Effect of scanner lens on lateral response artefact in radiochromic film dosimetry.

Radiochromic film is a good dosimeter choice for patient QA for complex treatment techniques because of its near tissue equivalency, high spatial resolution and established method of use. Commercial scanners are typically used for film dosimetry, with Epson scanners being the most common. Radiochromic film dosimetry is not straightforward having some well-defined problems which must be considered, one of the main ones being the Lateral Response Artefact (LRA) effect. Previous studies showed that the contributing factors to LRA are from the structure of the active ingredients of the film and the components and construction of the flatbed scanner. This study investigated the effect of the scanner lens on the LRA effect, as part of a wider investigation of scanner design effects and uncertainties. Gafchromic EBT3 films were irradiated with 40 × 40 cm2 field size 6 MV beams. Films were analysed using images captured by a Canon 7D camera utilising 18 mm, 50 mm and 100 mm focal length lenses compared to images scanned with a conventional Epson V700 scanner. The magnitude of the LRA was observed to be dependent on the focal length of the lens used to image the film. A substantial reduction in LRA was seen with the use of the 50 mm and 100 mm lenses, by factors of 3-5 for the 50 mm lens and 4-30 for the 100 mm lens compared to conventional desktop scanner techniques. This is expected to be from the longer focal length camera lens system being able to collect more light from distant areas compared to the scanner-based system. This provides an opportunity to design film dosimetry systems that minimise this artefact.

A novel extrapolation method using OSL detectors for very small field output factor measurement for stereotactic radiosurgery.

Appropriate methods for the determination of very small X-ray beam output factors are essential to ensure correct clinical outcomes for stereotactic radiosurgery. To date, substantial work has been performed in identifying and quantifying suitable dosimeters for relative output factor (ROF) measurements including recent IAEA published recommendations. In this work, we provide a novel method using optically stimulated luminescent dosimeters (OSLDs) with different effective sizes of the readout area to determine ROFs. This involves applying an extrapolation technique to assess ROFs for 6MV SRS X-ray beams with field diameters ranging from 4 to 30 mm as defined by the Brainlab SRS cones. By combining the use of multiple sized OSLDs and water droplets to remove air gaps located around the OSLD detectors, both volume averaging and density variation effects were minimised to estimate ROFs for an extrapolated zero volume detector. The measured results showed that for a 4 mm diameter cone, the ROF was 0.660 ± 0.032 (2SD) as compared to 0.661 ± 0.01 and 0.651 ± 0.018 for the PTW 600019 microDiamond detector and Gafchromic EBT3 film respectively. Whilst the uncertainties were larger than conventional detectors, the technique shows promise and improvements in accuracy may be obtained by higher quality manufacturing techniques. Based on these results, using OSLDs with different effective sizes of readout area and an extrapolation technique shows promise for use as an independent verification tool for very small X-ray field ROFs in the clinical department.

Monte Carlo calculated output correction factors for Gafchromic EBT3 film for relative dosimetry in small stereotactic radiosurgery fields.

To calculate small field output correction factors, [Formula: see text], for Gafchromic EBT3 film using Monte Carlo simulations. These factors were determined for a Novalis Trilogy linear accelerator equipped with Brainlab circular cones with diameters of 4.0 to 30.0 mm. The BEAMnrc Monte Carlo code was used to simulate the Novalis Trilogy linear accelerator and the Brainlab cones with diameters 4.0 to 30 mm. The DOSXYZnrc code was used to simulate Gafchromic EBT3 film with the atomic composition specified by the manufacturer. Small field correction factors were calculated according to new IAEA TRS-483 Code of Practice for small field dosimetry. The depth of calculation was 10 cm and a source to surface distance of 100 cm. The X-ray beam used in the simulations was a 6 MV SRS. The correction factors were then used to determine field output factors with Gafchromic EBT3 film. These field output factors were validated using three solid state detectors and applying correction factors from the TRS-483 Code of Practice. The solid state detectors were IBA SFD diode, PTW 60018 SRS diode and PTW 60019 microDiamond. The Monte Carlo calculated output correction factors, [Formula: see text], for Gafchromic EBT3 film ranged between 0.998 to 1.004 for Brainlab circular cones with diameters between 4.0 and 30.0 mm. The uncertainty for these factors was 2.0%. The field output factors obtained with Gafchromic EBT3 film were within 2% of the mean results obtained with the three solid state detectors. For field sizes 4 mm diameter and above, Gafchromic EBT3 film has field output correction factors within 1% of unity. Therefore, Gafchromic EBT3 film can be considered to be correction less and supports the assumption made about this film in the TRS-483 Code of Practice.

