Visible absorption properties of radiation exposed XR type-T radiochromic film

The visible absorption spectra of Gafchromic XR type-T radiochromic film have been investigated to analyse the dosimetry characteristics of the film with visible light densitometers. Common densitometers can use photospectrometry, fluorescent light (broad-band visible), helium neon (632 nm), light emitting diode (LED) or other specific bandwidth spectra. The visible absorption spectra of this film when exposed to photon radiation show peaks at 676 nm and 618 nm at 2 Gy absorbed doses which shift to slightly lower wavelengths (662 nm and 612 nm at 8 Gy absorbed dose) at higher doses. This is similar to previous models of Gafchromic film such as MD-55-2 and HS but XR type-T also includes a large absorption at lower visible wavelengths due to 'yellow' dyes placed within the film to aid with visible recognition of the film exposure level. The yellow dye band pass is produced at approximately 520 nm to 550 nm and absorbs wavelengths lower than this value within the visible spectrum. This accounts for the colour change from yellow to brown through the added absorption in the red wavelengths with radiation exposure. The film produces a relatively high dose sensitivity with up to 0.25 OD units per Gy change at 672 nm at 100 kVp x-ray energy. Variations in dose sensitivity can be achieved by varying wavelength analysis.     

Absorption spectra of irradiated XRCT radiochromic film

Gafchromic XRCT radiochromic film is a self-developing high sensitivity radiochromic film product which can be used for assessment of delivered radiation doses which could match applications such as computed tomography (CT) dosimetry. The film automatically changes colour upon irradiation changing from a yellow to green/brown colour. The absorption spectra of Gafchromic XRCT radiochromic film as measured with reflectance spectrophotometry have been investigated to analyse the dosimetry characteristics of the film. Results show two main absorption peaks produced from irradiation located at 636 nm and 585 nm. This is similar to EBT Gafchromic film. A high level of sensitivity is found for this film with a 1 cGy applied dose producing an approximate net optical density change of 0.3 at 636 nm. This high sensitivity combined with its relatively energy independent nature around the 100 kVp to 150 kVp x-ray energy range provides a unique enhancement in dosimetric measurement capabilities over currently available dosimetry films for CT applications.     

Visible absorption spectra of radiation exposed SIRAD dosimeters

SIRAD badge dosimeters are a new type of personal dosimeter designed to measure radiation exposure up to 200 R and give a visual qualitative measurement of exposure. This is performed using the active dosimeter window, which contains a radiochromic material amalgamated in the badge assembly. When irradiated, the badges active window turns blue, which can be matched against the given colour chart for a qualitative assessment of the exposure received. Measurements have been performed to analyse the absorption spectra of the active window, and results show that the window automatically turns a blue colour upon irradiation and produces two peaks in the absorption spectra located at 617 nm and 567 nm. When analysed with a common computer desktop scanner, the optical density response of the film to radiation exposure is non-linear but reproducible. The net OD of the film was 0.21 at 50 R exposure and 0.31 at 200 R exposure when irradiated with a 6 MV x-ray energy beam. When compared to the calibration colour strips at 6 MV x-ray energy the film's OD response matches relatively well within 3.5%. An approximate 8% reduction in measured OD to exposure was seen for 250 kVp x-rays compared to 6 MV x-rays. The film provides an adequate measurement and visually qualitative assessment of radiation exposure for levels in the range of 0 to 200 R.     

Corresponding dose response of radiographic film with layered gafchromic film

This note investigates the dose response of layered HS Gafchromic film compared to Kodak EDR-2 radiographic film. Using five layers of HS type Gafchromic film a dose response greater than EDR-2 film is achieved at the peak wavelength (0.55 OD/Gy versus 0.3 OD/Gy for EDR-2 film). Even over a broader waveband of 30 nm, which is similar to that found in ultra bright LED scanners, the response was found to be 0.38 OD/Gy as opposed to 0.29 OD/Gy. Measurements averaged over the entire visible spectrum produce a relative dose response of 0.165 OD/Gy for five layer HS and 0.29 OD/Gy for EDR-2 film. Due to this high dose response that is achievable, the five layer HS could be used in applications where small doses are delivered to certain areas and a low dependence of energy response is required for measurement.     

