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.     

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.     

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.     

Normalized sensitometric curves for the verification of hybrid IMRT treatment plans with multiple energies

With the clinical implementation of time-variable dose patterns and intensity modulated radiotherapy (IMRT) film dosimetry has regained popularity. Films are currently the most frequently used dosimetric means for patient specific quality assurance in IMRT. A common method is to verify a so-called hybrid IMRT plan, which is the patient specific treatment plan with unmodified fluence patterns recalculated in a dedicated phantom. For such applications the sensitometric curve, i.e., the relation between optical density (OD) and absorbed dose, should not depend critically on beam energy, field size and depth, or film orientation. In order to minimize the influence of all these variables a normalization of sensitometric curves is performed at various photon beam energies (6 MV, 10 MV, 25 MV). By doing so one unique sensitometric curve can be used for these three beam qualities. This holds for both film types investigated: Kodak X-Omat V films and EDR-2 films. Additionally, the influence of field size, depth and film orientation on a normalized sensitometric curve is determined for both film types. For doses smaller than 0.8 Gy for X-Omat V and doses smaller than 3 Gy for EDR-2 films the field size variation of normalized sensitometric curves is much smaller than 3% for fields up to 20 x 20 cm2. For X-Omat V films all differences between sensitometric curves determined at depths of 5, 10, and 15 cm are smaller than 3%. For EDR-2 films deviations larger than 3% are only observed at low net OD smaller than 0.25. The dependence of film orientation (parallel versus perpendicular) on a normalized sensitometric curve is found to be not critical. However, processing conditions have the largest influence and can result in differences up to 20% for sensitometric curves derived from films of the same batch but using different film processors. When normalizing sensitometric curves to the dose value necessary to obtain a net OD=1 for that respective geometry and energy the large energy dependence of sensitometric curves can be almost eliminated. This becomes especially important for the verification of hybrid IMRT plans with multiple energies. Additionally, such a normalization minimizes other influences such as field size, depth, and film orientation. This method is generally applicable to both Kodak X-Omat V and EDR-2 films. In order to achieve the highest accuracy level an upper dose limit of 0.8 Gy for X-Omat V films and 3 Gy for EDR-2 films should be taken into account. However, these dose limits may vary with film reading instrument and film processor.     

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.     

Dosimetric characterization of GafChromic EBT film and its implication on film dosimetry quality assurance

The suitability of radiochromic EBT film was studied for high-precision clinical quality assurance (QA) by identifying the dose response for a wide range of irradiation parameters typically modified in highly-conformal treatment techniques. In addition, uncertainties associated with varying irradiation conditions were determined. EBT can be used for dose assessment of absorbed dose levels as well as relative dosimetry when compared to absolute absorbed dose calibrated using ionization chamber results. For comparison, a silver halide film (Kodak EDR-2) representing the current standard in film dosimetry was included. As an initial step a measurement protocol yielding accurate and precise results was established for a flatbed transparency scanner (Epson Expression 1680 Pro) that was utilized as a film reading instrument. The light transmission measured by the scanner was found to depend on the position of the film on the scanner plate. For three film pieces irradiated with doses of 0 Gy, approximately 1 Gy and approximately 7 Gy, the pixel values measured in portrait or landscape mode differed by 4.7%, 6.2% and 10.0%, respectively. A study of 200 film pieces revealed an excellent sheet-to-sheet uniformity. On a long time scale, the optical development of irradiated EBT film consisted of a slow but steady increase of absorbance which was not observed to cease during 4 months. Sensitometric curves of EBT films obtained under reference conditions (SSD = 95 cm, FS = 5 x 5 cm(2), d = 5 cm) for 6, 10 and 25 MV photon beams did not show any energy dependence. The average separation between all curves was only 0.7%. The variation of the depth d (range 2-25 cm) in the phantom did not affect the dose response of EBT film. Also the influence of the radiation field size (range 3 x 3-40 x 40 cm(2)) on the sensitometric curve was not significant. For EDR-2 films maximum differences between the calibration curves reached 7-8% for X6MV and X25MV. Radiochromic EBT film, in combination with a flatbed scanner, presents a versatile system for high-precision dosimetry in two dimensions, provided that the intrinsic behaviour of the film reading device is taken into account. EBT film itself presents substantial improvements on formerly available models of radiographic and a radiochromic film and its dosimetric characteristics allow us to measure absorbed dose levels in a large variety of situations with a single calibration curve.     