A comparison between EPSON V700 and EPSON V800 scanners for film dosimetry.

Radiochromic film is a good dosimeter choice for patient QA for complex treatment techniques (IMRT, VMAT, SABR, SBRT) because of its near tissue equivalency, very high spatial resolution and established method of use. Commercial scanners are usually used for film dosimetry, among which EPSON scanners are the most common. NCCI have used an EPSON V700 scanner, but recently acquired a new model EPSON V800 scanner. The purpose of this work was to evaluate any differences between these two scanners to consider whether they can be used interchangeably or not. Different aspects of film dosimetry, e.g. lateral response artefact (LRA) effect, orientation effect, scanner response etc., were compared. EBT3 films were irradiated with 40 × 40 cm2 field size 6 MV beams and scanned in both the scanners. The scanned images were read in ImageJ V1.49 software. The data obtained was then copied in MS Excel to compare the scanners. The V800 scanner causes more polarisation, which results in more LRA effect than for the V700 scanner. The responses of the scanners in all three colour channels are not the same for the same film and irradiation. The V800 scanner shows an increase of response of up to 1.6% compared to 3.7% increase in the V700 scanner after scanning a piece of irradiated film 20 times. The scanners cannot be used interchangeably. The correction factors for LRA effect and the calibration curves are different. Further characterisation, evaluation and commissioning is required before clinical use.

Skin and build up dose determination for a 2.5 MV medical linear accelerator imaging beam.

The 2.5 MV Imaging beam produced by a Varian TrueBeam linear accelerator produces a dose build up effect at the beam entrance similar to other high energy photon beams. The surface dose values were found to range from 39% of maximum dose at a 5 cm × 5 cm field size up to 69% of maximum at a 40 cm × 40 cm field. The depth of maximum dose deposition was found to range from 5 mm at smaller field sizes to 4 mm at larger field sizes. Whilst large absorbed doses will not be delivered utilizing these beams, the data provided will allow the medical physics community to assess and estimate doses to patient's skin and subcutaneous tissue from low energy MV imaging beams.

Characterization of a novel scale maille contralateral breast shield: SMART Armor.

During breast radiotherapy treatment, the contralateral breast receives radiation doses to the skin and subcutaneous tissue caused mainly from incident electron contamination and low energy photon scatter radiation. Measurements have shown that for a typical hybrid tangential treatment, these dose levels can be up to 17% of maximum applied prescription dose if no shielding is used during the treatment process. This work examined the use of different shielding metals, aluminum, copper, and lead to reduce peripheral radiation dose to evaluate the optimal metal to form the basis of a contralateral breast radiation shield. This work also shows a simple but novel method to substantially reduce this unwanted radiation dose with the use of a copper scale maille sheet which can be easily and accurately draped over a patient's contralateral breast during treatment. The copper scale maille is flexible and can thus conform around typical breast shapes. It can also form irregular shaped edges to match those outlined by typical tangential treatment fields. As the shield is made from copper, it is nontoxic and can potentially be used directly on patients for treatment. The designed copper scale maille has shown to reduce contralateral breast skin and subcutaneous dose by up to 80% for typical radiation fields used in breast radiotherapy.

Practical time considerations for optically stimulated luminescent dosimetry (OSLD) in total body irradiation.

Total body irradiation (TBI) treatments are used to treat the whole body in preparation for hematopoietic stem cell (or bone marrow) transplantation. Our standard clinical regimen is a 12 Gy in 6 fraction, bi-daily technique using 6 MV X-rays at an extended Source-to-Surface distance (SSD) of 300 cm. Utilizing these characteristics, the beam dose rate is reduced below 7 cGy/min as is standard for TBI treatment. Dose received by the patient is monitored using optically stimulated luminescent dosimetry (OSLD). This work presents some practical calibration corrections based on time-dependant factors for OSLD calibration related to TBI procedure. Results have shown that a negligible difference is seen in OSL sensitivity for 6 MV X-rays irradiated in standard SSD (100 cm) and high dose rate (600 cGy/min) conditions compared to extended SSD (300 cm) and low TBI dose rate (6 cGy/min) conditions. Results have also shown that whilst short term signal fading occurs in the OSL after irradiation at a high dose rate (37% reduction in signal in the first 15 min), thereafter, negligible differences are seen in the OSL signal between 600 and 7 cGy/min irradiations. Thus a direct comparison can be made between calibration OSLs and clinical TBI OSLs between 15 min and 2 h. Finally a table is presented to provide corrections between calibration OSL readout and clinical TBI dose readout for a period up to 7 days. Combining these three results allows users to pre-irradiate their calibration OSLs at standard dose rate and SSD, up to 1 week prior to clinical treatment, and still provide accurate in-vivo dosimetry. This can help with time saving and work efficiency in the clinic.

