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 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.

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.

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.

Prostate dosimetry in an anthropomorphic phantom

Four field prostate treatments are a standard treatment procedure in radiotherapy. Dose in the prostate and rectum region were calculated for 6MV and 18MV photon beams on an anthropomorphic phantom with a collapsed cone convolution method using a 3-D planning system. Validation has been performed with radiographic film and thermoluminescent dosimeters. Results have shown that the pinnacle planning system has accurately modelled doses delivered to a heterogeneous phantom with calculations and measurements agreeing within +/-3% over most areas. When treating clinically, considerations such as the volume of bowel gas should be taken into account when planning. A sample of patient CT scans showed that in the absence of a heterogeneity correction, the error in estimated dose through the rectum could be as high as 8% in the presence of large volumes of rectal gas. Considerations, such as whether the patient undergoes another CT scan, the bowel gas volume ignored or assigned a specific density needs to be taken into account and brought to the attention of the radiation oncologists for accurate treatment.

Variations in skin dose using 6MV or 18MV x-ray beams

This research aimed to quantitatively evaluate the differences in percentage dose of maximum for 6MV and 18MV x-ray beams within the first 1 cm of interactions. Thus provide quantitative information regarding the basal, dermal and subcutaneous dose differences achievable with these two types of high-energy x-ray beams. Percentage dose of maximum build up curves are measured for most clinical field sizes using 6MV and 18MV x-ray beams. Calculations are performed to produce quantitative results highlighting the percentage dose of maximum differences delivered to various depths within the skin and subcutaneous tissue region by these two beams. Results have shown that basal cell layer doses are not significantly different for 6MV and 18MV x-ray beams. At depths beyond the surface and basal cell layer there is a measurable and significant difference in delivered dose. This variation increases to 20% of maximum and 22% of maximum at 1 mm and 1 cm depths respectively. The percentage variations are larger for smaller field sizes where the photon in phantom component of the delivered dose is the most significant contributor to dose. By producing graphs or tables of % dose differences in the build up region we can provide quantitative information to the oncologist for consideration (if skin and subcutaneous tissue doses are of importance) during the beam energy selection process for treatment.

Polarity effect on surface dose measurement for an attix parallel plate ionisation chamber

The effects of chamber polarity have been investigated for the measurement of 6MV and 18MV x-ray surface dose using a parallel plate ionization chamber. Results have shown that a significant difference in measured ionization is recorded between polarities at 6MV and 18MV at the phantom surface. A polarity ratio ranging from 1.062 to 1.005 is seen for 6MV x-rays at the phantom surface for field sizes 5 cm x 5 cm to 40 cm x 40 cm when comparing positive to negative polarity. These ratio's range from 1.024 to 1.004 for 18MV x-rays with the same field sizes. When these charge reading are compared to the Dmax readings of the same polarity it is found that these polarity effects are minimal for the calculation of percentage dose results with variations being less than 1% of maximum.

MOSFET dosimetry in-vivo at superficial and orthovoltage x-ray energies

This note investigates in-vivo dosimetry using a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) for radiotherapy treatment at superficial and orthovoltage x-ray energies. This was performed within one fraction of the patients treatment. Standard measurements along with energy response of the detector are given. Results showed that the MOSFET measurements in-vivo agreed with calculated results on average within +/- 5.6% over all superficial and orthovoltage energies. These variations were slightly larger than TLD results with variations between measured and calculated results being +/- 5.0% for the same patient measurements. The MOSFET device provides adequate in-vivo dosimetry for superficial and orthovoltage energy treatments with the accuracy of the measurements seeming to be relatively on par with TLD in our case. The MOSFET does have the advantage of returning a relatively immediate dosimetric result after irradiation.