High sensitivity radiochromic film dose comparisons

This short note investigates the dose characteristics of a relatively new high sensitivity radiochromic film (Gafchromic HS) and compares dose and energy response to various Gafchromic film types and radiographic (EDR-2) film. The original MD-55-1 and two improved sensitivity films, MD-55-2 and HS film, were investigated for energy and dose response. Results show that the energy response of the new HS film is relatively the same as the original MD-55-1 and MD-55-2 films with a decrease in sensitivity at lower x-ray energies, with response decreasing down to approximately 0.64 (normalized to 1 for a 6 MV beam) for a 28 keV effective energy beam. This is compared to an over response of 9.2 at the same energy for EDR-2 film. The dose response at the maximum absorption peak was found to be approximately 3.8 and 1.9 times more sensitive than MD-55-1 and MD-55-2 films, respectively. At the absorption peak yielding the maximum optical density change, HS was found to be approximately 0.2 to 0.25 times the sensitivity of EDR-2.     

Post-irradiation colouration of Gafchromic EBT radiochromic film

Gafchromic EBT (International Specialty Products, NJ, USA), radiochromic film is one of the newest radiation-induced auto-developing x-ray analysis films available for therapeutic radiation dosimetry in radiotherapy applications. Part of any radiochromic film product which undergoes a polymerization reaction for automatic darkening is an associated post-irradiation colouration whereby the film continues to darken after irradiation has ceased. The Gafchromic EBT film has been shown to produce an approximate 6% to 9% increase in post-irradiation optical density within the first 12 h of irradiation within the 1 Gy to 5 Gy dose range. This is compared to approximately 13%, 15% and 19% for MD-55-2, XR type T and HS radiochromic film, respectively. It is also shown that the EBT film's post-irradiation growth stabilizes to within 1% within the first 6 h. Thus EBT provides a reduced post-irradiation growth effect. However, to increase the accuracy of the film analysis, it is recommended that films be left for a significant period (at least 6 h) before the analysis is performed to provide a high level of accuracy. Also, calibration films must be read out with the same post-irradiation time to further enhance the accuracy of dosimetry.     

XR type-R radiochromic film x-ray energy response

Gafchromic XR type-R radiochromic film is a relatively new product designed for use at clinical diagnostic x-ray energies both qualitatively and quantitatively. This short note investigates the energy response characteristics of this high-sensitivity radiochromic film for both diagnostic and therapeutic x-ray energies. Results are also compared to conventional silver halide x-ray film for energy response. Results show that the energy response of the new XR type-R film is minimal over the 75-125 kVp range (9% variation with +/-3% error in measurement to 1 SD). This is compared to a 27% variation for X-Omat V radiographic film for the same energy range. XR type-R film does, however, produce a larger energy response variation when compared over a larger therapeutic x-ray range (50 kVp superficial to 10 MV megavoltage) with a relative response of 10.4 at 125 kVp compared to 1 at 6 MV. This is significantly different to MD-55-2 and HS Gafchromic film which has a lower energy response at lower energies. XR type-R film is ideal for a quantitative dosimeter in the low energy range due to its relative energy independence and high sensitivity compared to conventional radiochromic film.     

Experimental energy response verification of XR type T radiochromic film

This short note investigates the energy response characteristics of a relatively new high sensitivity radiochromic film (XR type T) and compares it to other radiochromic and radiographic films and thermoluminescent dosimeters. Results show that the energy response of the new XR type T film is relatively large over the range of therapeutic energies from 50 kVp superficial x-ray treatment to 18 MV high energy radiotherapy treatment. When normalized to 1 at a standard 6 MV radiotherapy x-ray energy the XR type T film produced a normalized dose response of approximately 6 in the energy range of 30 keV to 70 keV thus representing an increase in sensitivity at lower energies similar to that observed for radiographic x-ray films. This is quite different from previous versions of Gafchromic film where the energy response of the film decreases at lower energies down to levels approximately 0.6-0.7 for the same effective energies. This type of film has been optimized for use in diagnostic energy ranges producing a relatively uniform dose response in the 30 keV to 70 keV range.     

Absorption spectroscopy of EBT model GAFCHROMIC film

The introduction of radiochromic films has solved some of the problems associated with conventional 2D radiation detectors. Their high spatial resolution, low energy dependence, and near-tissue equivalence make them ideal for measurement of dose distributions in radiation fields with high dose gradients. Precise knowledge of the absorption spectra of these detectors can help to develop more suitable optical densitometers and potentially extend the use of these films to other areas such as the measurement of the radiation beam spectral information. The goal of this study is to present results of absorption spectra measurements for the new GAFCHROMIC film, EBT type, exposed to 6 MV photon beam in the dose range from 0 to 6 Gy. Spectroscopic analysis reveals that in addition to the two main absorption peaks, centered at around 583 and 635 nm, the absorption spectrum in the spectral range from 350 to 800 nm contains six more absorption bands. Comparison of the absorption spectra reveals that previous HD-810, MD-55, as well as HS GAFCHROMIC film models, have nearly the same sensitive layer base material, whereas the new EBT model, GAFCHROMIC film has a different composition of its sensitive layer. We have found that the two most prominent absorption bands in EBT model radiochromic film do not change their central wavelength position with change in a dose deposited to the film samples.     