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.     

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.     

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.     

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.     

Precautions and strategies in using a commercial flatbed scanner for radiochromic film dosimetry

The purpose of this study was to investigate the value of a commercially available flatbed scanner for film dosimetry with radiochromic film for external radiotherapy. The EPSON Pro 1680 Expression scanner was examined as a densitometer for two-dimensional film dosimetry with Gafchromic EBT film. An accurate and efficient scanning procedure was established. Possible drift and warm-up effects of the scanner were studied and the direct physical influence of the scanner light on the radiochromic film was assessed. Next, we investigated the scan field uniformity. Also, we examined if the accuracy of radiochromic film was improved by subtracting the optical density of the unirradiated blank film from the optical density of the irradiated film. To assess the accuracy of Gafchromic EBT film when the EPSON scanner was used as a densitometer, the depth dose of a 2 x 15 cm(2) field and the in-plane and cross-plane profiles of a 15 x 15 cm(2) field were measured and compared with diamond detector measurements. When taking consecutive scans, we found that the optical density taken from the first scan was about 1% higher than the optical density taken from subsequent scans. We attribute this to the warming up of the lamp of the scanner. Longer-term drift of the scanner was found to be absent. We found that the use of a correction matrix was necessary to correct for the non-uniform scanner response over the scan field. Subtracting the optical density of the unirradiated blank film from the irradiated film improves the precision of the Gafchromic EBT film. Depth dose and profile measurements with Gafchromic EBT film and the diamond detector are in agreement within 2.5%. The EPSON Pro 1680 Expression scanner is an excellent tool for accurate two-dimensional film dosimetry with Gafchromic EBT film provided that some precautions and corrections are taken into account.     

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.     

Optical density changes of Gafchromic MD-55 film resulting from laser light exposure at wavelengths of 671 nm and 633 nm

Laser-based scanners provide a sensitive means for measuring optical density (OD) of Gafchromic films. Such instruments were reviewed in a recent AAPM report (task group 55) which provided recommendations and information on OD measurements (effect of wavelength, temperature, etc.). The present article reports that variable rate scanners and spot densitometers using laser diodes (671 nm) and HeNe lasers (633 nm) can cause polymerization of Gafchromic film. The light induced polymerization depends on light power, wavelength, beam spot size, dwell time, and prior radiation dose of the film. Measurements were made with a custom built scanner that provided accurate control of light power, light polarization, dwell time, and film position in relation to the beam focus. The results demonstrate that lasers operating with powers of 0.1, 0.5, 1.0, and 1.5 mW produce a nonlinear increase in OD of Gafchromic film. The measured change in OD after 1 min of exposure ranges from 0.150 to 0.244 for a laser diode operating at 0.5 and 1.5 mW, respectively. Tables are included that tabulate the increase in OD for laser power, dwell time, and prior dose. Laser light induced polymerization can have a significant impact on dosimetry measurements acquired using these laser-based systems.     

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.     

On the calibration process of film dosimetry: OLS inverse regression versus WLS inverse prediction

The purpose of this study was both putting forward a statistically correct model for film calibration and the optimization of this process. A reliable calibration is needed in order to perform accurate reference dosimetry with radiographic (Gafchromic) film. Sometimes, an ordinary least squares simple linear (in the parameters) regression is applied to the dose-optical-density (OD) curve with the dose as a function of OD (inverse regression) or sometimes OD as a function of dose (inverse prediction). The application of a simple linear regression fit is an invalid method because heteroscedasticity of the data is not taken into account. This could lead to erroneous results originating from the calibration process itself and thus to a lower accuracy. In this work, we compare the ordinary least squares (OLS) inverse regression method with the correct weighted least squares (WLS) inverse prediction method to create calibration curves. We found that the OLS inverse regression method could lead to a prediction bias of up to 7.3 cGy at 300 cGy and total prediction errors of 3% or more for Gafchromic EBT film. Application of the WLS inverse prediction method resulted in a maximum prediction bias of 1.4 cGy and total prediction errors below 2% in a 0-400 cGy range. We developed a Monte-Carlo-based process to optimize calibrations, depending on the needs of the experiment. This type of thorough analysis can lead to a higher accuracy for film dosimetry.     