Variations in 6MV x-ray radiotherapy build-up dose with treatment distance

Dose in the build up region for high energy x-rays produced by a medical linear accelerator is affected by the x-ray source to patient surface distance (SSD). The use of isocentric treatments whereby the tumour is positions 100 cm from the source means that depending of the depth of the tumour and the size of the patient, the SSD can vary from distances of 80 cm to 100 cm. To achieve larger field sizes, the SSD can also be extended out to 120 cm at times. Results have shown that open fields are not significantly affected by SSD changes with deviations in percentage dose being less than 4% of maximum dose for SSD's from 80 cm to 120 cm SSD. With the introduction of beam modifying devices such as Perspex blocking trays, the effects are significant with a deviation of up to 22% measured at 6MV energy with a 6 mm Perspex tray for SSD's from 80 cm to 120 cm. These variations are largest at the skin surface and reduce with depth. The use of a multi leaf collimator for blocking removes extra skin dose caused by the Perspex block trays with decreasing SSD.

Spatial resolution of a stacked radiochromic film dosimeter

PURPOSE

The spatial resolution of stacked radiochromic film dosimeters, which have increased sensitivity from a single layer radiochromic film detector, has been studied.

METHODS

A 5-layer film which can easily be constructed provided a 4.3-times increase in sensitivity over a single layer film at 670 nm readout wavelength which meant that doses as low as 0.6 Gy could be measured with an accuracy of +/-4% with the stacked dosimeter. The spatial resolution was tested by comparison of the 80%/20% penumbral widths of a 5 x 5 cm 6 MV X-ray field.

RESULTS

The MD-55-2 film measured the penumbral width as 3.0 mm whereas the 5-layer stack dosimeter measured the same penumbra as 3.2 mm.

CONCLUSION

The stack dosimeter can provide useful in vivo information such as the position of a diverging beam edge for treatments around critical structures such as eyes during the first fraction of treatment.

Visible dye light absorption properties of processed radiographic film

The visible absorption spectra of Kodak X-Omat V film, which had been exposed to various doses of radiation, have been investigated to analyse the dosimetry characteristics of the film with various densitometers. Common densitometers can use fluorescent light (broad band visible), helium-neon (632 nm) or other spectra of specific bandwidth. The visible absorption spectra show a slight peak in absorption at approximately 580 nm and another at 630 nm caused by the base material of the film. The optical density of the film is shown to increase almost equally at all wavelengths within the visible region with increases in applied dose. By evaluating the results for the broad band spectra and specific wavelength optical density it is shown that a relatively uniform response is expected for all densitometers that work within the visible region as well as in selected infrared wavelengths. Thus similar optical density to dose response curves for X-Omat V radiographic film should be produced for all types of densitometers, no matter what type of light source is used for illumination. Thus it is most efficient to have a densitometer with a light source suitable for radiochromic film, which can also be used with radiographic film.

Radiochromic film dosimetry in water phantoms

Radiochromic film is investigated for use in dosimetry in water phantoms as opposed to solid phantoms. Investigations are performed to measure the penetration rates of water into radiochromic film and to assess the effects on optical density that this penetration causes. The effects of film orientation during irradiation in water are also tested. Results show that only a small penetration rate is seen from water into the film which only affects the outer areas of the film, with penetration being less than 0.5 mm per hour. The optical density measurements of the film at 660 nm remain unchanged in the unaffected regions of the radiochromic film. Minimal effects are seen due to beam orientation in a water phantom as opposed to solid water phantoms in which an overestimation in dose is normally seen for parallel irradiation. Radiochromic film seems to be an adequate detector for dosimetry in a water phantom where high spatial resolution is needed and angle of beam incidence at the point of interest is important.

Verification of lung dose in an anthropomorphic phantom calculated by the collapsed cone convolution method

Verification of calculated lung dose in an anthropomorphic phantom is performed using two dosimetry media. Dosimetry is complicated by factors such as variations in density at slice interfaces and appropriate position on CT scanning slice to accommodate these factors. Dose in lung for a 6 MV and 10 MV anterior-posterior field was calculated with a collapsed cone convolution method using an ADAC Pinnacle, 3D planning system. Up to 5% variations between doses calculated at the centre and near the edge of the 2 cm phantom slice positioned at the beam central axis were seen, due to the composition of each phantom slice. Validation of dose was performed with LiF thermoluminescent dosimeters (TLDs) and X-Omat V radiographic film. Both dosimetry media produced dose results which agreed closely with calculated results nearest their physical positioning in the phantom. The collapsed cone convolution method accurately calculates dose within inhomogeneous lung regions at 6 MV and 10 MV x-ray energy.