An experimental extrapolation technique using the Gafchromic EBT3 film for relative output factor measurements in small x-ray fields.

PURPOSE: An experimental extrapolation technique is presented, which can be used to determine the relative output factors for very small x-ray fields using the Gafchromic EBT3 film. METHODS: Relative output factors were measured for the Brainlab SRS cones ranging in diameters from 4 to 30 mm(2) on a Novalis Trilogy linear accelerator with 6 MV SRS x-rays. The relative output factor was determined from an experimental reducing circular region of interest (ROI) extrapolation technique developed to remove the effects of volume averaging. This was achieved by scanning the EBT3 film measurements with a high scanning resolution of 1200 dpi. From the high resolution scans, the size of the circular regions of interest was varied to produce a plot of relative output factors versus area of analysis. The plot was then extrapolated to zero to determine the relative output factor corresponding to zero volume. RESULTS: Results have shown that for a 4 mm field size, the extrapolated relative output factor was measured as a value of 0.651 ± 0.018 as compared to 0.639 ± 0.019 and 0.633 ± 0.021 for 0.5 and 1.0 mm diameter of analysis values, respectively. This showed a change in the relative output factors of 1.8% and 2.8% at these comparative regions of interest sizes. In comparison, the 25 mm cone had negligible differences in the measured output factor between zero extrapolation, 0.5 and 1.0 mm diameter ROIs, respectively. CONCLUSIONS: This work shows that for very small fields such as 4.0 mm cone sizes, a measureable difference can be seen in the relative output factor based on the circular ROI and the size of the area of analysis using radiochromic film dosimetry. The authors recommend to scan the Gafchromic EBT3 film at a resolution of 1200 dpi for cone sizes less than 7.5 mm and to utilize an extrapolation technique for the output factor measurements of very small field dosimetry.

Build-up material requirements in clinical dosimetry during total body irradiation treatments.

Total body irradiation (TBI) treatments are mainly used in a preparative regimen for hematopoietic stem cell (or bone marrow) transplantation. Our standard clinical regimen is a 12 Gy/6 fraction bi-daily technique using 6MV X-rays at a large extended source to surface distance (SSD). This work investigates and quantifies the dose build-up characteristics and thus the requirements for bolus used for in vivo dosimetry for TBI applications. Percentage dose build-up characteristics of photon beams have been investigated at large extended SSDs using ionization chambers and Gafchromic film. Open field measurements at different field sizes and with differing scatter conditions such as the introduction of standard Perspex scattering plates at different distances to the measurement point were made in an effort to determine the required bolus/build-up material required for accurate determination of applied dose. Percentage surface dose values measured for open fields at 300 cm SSD were found to range from 20% up to 65.5% for fields 5 cm × 5 cm to 40 cm × 40 cm, respectively. With the introduction of 1 cm Perspex scattering plates used in TBI treatments, the surface dose values increased up to 83-90% (93-97% at 1 mm depth), depending on the position of the Perspex scattering plate compared to the measurement point. Our work showed that at least 5 mm water equivalent bolus/scatter material should be placed over the EBT3 film for accurate dose assessment for TBI treatments. Results also show that a small but measurable decrease in measured dose occurred with 5 mm water equivalent thick bolus material of areas '3 cm(2). As such, we recommend that 3 cm × 3 cm × 5 mm bolus build-up is the smallest size that should be placed over EBT3 Gafchromic film when used for accurate in vivo dosimetry for TBI applications.

Reducing shield thickness and backscattered radiation using a multilayered shield for 6­10 MeV electron beams.

