High Sensitivity Radiochromic Film Research Articles

A prototype quantitative film scanner for radiochromic film dosimetry

We have developed a high resolution, quantitative, two-dimensional optical film scanner for use with a commercial high sensitivity radiochromic film (RCF) for measuring single fraction external-beam radiotherapy dose distributions. The film scanner was designed to eliminate artifacts commonly observed in RCF dosimetry. The scanner employed a stationary light source and detector with a moving antireflective glass film platen attached to a high precision computerized X-Y translation stage. An ultrabright red light emitting diode (LED) with a peak output at 633 nm and full width at half maximum (FWHM) of 16 nm was selected as the scanner light source to match the RCF absorption peak. A dual detector system was created using two silicon photodiode detectors to simultaneously measure incident and transmitted light. The LED light output was focused to a submillimeter (FWHM 0.67 mm) spot size, which was determined from a scanning knife-edge technique for measuring Gaussian optical beams. Data acquisition was performed with a 16-bit A/D card in conjunction with commercial software. The linearity of the measured densities on the scanner was tested using a calibrated neutral-density step filter. Sensitometric curves and three IMRT field scans were acquired with a spatial resolution of 1 mm for both radiographic film and RCF. The results were compared with measurements taken with a commercial diode array under identical delivery conditions. The RCF was rotated by 90 deg and rescanned to study orientation effects. Comparison between the RCF and the diode array measurements using percent dose difference and distance-to-agreement criteria produced average passing rates of 99.0% using 3%/3 mm criteria and 96.7% using 2%/2 mm criteria. The same comparison between the radiographic film and diode array measurements resulted in average passing rates 96.6% and 91.6% for the above two criteria, respectively. No measurable light-scatter or interference scanner artifacts were observed. The RCF rotated by 90 deg showed no measurable orientation effect. A scan of a 15 x 15 cm2 area with 1 mm resolution required 22 min to acquire. The LED densitometer provides an accurate film dosimetry system with 1 mm or better resolution. The scanner eliminates the orientation dependence of RCF dosimetry that was previously reported with commercial flatbed scanners.

SU-FF-T-321: Off-Axis Dose Measurement Using New High Sensitivity Radiochromic Film: Potential Application to Pacemaker Dosimetry in a Clinical Setting

Purpose: To study the feasibility of using new high sensitivity radiochromic film, GAFCHROMIC® Low Dose Film, in measuring the doses to off-axis points for pacemaker. Method and Materials: Each experimental radiochromic film (ISP) was positioned at various off-axis points from the field edge of a 6MV beam of a Varian Clinac-iX. The off-axis points were at x=3,5,8,10,15 cm from the field edge, and at depths of 0, 0.5, 1.5cm. Calibration films were positioned at the central axis of the beam at 1.5cm depth of a 10×10 cm field. Dose was calibrated against the ion chamber measurement (Standard Imaging Exradin A-12 with ADCL calibration) at the same location and depth. All experimental, calibration, and background films were scanned at the same location and orientation of an Epson Perfection 4870 flatbed scanner with reflection mode, 48 bit color, and 150 dpi spatial resolution. The ImageJ software from NIH was used to extract and analyze the data for red and green channels. For each film, a 1cm circular area at the film center was sampled and the mean value in this circle was obtained. Calibration curves for red and green channels were established from the calibration films. Conversion from net optical density readings to doses was achieved based on the polynomial fit to the calibration curve established for each light channel. Results: The doses at the off-axis measurement points were determined, and compared with those obtained using ion chamber at respective locations. The ratio of the radiochromic film dose to that from ion chamber measurement at each measurement point was obtained. Conclusion: It is feasible to accurately measure off-axis doses of megavoltage photon beams using the new high sensitivity radiochromic film. This fast and inexpensive method can be applied in a clinical setting for dose measurement to pacemaker and other critical organs.

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

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

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.

Experimental determination of the radial dose function of Sr90∕Y90 IVBT sources

A series of measurements were undertaken using both high sensitivity radiochromic film and new lithium fluoride thermoluminescent dosimeters in a liquid water medium to define the radial dose function of 90Sr/90Y beta emitting intravascular brachytherapy sources more accurately. These measurements of a single 5 French source pellet served to verify current Monte Carlo transport models and extrapolation chamber measurements of the radial dose function, thus providing the recommended independent published measurements for g(r) of these sources. A slight deviation in the published radial dose function at depth leads the authors to recommend that treatment planning be performed using updated g(r) values from current Monte Carlo transport models verified by measurements such as those shown in this investigation.

