0 cm2 beam arrangement. The ratio of skin dose to dose at d(max) for the clinical IMRT field ranged from 0.093 to 0.284. The results indicated that an IMRT field produced only minor changes in the relative skin dose, with variations potentially attributable to fluctuations in the electron contamination produced by neighbouring regions of different intensity. The use of an individual IMRT field does not significantly increase the skin dose above that of a conventional photon field.

Scanning orientation effects on Gafchromic EBT film dosimetry.

Gafchromic EBT film, a new high sensitivity radiochromic film has been tested for variations in optical properties due to scanning orientation. Gafchromic EBT film has been shown to produce a scanning orientation effect whereby variations in measured relative optical density are found due to the films orientation relative to the scanner direction. This relative optical density change was found to be relatively consistent for different films exposed to varying dose levels ranging from 0 Gy to 3 Gy. A maximum variation of 0.0157 +/- 0.0035 in optical density (OD) was found. This relates to an approximate 15% variation in net OD for a 50 cGy irradiated film and 4% variation for a 3 Gy irradiated film. No noticeable effects or variations were seen with changing scanning resolution or with the film placed "up or down" during scanning. Other Gafchromic film types were tested and compared to EBT for unirradiated film to assess the magnitude of this orientation effect on the scanner used and results showed that EBT produced a significantly higher effect that MD-55-2, HS, XR type T and XR type R film by up to 3 times. As such, providing the same orientation of EBT film when scanning for dosimetric analysis becomes an essential part of EBT film dosimetry.

Does mechanical pressure on radiochromic film affect optical absorption and dosimetry?

EBT Gafchromic film, a new high sensitivity radiochromic film has been tested to evaluate if external pressure on the film can affect absorption spectra analysis and thus radiation dosimetry. This question arises from the fact that Gafchromic film is often cut into smaller pieces or to certain shapes for dosimetric analysis using scissors which can apply significant pressure to the sides of the film and small film pieces are placed within a solid phantom at depth which can produce significant pressure on the film if appropriate weight distribution procedures are not performed. As expected, results have shown that films cut by scissors can produce a large increase in OD near the film edge up to 5-10 mm away due to physical damage to the EBT film layers. Films placed within a solid phantom receiving up to 39.5 kg/cm2 pressure showed negligible differences in measured absorption spectra compared with control films subject to no external pressure. This equates to negligible external pressure effects for as much as 44 cm of 30 cm x 30 cm solid water placed on a 1 cm2 area film piece. As such, we recommend based on results herein, that film analysis should be performed with a boundary around every film edge, which can be defined visually based in the film. Also film dosimetry in a phantom can be performed with weights up to 39.5 kg/cm2 (or 44 cm of 30 cm x 30 cm solid water or equivalent) placed on the film without effecting the absorption spectra and thus dosimetry of radiation beams.

Measurement of magnetic fields produced by a "magnetic deflector" for the removal of electron contamination in radiotherapy.

Electron contamination generated from interactions of x-rays with components in a medical linear accelerator's head can increase damage to skin and subcutaneous tissue during radiotherapy through increased dose deposition. Skin and subcutaneous dose from high energy x-rays can be reduced using magnetic fields to sweep the electron contamination away from the radiation treatment field. This work is aimed at investigating the magnetic fields generated by an improved magnetic deflector which utilizes Nd2Fe14B magnets. Magnetic field strengths generated by the deflector have been simulated using Vizimag 3.0 magnetic modelling software. The improved deflector has a more uniform magnetic field strength than its predecessor and is optimised to easily fit on a clinical linear accelerator. Experimental measurements of the magnetic field strengths produced have also been performed for comparison. Results show a relatively good match to Vizimag modelling in the central regions of the deflector. Reductions of skin and subcutaneous dose up to 34% of original values were seen for a 20 x 20 cm2 field at 6MV x-ray energy.

Absorption spectra variations of EBT radiochromic film from radiation exposure.

Gafchromic EBT radiochromic film is one of the newest radiation-induced auto-developing x-ray analysis films available for therapeutic radiation dosimetry in radiotherapy applications. The spectral absorption properties in the visible wavelengths have been investigated and results show two main peaks in absorption located at 636 nm and 585 nm. These absorption peaks are different to many other radiochromic film products such as Gafchromic MD-55 and HS film where two peaks were located at 676 nm and 617 nm respectively. The general shape of the absorption spectra is similar to older designs. A much higher sensitivity is found at high-energy x-rays with an average 0.6 OD per Gy variation in OD seen within the first Gy measured at 636 nm using 6 MV x-rays. This is compared to approximately 0.09 OD units for the first Gy at the 676 nm absorption peak for HS film at 6 MV x-ray energy. The film's blue colour is visually different from older varieties of Gafchromic film with a higher intensity of mid-range blue within the film. The film provides adequate relative absorbed dose measurement for clinical radiotherapy x-ray assessment in the 1-2 Gy dose range which with further investigation may be useful for fractionated radiotherapy dose assessment.

Assessment of large single-fraction, low-energy X-ray dose with radiochromic film.

PURPOSE: To investigate the accuracy of in vivo dosimetry using radiochromic film for large single-fraction, low-energy irradiations. METHODS AND MATERIALS: Gafchromic MD-55-2 radiochromic film and LiF thermoluminescent dosimeters (TLDs) were placed in vivo on 25 patients to ascertain their effectiveness for assessment of dose. All patients received 10 Gy single fractions at energies ranging from 100 kVp (half-value layer [HVL] = 3.5 mm Al) up to 250 kVp (HVL = 2.3 mm Cu). Effects of small air gaps were also investigated using LiF TLDs and radiochromic film. RESULTS: Radiochromic film adequately measured applied dose for 25 patients in vivo with a standard deviation of 5.5% from prescribed dose. LiF TLDs recorded a standard deviation of 4.1% from measured to applied dose. Small air gaps which can be created under the film or TLDs during in vivo dosimetry were shown to have a measurable but minimal effect on results for gaps less than 5 mm. CONCLUSIONS: Gafchromic film has adequately measured applied dose in vivo at low energy for large 10 Gy single-fraction irradiation.