Intraoral and external electron shields used in radiotherapy are designed to minimize radiation exposure to non-treatment tissue. Sites where shields are used include but are not limited to, the treatment of lips, cheeks and ears whilst shielding the underlying oral cavity, tongue, gingival or temporal region. A commonly known and published effect, concerns the enhancement in dose that can occur on the beam side on an electron shield caused by an increase in electron backscatter radiation. In this work a lead shield has been designed incorporating copper, aluminium and wax in a step down filter arrangement to minimise backscatter whilst minimizing overall shield thickness for better clinical setup and ease of use. For electron beams ranging from 6 to 10 MeV, a standard shield design of 4 mm lead, 0.6 mm copper, 1.0 mm aluminium and 1.5 mm wax (3.1 mm added filtration, 7.1 mm total thickness) provided adequate backscatter and transmission reduction to match a standard 4.5 mm lead and 10 mm wax (total thickness 14.5 mm) electron shield. Dose enhancement values of no more than 10 % were measured utilising this shield design with a 50 % reduction in shield thickness. The thinner shield will not only allow easier patient set up but should be tolerated better by patients when mucosal reactions occur as they place less physical pressure on these sites during treatment due to their smaller size.

Solar ultraviolet radiation response of EBT2 Gafchromic, radiochromic film.

Measurement of solar ultraviolet (UV) radiation is an important aspect of dosimetry for the improved knowledge of UV exposure and its associated health related issues. EBT2 Gafchromic film has been designed by its manufacturers as an improved tool for ionizing radiation dosimetry. The film is stated as exhibiting a significant reduction in UV response. However, results have shown that when exposed to UV from the 'bottom side' i.e. from the thick laminate side, the film exhibits a sensitivity to solar UV radiation which is both measurable and accurate for UV dosimetry. Films were irradiated in this position to known solar UV exposures and results are quantified showing a reproducibility of measurement to within ±7% (1 SD) when compared to calibrated UV meters. With an exposure of 20 J cm(-2) broad spectrum solar UV, the films net OD change was found to be 0.248 OD ± 0.021 OD when analysing the results using the red channel region of an Epson V700 desktop scanner. This was compared to 0.0294 OD ± 0.0053 OD change with exposure to the same UV exposure from the top side. This means that solar UV dosimetry can be performed using EBT2 Gafchromic film utilizing the underside of the film for dosimetry. The main advantages of this film type for measurement of UV exposure is the visible colour change and thus easy analysis using a desktop scanner as well as its uniformity in response and its robust physical strength for use in outside exposure situations.

Measurement of effects of nasal and facial shields on delivered radiation dose for superficial x-ray treatments.

Kilovoltage x-ray beams are used for the treatment of facial cancers when located on the patient's skin or subcutaneous tissue. This is of course due to the sharp depth dose characteristics of these beams delivering much lower doses at depth, than high energy x-ray beams. When treatment is performed, lead shields are often used within the nasal passage, or behind the lips and ears. These shields affect the backscattering patterns of the x-ray beams producing perturbations to upstream dose thus reducing delivered dose to the tumour. Experimental results using radiochromic films have shown that up to 10.5% ± 1.9% reduction in tumour dose can occur for field sizes less than 5 cm circle diameter for x-ray beams of 50 to 150 kVp. These results were confirmed using EGSnrc Monte Carlo techniques. Clinically more than 70% of treatments used fields of diameters less than 3 cm where the reductions were up to 6% ± 1.3%. Using a 1 cm diameter field, which can be used for skin cancer treatment on the nose, reductions up to 2.5% ± 1.3% were seen. Thus corrections need to be applied for dose calculations when underlying lead shields are used clinically in kilovoltage x-rays. The size of the reduction was also found to be dependent on the depth of the shield which will normally clinically vary from approximately 0.5 cm for nasal shields or behind eye lobes and up to approximately 1 cm for lips or cheek areas. We recommend that clinics utilize data for corrections to delivered dose in kilovoltage x-ray beams when lead shields are used in nasal passages, behind lips or behind ears for dose reduction. This can be easily and accurately measured with EBT2 Gafchromic film.

Variations in daily quality assurance dosimetry from device levelling, feet position and backscatter material.