Blood irradiation with accelerator produced electron beams

Blood and blood products are irradiated with gamma rays to reduce the risk of graft versus host disease (GVHD). A simple technique using electron beams produced by a medical linear accelerator has been studied to evaluate irradiation of blood and blood products. Variations in applied doses for a single field 20 MeV electron beam are measured in a phantom study. Doses have been verified with ionization chambers and commercial diode detectors. Results show that the blood product volume can be given a relatively homogeneous dose to within 6% using 20 MeV electrons without the need to rotate the blood bags or the beam entry point. The irradiation process takes approximately 6.5 minutes for 30 Gy applied dose to complete as opposed to 12 minutes for a dual field x-ray field irradiation at our centre. Electron beams can be used to satisfactorily irradiate blood and blood products in a minimal amount of time.

Measurement of radiotherapy x-ray skin dose on a chest wall phantom

Sufficient skin dose needs to be delivered by a radiotherapy chest wall treatment regimen to ensure the probability of a near surface tumor recurrence is minimized. To simulate a chest wall treatment a hemicylindrical solid water phantom of 7.5 cm radius was irradiated with 6 MV x-rays using 20x20 cm2 and 10x20 cm2 fields at 100 cm source surface distance (SSD) to the base of the phantom. A surface dose profile was obtained from 0 to 180 degrees, in 10 degrees increments around the circumference of the phantom. Dosimetry results obtained from radiochromic film (effective depth of 0.17 mm) were used in the investigation, the superficial doses were found to be 28% (of Dmax) at the 0 degrees beam entry position and 58% at the 90 degrees oblique beam position. Superficial dose results were also obtained using extra thin thermoluminescent dosimeters (TLD) (effective depth 0.14 mm) of 30% at 0 degrees, 57% at 90 degrees, and a metal oxide semiconductor field effect transistor (MOSFET) detector (effective depth 0.5 mm) of 43% at 0 degrees, 62% at 90 degrees. Because the differences in measured superficial doses were significant and beyond those related to experimental error, these differences are assumed to be mostly attributable to the effective depth of measurement of each detector. We numerically simulated a bolus on/bolus off technique and found we could increase the coverage to the skin. Using an alternate "bolus on," "bolus off" regimen, the skin would receive 36.8 Gy at 0 degrees incidence and 46.4 Gy at 90 degrees incidence for a prescribed midpoint dose of 50 Gy. From this work it is evident that, as the circumference of the phantom is traversed the SSD increases and hence there is an inverse square fluence fall-off, this is more than offset by the increase in skin dose due to surface curvature to a plateau at about 90 degrees. Beyond this angle it is assumed that beam attenuation through the phantom and inverse square fall-off is causing the surface dose to reduce.

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.

Comparison of effective source-surface distances for electron beams derived from measurements made under different scatter conditions

The purpose of this paper is to investigate the effect of scatter conditions around the chamber on the effective source-surface distance (SSDeff) calculation. Three setups were considered, viz with the chamber: in air, in solid water at the surface, and in solid water at dmax. Ionization measurements for electron beams from a Varian Clinac-210 degrees C were recorded from 100 cm to 120 cm nominal SSD in 5 cm increments, using a parallel-plate Markus chamber. Two electron energies, 6 and 12 MeV, were investigated with a range of electron applicators (Series-III, 10 x 10 to 25 x 25 cm2) and different cerrobend inserts (square and rectangular). The study was undertaken by placing the chamber in air and in solid water, respectively. SSDeff was calculated from the ionisation chamber measurements using the method of Khan (1984). The results are summarised in the table 1. In almost all cases the SSDeff calculated using the in-air data is less. This is likely to be due to a larger proportion of scatter off the applicators reaching the chamber when it is in air than when it is at dmax in the phantom. The results also show the difference is reduced for a larger applicator, probably because the applicator scatter component reaching the in-air measurement point is reduced.