Rounded end multi-leaf penumbral measurements with radiochromic film

Multi-leaf penumbral doses have been investigated for 6 MV x-rays and a Varian millennium multi-leaf collimator (MLC) using Gafchromic MD-55-2, radiochromic film and X-omat V radiographic film. An advantage of Gafchromic film for multi-leaf penumbral dose measurement is the relatively low energy dependence of the film. A comparison of penumbral dose measurements has also ascertained the effects of energy response on radiographic film in this region. Similar 80%/20% penumbral doses have been measured with both types of films. Thus there is a relatively low energy effect on penumbral dose measurements in film dosimetry. The 80%/20% dose penumbral distances for rounded leaf end multi-leafs for a 10 cm x 10 cm field at Dmax were found to be 4.6 mm and 4.3 mm for radiochromic and radiographic film respectively. This is compared to 2.6 mm and 2.6 mm for the leaf edge penumbra. Radiochromic film also measured leaf end/interleaf leakage doses in the penumbral region, which was shown to produce approximately 4% of maximum dose wave across the penumbral region with maximum doses delivered at the MLC leaf interfaces.     

Polarization effects on a high-sensitivity radiochromic film

A new high-sensitivity radiochromic film has been tested for its polarization properties. Gafchromic HS film has been shown to produce a relatively small (less than 3%) variation in the optical density measured at 660 nm wavelength when the light source is fully linear polarized and the film is rotated through a 360 angle. Similar variations are seen when the detector is linearly polarized. If both the light source and the detector are linearly polarized, variations in the measured optical density can reach 15% when the film is rotated through a 360 angle. This seems to be due to a phase shift in polarized light caused by the radiochromic film resulting in the polarized light source becoming out of phase with the polarized detector. Gafchromic HS radiochromic film produces a minimal polarization response with varying angle of rotation; however, we recommend that a polarization test be performed on a densitometry system to establish the extent of its polarization properties before accuracy dosimetry is performed with radiochromic HS film.     

Absorption spectra analysis of exposed FWT-60 radiochromic film

The visible absorption spectra of Radiachromic FWT-60 radiochromic film have been investigated to analyse the dosimetry characteristics of the film. The film is radiation sensitive to high absorbed doses. The visible absorption spectra of this film when exposed to photon radiation show a peak at 605 nm which is stable over the dose range of 0 Gy to 20 kGy. The radiation sensitive absorption spectra are present over the wavelength range of approximately 500 nm to 660 nm. Negligible dose response is seen in the infrared region or the UV region of wavelength readout. Variation of sensitivity of response can be achieved by varying the wavelength of readout with the maximum measured response of 0.077 OD units per kGy. The film can be an ideal dosimeter for areas where high dose levels need to be measured.     

Measurement of energy dependence for XRCT radiochromic film

Gafchromic XRCT, radiochromic film is assessed over a broad energy range, from kilovoltage to megavoltage x rays for variations in reflected optical density to dose response. A large energy dependence was found with reflected optical density output for the same delivered dose varying from 7.8 +/- 0.35 at 25.5 keV (50 kVp) peaking at 12.1 +/- 0.5 at 54 keV (125 kVp) to 0.975 +/- 0.03 at 2300 keV (10 MV) when normalized to 1 at 1400 keV (6 MV) energy. The response is constant (within 3%) in the 36-69 keV equivalent photon energy range, which corresponds to x-ray tube generating potentials of approximately 100-150 kVp. This matches well with beam qualities for diagnostic computed topography applications.     

Energy dependence of response of new high sensitivity radiochromic films for megavoltage and kilovoltage radiation energies