Daily quality assurance procedures are an essential part of radiotherapy medical physics. Devices such as the Sun Nuclear, DQA3 are effective tools for analysis of daily dosimetry including flatness, symmetry, energy, field size and central axis radiation dose measurement. The DQA3 can be used on the treatment couch of the linear accelerator or on a dedicated table/bed for superficial and orthovoltage x-ray machines. This device is levelled using its dedicated feet. This work has shown that depending on the quantity of backscatter material behind the DQA3 device, the position of the levelling feet can affect the measured central axis dose by up to 1.8 % (250 kVp and 6 MV) and that the introduction of more backscatter material behind the DQA3 can lead to up to 7.2 % (6 MV) variations in measured central axis dose. In conditions where no backscatter material is present, dose measurements can vary up to 1 %. As such this work has highlighted the need to keep the material behind the DQA3 device constant as well as maintaining the accuracy of the feet position on the device to effectively measure the most accurate daily constancy achievable. Results have also shown that variations in symmetry and energy calculations of up to 1 % can occur if the device is not levelled appropriately. As such, we recommend the position of the levelling feet on the device be as close as possible to the device so that a constant distance is kept between the DQA3 and the treatment couch and thus minimal levelling variations also occur. We would also recommend having no extra backscattering material behind the DQA3 device during use to minimise any variations which might occur from these backscattering effects.

Comparison of Epson scanner quality for radiochromic film evaluation.

Epson Desktop scanners have been quoted as devices which match the characteristics required for the evaluation of radiation dose exposure by radiochromic films. Specifically, models such as the 10000XL have been used successfully for image analysis and are recommended by ISP for dosimetry purposes. This note investigates and compares the scanner characteristics of three Epson desktop scanner models including the Epson 10000XL, V700, and V330. Both of the latter are substantially cheaper models capable of A4 scanning. As the price variation between the V330 and the 10000XL is 20-fold (based on Australian recommended retail price), cost savings by using the cheaper scanners may be warranted based on results. By a direct comparison of scanner uniformity and reproducibility we can evaluate the accuracy of these scanners for radiochromic film dosimetry. Results have shown that all three scanners can produce adequate scanner uniformity and reproducibility, with the inexpensive V330 producing a standard deviation variation across its landscape direction of 0.7% and 1.2% in the portrait direction (reflection mode). This is compared to the V700 in reflection mode of 0.25% and 0.5% for landscape and portrait directions, respectively, and 0.5% and 0.8% for the 10000XL. In transmission mode, the V700 is comparable in reproducibility to the 10000XL for portrait and landscape mode, whilst the V330 is only capable of scanning in the landscape direction and produces a standard deviation in this direction of 1.0% compared to 0.6% (V700) and 0.25% (10000XL). Results have shown that the V700 and 10000XL are comparable scanners in quality and accuracy with the 10000XL obviously capable of imaging over an A3 area as opposed to an A4 area for the V700. The V330 scanner produced slightly lower accuracy and quality with uncertainties approximately twice as much as the other scanners. However, the results show that the V330 is still an adequate scanner and could be used for radiation dosimetry purposes. As such, if budgetary requirements are limited, the V700 scanner would be the recommended option at a price eight times cheaper than the 10000XL; however, the V330 produces adequate results at a price which is 2.5 times cheaper again. This may be a consideration for smaller institutions or individuals working with radiochromic film dosimetry.

Measurement and effects of MOSKIN detectors on skin dose during high energy radiotherapy treatment.

During in vivo dosimetry for megavoltage X-ray beams, detectors such as diodes, Thermo luminescent dosimeters (TLD's) and MOSFET devices are placed on the patient's skin. This of course will affect the skin dose delivered during that fraction of the treatment. Whilst the overall impact on increasing skin dose would be minimal, little has been quantified concerning the level of increase in absorbed dose, in vivo dosimeters produce when placed in the beams path. To this extent, measurements have been made and analysis performed on dose changes caused by MOSKIN, MOSFET, skin dose detectors. Maximum increases in skin dose were measured as 15 % for 6 MV X-rays and 10 % for 10 MV X-rays at the active crystal of the MOSKIN device which is the thickest part of the detector. This is compared to 32 and 26 % for a standard 1 mm thick LiF TLD at 10 × 10 cm(2) field size for 6 and 10 MV X-rays respectively. Radiochromic film, EBT2 has been shown to provide a high resolution 2 dimensional map of skin dose from these detectors and measures the effects of in vivo dosimeters used for radiotherapy dose assessment.

Scanner uniformity improvements for radiochromic film analysis with matt reflectance backing.