Dosimetry of blood irradiation with radiochromic film

It has been shown that radiochromic film is an ideal dosimeter for assessment and verification of delivered dose to irradiated blood products. Using a parallel opposing two-field technique on a medical linear accelerator, blood is irradiated to diminish the risk of transfusion-associated graft vs. host disease (TA-GVHD). The blood products are irradiated in a Perspex blood box to an applied dose of 29.5-31.7 Gy. Verification of applied dose has been performed with thimble ionization chambers and radiochromic film. Radiochromic film results have matched absorbed dose measurements from ionization chambers at all sites within the 'active' treatment volume within +/-6% for a 95% confidence limit. Using a sample of 100 in-vitro measurements, radiochromic film has measured the average applied dose to blood products to be 30.95+/-2.6 Gy for two standard deviations. Like currently available 'irradiated' film labels, the radiochromic film also serves as a visible reminder that the blood products have been irradiated.

Measurement of off-axis and peripheral skin dose using radiochromic film

A radiotheraphy skin dose profile can be obtained with radiochromic film. The central axis skin dose relative to Dmax for a 10 x 10 cm2 field size was found to be 22%, 17% and 15.5% for 6 MV, 10 MV and 18 MV photon beams. Peripheral dose increased with increasing field size. At 10 MV the skin dose 2 cm outside the geometric field edge was measured as 6%, 10% and 17% for 10 x 10 cm2, 20 x 20 cm2 and 30 x 30 cm2 field sizes respectively. Off-axis skin dose decreased as distance increased from central axis for fields with Perspex block trays. For a 20 x 20 cm2 field, an approximately 5-8% drop in percentage skin dose was observed from central axis to the beam edge.

Effects of read-out light sources and ambient light on radiochromic film

Both read-out light sources and ambient light sources can produce a marked effect on coloration of radiochromic film. Fluorescent, helium neon laser, light emitting diode (LED) and incandescent read-out light sources produce an equivalent dose coloration of 660 cGy h(-1), 4.3 cGy h(-1), 1.7 cGy h(-1) and 2.6 cGy h(-1) respectively. Direct sunlight, fluorescent light and incandescent ambient light produce an equivalent dose coloration of 30 cGy h(-1), 18 cGy h(-1) and 0 cGy h(-1) respectively. Continuously on, fluorescent light sources should not be used for film optical density evaluation and minimal exposure to any light source will increase the accuracy of results.

Conversion of an infrared densitometer for radiochromic film analysis

By the simple incorporation of a high intensity red LED into a typical infrared film densitometer, radiochromic film can be analysed using existing detectors and scanning software. Results show an accurate dose measurement using radiochromic film and this system compared to conventional detectors for percentage depth dose and penumbral measurements in high and low energy x-ray beams. A small circuit including a red Light Emitting Diode (LED) was positioned inside the film densitometer which does not obscure the infrared source. The red and infrared diodes work independently. For 6 MV x-rays, the 80%/20% penumbral width at 15 mm depth for a 10 x 10 cm field at 100 cm SSD was measured to be 3.5 mm with radiochromic film as compared to 3.3 with corrected diode measurements. Percentage depth doses were measured to within +/- 3% of ionisation chamber data at 6MV and within +/- 2% for 250 kVp x-ray with the film placed parallel to the beam direction in both cases.

Skin dose reduction by a clinically viable magnetic deflector

A variable magnetic deflector which attaches onto the treatment head of a linear accelerator has reduced skin dose by as much as 65% for 6MV x-rays. The magnetic deflector is constructed from Neodymium Iron Boron (NdFeB) rare earth magnets. It weighs approximately 15 kg and is designed to easily fit onto the accessory mount of a clinical linear accelerator. All field sizes are attainable up to 35 cm x 35 cm at 100 cm SSD. The gap between the magnetic poles can be adjusted, providing the highest field strength for each field size. Magnetic field strengths up to 0.55 Tesla are attainable. For a 6MV x-ray beam with a 10 mm perspex block tray, surface dose is reduced from 29% to 14% and from 59% to 37% for a 20 cm x 20 cm and 35 cm x 35 cm field size, respectively. Results at varying SSD's have shown at least 10 cm of space must be allowed between the magnets and patient for adequate reduction of skin dose through removal of electron contaminants.