The purpose of this paper is to evaluate the energy dependence of the response of two new high sensitivity models of radiochromic films EBT and XR-QA. We determined the dose response curves of these films for four different radiation sources, namely, 6 MV photon beams (6 MVX), Ir-192, I-125, and Pd-103. The first type (EBT) is designed for intensity modulated radiation therapy (IMRT) dosimetry, and the second type (XR-QA) is designed for kilovoltage dosimetry. All films were scanned using red (665 nm) and green (520 nm) light sources in a charge-coupled device-based densitometer. The dose response curves [net optical density (NOD) versus dose] were plotted and compared for different radiation energies and light sources. Contrary to the early GAFCHROMIC film types (such as models XR, HS, MD55-2, and HD810), the net optical densities of both EBT and XR-QA were higher with a green (520 nm) than those with a red (665 nm) light source due to the different absorption spectrum of the new radiochromic emulsion. Both film types yield measurable optical densities for doses below 2 Gy. EBT film response is nearly independent of radiation energy, within the uncertainty of measurement. The NOD values of EBT film at 1 and 2 Gy are 0.13 and 0.25 for green, and 0.1 and 0.17 for red, respectively. In contrast, the XR-QA film sensitivity varies with radiation energy. The doses required to produce NOD of 0.5 are 6.9, 5.4, 0.7, and 0.9 Gy with green light and 19, 13, 1.7, and 1.5 Gy with red light, for 6 MVX, Ir-192, I -125, and Pd-103, respectively. EBT film was found to have minimal photon energy dependence of response for the energies tested and is suitable for dosimetry of radiation with a wide energy spectrum, including primary and scattered radiation. XR-QA film is promising for kilovoltage sources with a narrow energy spectra. The new high sensitivity radiochromic films are promising tools in radiation dosimetry.     

Low dose fraction behavior of high sensitivity radiochromic film

A high sensitivity (HS) model of radiochromic film is receiving increasing use. The film's linear sensitometric response in the range of 0.5-40 Gy would make this film an ideal candidate for complex dosimetry applications that require tissue equivalence. This study investigates the potential use for clinical dosimetry of typical radiotherapy fractions at relatively low doses (0.5-5 Gy). The experiment involved exposing 25 pre-exposed pieces of HS film to five equal fractions of doses from 0.5 to 5 Gy 24 hours apart. The cumulative dose for each film was carefully monitored and optical density measurements were used as the sole determination of film response to dose. The average behavior of the various fractionation schemes was roughly consistent with previous observations of the MD-55 radiochromic film with about twice the overall sensitivity as expected. However, at low doses and low dose increments, unexpected variations beyond a well-documented low dose nonlinearity were observed. These unexpected variations may indicate complex polymer kinetics at low doses. This type of film would require extra care beyond that described in TG-55 for accurate use at low doses or low dose fraction schemes.     

Validation of a precision radiochromic film dosimetry system for quantitative two-dimensional imaging of acute exposure dose distributions

We present an evaluation of the precision and accuracy of image-based radiochromic film (RCF) dosimetry performed using a commercial RCF product (Gafchromic MD-55-2, Nuclear Associates, Inc.) and a commercial high-spatial resolution (100 microm pixel size) He-Ne scanning-laser film-digitizer (Personal Densitometer, Molecular Dynamics, Inc.) as an optical density (OD) imaging system. The precision and accuracy of this dosimetry system are evaluated by performing RCF imaging dosimetry in well characterized conformal external beam and brachytherapy high dose-rate (HDR) radiation fields. Benchmarking of image-based RCF dosimetry is necessary due to many potential errors inherent to RCF dosimetry including: a temperature-dependent time evolution of RCF dose response; nonuniform response of RCF; and optical-polarization artifacts. In addition, laser-densitometer imaging artifacts can produce systematic OD measurement errors as large as 35% in the presence of high OD gradients. We present a RCF exposure and readout protocol that was developed for the accurate dosimetry of high dose rate (HDR) radiation sources. This protocol follows and expands upon the guidelines set forth by the American Association of Physicists in Medicine (AAPM) Task Group 55 report. Particular attention is focused on the OD imaging system, a scanning-laser film digitizer, modified to eliminate OD artifacts that were not addressed in the AAPM Task Group 55 report. RCF precision using this technique was evaluated with films given uniform 6 MV x-ray doses between 1 and 200 Gy. RCF absolute dose accuracy using this technique was evaluated by comparing RCF measurements to small volume ionization chamber measurements for conformal external-beam sources and an experimentally validated Monte Carlo photon-transport simulation code for a 192Ir brachytherapy source. Pixel-to-pixel standard deviations of uniformly irradiated films were less than 1% for doses between 10 and 150 Gy; between 1% and 5% for lower doses down to 1 Gy and 1% and 1.5% for higher doses up to 200 Gy. Pixel averaging to form 200-800 microm pixels reduces these standard deviations by a factor of 2 to 5. Comparisons of absolute dose show agreement within 1.5%-4% of dose benchmarks, consistent with a highly accurate dosimeter limited by its observed precision and the precision of the dose standards to which it is compared. These results provide a comprehensive benchmarking of RCF, enabling its use in the commissioning of novel HDR therapy sources.     