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.

Measurement of centi-Gray X-ray dose levels with radiochromic film

X-ray doses in the range of centi-Gray can be detected and accurately measured on a relative scale for low energy X-rays using a high sensitivity radiochromic film stack. Gafchromic XR type T radiochromic film has been investigated to analyze its ability to measure relative doses below 50 cGy for 100-kVp X-rays using a layered technique. Investigations showed that a 4-layer film stack could produce reproducible results at 50-cGy relative applied dose to within 3% and to within 12% for 5-cGy applied dose. The combination of single sheet higher film sensitivity to low energy X-rays along with a layered film dosimetry technique provides the ability to adequately measure low relative absorbed doses less than 50 cGy if required.

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.

WE‐C‐T‐617‐03: Dosimetry of Beta Emitting Intravascular Brachytherapy Sources

Purpose: To verify the accuracy of current intravascular brachytherapy (IVBT) Sr‐90 calibration standards and to generate accurate and detailed IVBT beta dosimetry by analyzing dose distributions in a liquid water medium Method and Materials: High Sensitivity radiochromic film as well as lithium fluoride thermoluminescent dosimeter (TLD) 1×1×1mm microcubes were exposed in liquid water to a Novoste Beta‐Cath Sr‐90 IVBT source pellet. Calibration exposures were performed with a Tracerlab RA‐1 Sr‐90/Y‐90 ophthalmic applicator directly traceable to the National Institute of Standards and Technology (NIST) absorbed dose to water standard. Film and TLDs were read 24 hours post‐exposure using a Molecular Dynamics Personal Densitometer SI scanner (633nm HeNe laser light source) and a Harshaw 5500 unit, respectively. All exposures were made in a custom built water tank with computer controlled high precision Velmex tracks providing three axes for movement of measurement devices. Results: Preliminary radial depth dose measurement results with film and TLDs correspond within uncertainty estimates to data published by Soares et al, which is the current NIST standard for IVBT beta sources. The TLDs showed significant volume averaging over their 1mm thickness, requiring Monte Carlo correction to determine the effective point of measurement for each dosimeter. Conclusion: Liquid water measurements of IVBT beta dosimetry validate the results from previously published studies conducted solely in plastics and in air. These measurements also reduce experimental uncertainty by eliminating the conversion to water from data taken in water mimicking media.

SU-FF-T-259: Use of Radiochromic Films for I-125 Seed Dosimetry In Solid Water

Purpose: To evaluate the use of two new models of high sensitivity radiochromic film, GAFCHROMIC® EBT and XRT, for I‐125 seed dosimetry from 0.1 cm out to 5 cm. Method and Materials: Experimental films were positioned in contact with or at a distance of 0.5 or 1 cm from the 125I seed (Implant Sciences model 3500) in solid water phantom. A multiple film technique was employed, with the product of the air kerma strength and exposure time ranging from 5 Uh to 5500 Uh, using four seeds with NIST traceable air kerma strengths of 7 U. A series of 25 calibration films of each model were exposed to one 125I seed at 0.5 cm distance, one at a time. A CCDcamera based microdensitometer, with interchangeable green (520nm) and red (665nm) light boxes, was used to scan all the films with 0.2 mm pixel resolution. The dose to each calibration film center was calculated using the air kerma strength of the seed (incorporating decay), exposure time and TG43U1 recommended dosimetric parameters. Based on the established calibration curve, dose conversion from net optical density was achieved for each film model and light combination. Results: 2‐d isodose rate (in cGy/Uh) curves were plotted and compared among the film model and scanning light combinations. Radial dose function and anisotropy function were also determined. The results obtained from two film models corroborate each other. We found general agreement with the TG43U1 recommend values of dosimetric parameters. Conclusion: Radiochromic film dosimetry using GAFCHROMIC® EBT or XRT models is feasible and accurate in determining 2‐d dose distributions around low dose rate 125I seed. It is a viable alternative to TLDdosimetry for 125I seed dose characterization. Conflict of Interest: Prototype GAFCHROMIC® EBT and XRT films from ISP. Free I‐125 seeds from Implant Sciences.

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.

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.

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.

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.