Skin dose from radiotherapy X-ray beams: the influence of energy

Skin-sparing properties of megavoltage photon beams are compromised by electron contamination. Higher energy beams do not necessarily produce lower surface and basal cell layer doses due to this electron contamination. For a 5 x 5 cm field size the surface doses for 6 MVp and 18 MVp X-ray beams are 10% and 7% of their respective maxima. However, at a field size of 40 x 40 cm the percentage surface dose is 42% for both 6 MVp and 18 MVp beams. The introduction of beam modifying devices such block trays can further reduce the skin-sparing advantages of high energy photon beams. Using a 10 mm perspex block tray, the surface doses for 6 MVp and 18 MVp beams with a 5 x 5 cm field size are 10% and 8%, respectively. At 40 x 40 cm, surface doses are 61% and 63% for 6 MVp and 18 MVp beams, respectively. This trend is followed at the basal cell layer depth. At a depth of 1 mm, 18 MVp beam doses are always at least 5% smaller than 6 MVp doses for the same depth at all field sizes when normalized to their respective Dmax values. Results have shown that higher energy photon beams produce a negligible reduction of the delivered dose to the basal cell layer (0.1 mm). Only a small increase in skin sparing is seen at the dermal layer (1 mm), which can be negated by the increased exit dose from an opposing field.

Effect of block trays on skin dose in radiotherapy

Percentage dose at the surface and at 1 mm depth for megavoltage photon beams are increased through the influence of block trays. This represents a decrease in skin sparing properties for both the epidermal and dermal layers. The increase in percentage dose varies with type and thickness of block tray material. At 6MVp, 20cm x 20cm field size, the percentage surface dose is 26%, 26.5%, 33% and 35% for open, steel honeycomb tray, 6mm perspex and 10 mm perspex block trays respectively. At 1 mm depth these values are 52%, 52.5%, 61%, 60% respectively. A similar effect is seen at higher energies. Results show that care should be taken when selecting an appropriate block tray if skin sparing is of importance.

An analytical representation of equivalent square field size in relation to surface dose

Analytical representation of the build up characteristics of megavoltage photon beams is achievable, producing accurate percentage dose results in the build up region for typical patient treatments. This approach requires conversion of an irregular shaped field to a equivalent square field for mathematical analysis. Surface dose for rectangular fields is expressed by an extension to Stirling's 4A/P formula while irregular shaped fields follow an area integration technique. The equivalent square for a typical irregular treatment field is modelled within +/- 0.5 cm2 for all measured fields.

6MV x-ray dose in the build up region: empirical model and the incident angle effect

A simple and fast empirical model has been developed which accurately predicts central axis surface and build up dose for a 6MV radiotherapy x-ray beam. The model is based on fits to experimental data and accounts for open fields, block trays and wedges at normal incidence and at angle. The model separates the beam into components produced by primary photon interactions which have only interacted in the phantom at normal and oblique incidence and head scattered photons/electrons generated in the treatment head. The model quantifies these components for open unwedged fields and then the effect on each component by introducing beam modifying devices/ accessories or changing the angle of incidence is determined. Dose results at oblique incidence for Monte Carlo (electron contamination free) and experimental (electron contamination present) are sufficiently close to imply that the increase in build up dose with beam angle is mainly due to changes in photon interactions within the phantom and only a slight increase with angle is due to changes in the electron contamination. Electron contamination/ head scatter component was found to be measurable by three methods. These being TLD extrapolation in air, ionisation chamber measurements in air and Monte Carlo pure photon methods. These methods produced comparable electron contamination/head scatter dose results at all field sizes.

Radiochromic film as a radiotherapy surface-dose detector

Radiochromic film is shown to be a useful surface-dose detector for radiotherapy x-ray beams. Central-axis percentage surface-dose results as measured by Gafchromic film for a 6 MVp x-ray beam produced by a Varian 2100C Linac at 100 cm SSD are 16%, 25%, 35%, 41% for 10, 20, 30 and 40 cm square field sizes, respectively. Using a simple, uniform light source and a CCD camera connected to an image analysis system, quantitative 3D surface doses are accurately attainable in real time as either numerical data, a black-and-white image or a colour-enhanced image.