EDR-2 film response to charged particles

A useful tool for verifying segmental or dynamic treatments with multiple multi-leaf collimator positions, spinning range modulator propellors or magnetically scanned beams would be a film with a linear dose response up to several hundred centiGray, as typical for delivered treatments. Kodak has released an extended range film (EDR-2) that may satisfy this desire. In this study, dose response curves were obtained for several electron, proton, carbon ion and iron ion beams of different energies to determine the utility of this film.     

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.     

Dependence of radiochromic film optical density post-exposure kinetics on dose and dose fractionation

Radiochromic film (RCF) has been shown to be a precise and accurate secondary planar dosimeter for acute exposure radiation fields. However, its application to low dose-rate brachytherapy has been questioned because of possible dose-rate effects. To address this concern, we have measured the optical density (OD) of Model 55-2 RCF as a function of time (interval between the completion of irradiation and densitometry using a 633 nm laser scanner) following exposure (from less than 1 hour to 90 days) for single and split doses from 1 Gy to 100 Gy. Our work demonstrates that film darkening as a function of post-irradiation time depends significantly on total dose, with films exposed to lower doses developing faster than films given higher doses. At 1 Gy, the OD 90 days after exposure is 200% larger than that measured 1 h after exposure compared to a 20% increase over 90 days for doses larger than 20 Gy. An empirical model with time-independent, fast and slow growth terms was used to fit single exposure data. The dependence of the resulting best-fit parameters on dose was investigated. Splitting the dose into two fractions (20 Gy followed by doses of 1-80 Gy 24 h later) results in modest post-irradiation time-dependent changes in the total optical density (at most 15% at small doses), which dissipates within 20 hours following the second exposure. This experimental finding is consistent with the predictions of a simple cumulative dose superposition model. Overall, both experimental and empirical modeling suggest that dose-rate effects may be relatively small despite the strong dependence of film darkening kinetics on total dose. However, more experimental evaluation of radiochromic film response dependence on dose rate and dose-time-fractionation patterns is needed.     

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.     

An application of GafChromic MD-55 film for 67.5 MeV clinical proton beam dosimetry

The purpose of this study is to explore the use of GafChromic MD-55 (RC) film for 67.5 MeV clinical proton beam dosimetry at the Crocker Nuclear Laboratory, University of California, Davis. Several strips of RC film 6 cm x 6 cm in dimension were irradiated at a depth of 18.2 mm corresponding to the middle of a 24 mm spread-out Bragg peak (SOBP). The films were irradiated to a proton dose in the range of 0.5 Gy to 100 Gy. The beam profiles were also measured at the middle of the 24 mm SOBP. The Bragg peak was measured by using a wedge shaped phantom made of Lucite. The Bragg peak measured with RC film was compared with diode and ionization chamber measurements. After background subtraction, the calibration of the dose response of RC film showed, to a maximum deviation of 10%, a linear increase of optical density (OD) with dose from 0.5 to 100 Gy. The uniformity of OD over a single sheet of film showed a variation of +/-6%. The distal-fall off between 90% and 20% measured with GafChromic film for the Bragg peak was 1.3 mm as compared to 1.1 mm for a diode measurement and 1.4 mm for an ionization chamber measurement. The FWHM of the Bragg peak was 7.5 mm when measured with GafChromic film, 5.3 mm when measured with a diode and 8.1 mm as measured by an ionization chamber. The peak/plateau ratio with GafChromic film was 3.3 as compared to 3.7 with a diode and 3.2 with an ionization chamber. In conclusion, GafChromic MD-55 film may be a useful and convenient detector for dose measurement and quality assurance programmes of proton beams.