Energy dependence (75 kVp to 18 MV) of radiochromic films assessed using a real-time optical dosimeter

The response of radiochromic film, GafChromic EBT, was investigated for dependence on x-ray beam energy using a previously reported real-time optical readout approach. X-ray beams of energy from 75 kVp to 18 MV were employed. The dose-induced change in optical density for the EBT film was compared to values obtained for GafChromic HS and MD-55 films, exposed under the same conditions. All responses were normalized to that obtained for 60Co irradiation. While change in optical density for 1 Gy of applied dose as measured with HS and MD-55 films decreased by approximately 40% at low energies, the mean change in optical density of EBT film remained within 3% of that in the 60Co beam over the entire energy range.     

Use of multiple layers of Gafchromic film to increase sensitivity.

This note investigates the increase of the sensitivity of radiochromic film by layering the film together. Investigations with up to five layers of radiochromic film placed together as a single dosimeter have allowed accurate two-dimensional dosimetry at low radiotherapy doses. The sensitivity of the response of the layered film increases with the number of layers, with the five-layer dosimeter increasing the change in optical density by 4.3 +/- 0.2 times that for a single layer film at 670 nm readout wavelength. Measurements in the penumbral region of a 6 MV x-ray beam show that the spatial resolution is slightly reduced with the layered film. The distance across the region from 20 to 80% of the maximum dose at the beam edge (20/80% penumbra) measured at 1.5 cm depth was measured as 3.1 +/- 0.3 and 3.5 +/- 0.3 mm for one and five layers, respectively. Thus no major reduction in spatial resolution was found by layering the film to five layers but the sensitivity was increased by 430% at 670 nm readout wavelength.     

Matchline dosimetry in step and shoot IMRT fields: a film study

The Varian millennium 120 multileaf collimator has curved leaf ends. Transmission through the leaf ends generates a small asymmetric penumbral dose effect. This design can lead to hot spots between neighbouring beam segments during step and shoot IMRT dose delivery. We have observed some matchlines with film for clinical beams optimized using the pinnacle radiotherapy treatment planning system; hence we sought to verify the optimum leaf offset required to minimize the matchline effect. An in-house program was created to control the MLC leaf banks in 2 cm steps with a 2 cm gap. The gap was varied by the following offset values from 0.0 to 0.1 cm. Two types of radiographic films (Kodak EDR and XV films) and a radiochromic film (Gafchromic MD-55-2) were used to measure the optical density maps. The films were positioned in a solid water phantom perpendicular to the beam axis and irradiated at d(max) using a 6 MV photon beam. An ion chamber (IC4) was used to measure point doses for normalization in a beam umbral minima position. The relative mean peak to valley dose ratios measured with no leaf offset were 1.31, 1.30 and 1.31 for the XV, EDR2 and Gafchromic films, respectively. For a 0.07 cm gap per leaf and a performance of end leaf repeatability of 0.01 cm, the central matchline was reduced to about 1.0 for all dosimeters, with two mini-peaks measured as 1.05, 1.05 and 1.08 each side of the matchline, for XV, EDR2 and Gafchromic, respectively. The average relative dose across the umbra for this offset was XO-mat V = 1.01, EDR = 1.01 and radiochromic film = 1.02, respectively. While we expected the beam penumbral tails from segment neighbours to cause overprediction of the dose in the central valley regions due to the energy response of radiographic films, by normalizing all dosimeters to an ion chamber reading in the minimum we could not observe any major shape distortion between the radiographic film and radiochromic film results. In conclusion, relative doses measured by radiographic and radiochromic films agree well with IC4 within +/-2%.     

Measurement of high energy x-ray beam penumbra with Gafchromic EBT radiochromic film

High energy x-ray beam penumbra are measured using Gafchromic EBT film. Gafchromic EBT, due to its limited energy dependence and high spatial resolution provide a high level of accuracy for dose assessment in penumbral regions. The spatial resolution of film detector systems is normally limited by the scanning resolution of the densitometer. Penumbral widths (80%/20%) measured at Dmax were found to be 2.8, 3.0, 3.2, and 3.4 mm (+/- 0.2 mm) using 5, 10, 20, and 30 cm square field sizes, respectively, for a 6 MV linear accelerator produced x-ray beam. This is compared to 3.2 mm +/- 0.2 mm (Kodak EDR2) and 3.6 mm +/- 0.2 mm (Kodak X-Omat V) at 10 cm x 10 cm measured using radiographic film. Using a zero volume extrapolation technique for ionization chamber measurements, the 10 cm X 10 cm field penumbra at Dmax was measured to be 3.1 mm, a close match to Gafchromic EBT results. Penumbral measurements can also be made at other depths, including the surface, as the film does not suffer significantly from dosimetric variations caused by changing x-ray energy spectra. Gafchromic EBT film provides an adequate measure of penumbral dose for high energy x-ray beams.