Magnetic repulsion of linear accelerator contaminates

Neodymium Iron Boron (NdFeB) rare earth permanent magnets have unique properties that enable them to fit easily onto the accessory mount of a clinical linear accelerator to partially sweep away electron contamination produced by the treatment head and block trays and thus increase skin sparing. Using such magnets the central axis entrance surface dose has been reduced by 11% for a 20 x 30 cm field size from 32% to 21% of maximum dose by the magnetic device. A reduction of 14% from 32% to 18% was seen for a 20 x 20 cm field size with a 6 mm perspex block tray positioned above the magnet. The magnetic device is light weight and thus clinically usable.

A new radiotherapy surface dose detector:the MOSFET

Radiotherapy x-ray and electron beam surface doses are accurately measurable by use of a MOS-FET detector system. The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is approximately 200-microns in diameter and consists of a 0.5-microns Al electrode on top of a 1-microns SiO2 and 300-microns Si substrate. Results for % surface dose were within +/- 2% compared to the Attix chamber and within +/- 3% of TLD extrapolation results for normally incident beams. Detectors were compared using different energies, field size, and beam modifying devices such as block trays and wedges. Percentage surface dose for 10 x 10-cm and 40 x 40-cm field size for 6-MV x rays at 100-cm SSD using the MOSFET were 16% and 42% of maximum, respectively. Factors such as its small size, immediate retrieval of results, high accuracy attainable from low applied doses, and as the MOSFET records its dose history make it a suitable in vivo dosimeter where surface and skin doses need to be determined. This can be achieved within part of the first fraction of dose (i.e., only 10 cGy is required.)

Dose characteristics of a new 300kVp orthovoltage machine

The dose characteristics of a relatively new type of orthovoltage machine which displays different beam qualities compared to other orthovoltage machines has been studied. Surface dose, depth dose and dose profiles have been measured with various ionization chambers as well as diodes and thermoluminescent dosemeters. Profiles produced by the 300kVp x-ray machine's fixed and variable collimators show no significant differences. A slight depth dose build up effect of 2% is seen over the first 1mm with the variable collimator at 250kVp but this is not seen with the fixed collimators. Surface charge is increased to 110%, normalised to 100% at 0.3mm depth in solid water when the front perspex plate of the 10x10cm 50cm FSD fixed collimator is removed. The use of a magnetic field placed directly under the fixed collimator to sweep away any electrons produced has shown that the extra charge and thus dose is caused mainly by electrons produced from the fixed collimator material. The percentage surface charge was reduced from 110% to 102% at 50cm FSD with the magnetic field energised.

Surface doses from combined electron/photon fields in a radiotherapy

Using mixed modality treatments of photon and electron beams, some skin sparing can be acquired whilst administering a safer dose to crucial structures such as the spine or lung. The combination of 6MV X-rays and 12MeV electron beams in the treatment of breast nodes is a clinical example where such treatments are beneficial. By weighting the photon and electron beams accordingly, the surface dose and dose at depth can be changed whilst not dramatically varying the depth at which the 90% dose level is maintained. In order to accurately predict near surface dose, build up results were obtained using TLD extrapolation, Markus parallel plate and Attix parallel plate ionisation chambers in a solid water phantom. This data was then used to predict surface dose due to different beam weights. Depending on the weightings given to the photon and electron beams, the surface dose and dose at depth varies. For example, when 6MV X-rays and 12MeV electrons are combined the percentage dose at surface and 20cm depth is 46%/23%, 54%/20%, 61%/15% for 60/40, 50/50 and 40/60 X-ray/electron weightings respectively. For these weightings, the depth of the 90% level remained at 30mm. From a clinical point of view this data is important, showing that the 90% level of radiation does not vary in depth significantly provided the ratio of photon/electron weights is kept within a range of 60/40 to 40/60. However by varying the weightings, the ability to control dose to skin in particular to produce the optimum level for both areas whilst still delivering the required tumour dose is obtained.