A simple and reproducible method for increasing desktop scanner uniformity for the analysis of radiochromic films is presented. Scanner uniformity, especially in the non-scan direction, for transmission scanning is well known to be problematic for radiochromic film analysis and normally corrections need to be applied. These corrections are dependant on scanner coordinates and dose level applied which complicates dosimetry procedures. This study has highlighted that using reflectance scanning in combination with a matt, white backing material instead of the conventional gloss scanner finish, substantial increases in the scanner uniformity can be achieved within 90% of the scanning area. Uniformity within ±1% over the scanning area for our epsonV700 scanner tested was found. This is compared to within ±3% for reflection scanning with the gloss backing material and within ±4% for transmission scanning. The matt backing material used was simply 5 layers of standard quality white printing paper (80 g/m(2)). It was found that 5 layers was the optimal result for backing material however most of the improvements were seen with a minimum of 3 layers. Above 5 layers, no extra benefit was seen. This may eliminate the need to perform scanner corrections for position on the desktop scanners for radiochromic film dosimetry.

Measuring solar UV radiation with EBT radiochromic film.

Ultraviolet radiation dosimetry has been performed with the use of a radiochromic film dosimeter called Gafchromic EBT for solar radiation exposure. The film changes from a clear colour to blue colour when exposed to ultraviolet radiation and results have shown that the colour change is reproducible within ±10% at 5 kJ m(-2) UV exposure under various conditions of solar radiation. Parameters tested included changes in season (summer versus winter exposure), time of day, as well as sky conditions such as cloudy skies versus clear skies. As the radiochromic films' permanent colour change occurs in the visible wavelengths the film can be analysed with a desktop scanner with the most sensitive channel for analysis being the red component of the signal. Results showed that an exposure of 5 kJ m(-2) (approximately 1 h exposure in full sun during summer) produced an approximate 0.28 change in the net OD when analysed in reflection mode on the desktop scanner which is significant darkening. The main advantages of this film type, and thus the new EBT2 film which has replaced EBT for measurement of UV exposure, is the visible colour change and thus easy analysis using a desktop scanner, its uniformity in response and its robust physical strength for use in outside exposure situations.

Scanning orientation and polarization effects for XRQA radiochromic film.

Gafchromic XRQA radiochromic film, is an effective tool for quality assurance and dose assessment in kilovoltage radiotherapy and diagnostic applications. Like other Gafchromic film products, XRQA film exhibits a variation in dose to reflected optical density response with angle of rotation when analysed with a light source that is partially or fully polarised such as a desktop scanner. Although warnings are not given on manufacturers specifications, this can affect dosimetry accuracy and we recommend that it is essential to scan all XRQA films in the same orientation. The effect is not as pronounced as EBT Gafchromic film. The magnitude of this variation has been measured and shown to be up to 16 ± 2% (1SD) using a fully linear polarised light source was seen with a 90° angle rotation. This would be the maximum variation seen on a desktop scanner with a fully polarised light source. For our standard desktop scanner (Epson v700) a mean variation of 2 ± 1% from 0 cGy to 20 cGy applied dose was measured as compared to 8 ± 2% for EBT Gafchromic. We recommend that to decrease uncertainty in dose measurement, accurate alignment of the calibration films to experimental films be performed on a regular basis. This is especially important if your desktop scanner has a high degree of polarization of its light source.

Measurement of dose reductions for superficial x-rays backscattered from bone interfaces.

Accurate measurement and knowledge of dose delivered during superficial x-ray radiotherapy is required for patient dose assessment. Some tumours treated near the surface (within the first few centimetres) can have large posterior bone structures. This can cause perturbations to dose delivered due to changed backscatter contributions from the bony structure as compared to full water or tissue scattering conditions. Measured results have shown that up to 7.5% of Dmax reductions in dose can occur near the water/bone interface for 100 kVp, using 10 cm diameter field sizes when a 1 cm thick slab of bone is located at 2 cm depth. At smaller field sizes such as 2 cm diameter these values reduce to 2% for the same energy. Larger variations (up to 12.5% of maximum) have been seen at the phantom surface when the bone layer is directly behind the point of interest (within 0.5 mm) and smaller effects (up to 5% of maximum) at depths down to 5 cm. Interesting to note is the fact that for larger field sizes, an increase in percentage dose is found at the water/bone interface due to the production of low energy backscattered electrons similar to the effect found in lead. However, they are much smaller in magnitude and thus would not cause any significant dosimetric effects. In the case where large bony structures lie relatively close to the surface and the tissue above this region is being treated, a dosimeter such as radiochromic film can be used to estimate the dose reduction that may occur due to the changed backscatter conditions.