Linear energy transfer (LET) distribution outside small radiotherapy field edges produced by 6 MV X-rays.

In modern radiotherapy with photons, the absorbed dose outside the radiation field is generally investigated. But it is well known that the biological damage depends not only on the absorbed dose but also on LET. This work investigated the dose-average LET (LΔ,D) outside several small radiotherapy fields to provide information that can help for better evaluating the biological effect in organs at risk close to the tumour volume. The electron fluences produced in liquid water by a 6 MV X-rays Varian iX linac were calculated using the EGSnrc Monte Carlo code. With the electron spectra, LΔ,D calculations were made for eight open small square fields and the reference field at water depths of 0.15 cm, 1.35 cm, 9.85 cm and 19.85 cm and several off-axis distances. The variation of LΔ,D from the centre of the beam to 2 cm outside the field's edge depends on the field size and water depth. Using radiobiological data reported in the literature for chromosomal aberrations as an endpoint for the induction of dicentrics determined in Human Lymphocytes, we estimated the maximum low-dose relative biological effectiveness, (RBEM) finding an increase of up to 100% from the centre of the beam to 2 cm from the field's edge.

Future directions on low-energy radiation dosimetry.

For more than one century, low-energy (< 100 keV) photons (x-rays and gamma) have been widely used in different areas including biomedical research and medical applications such as mammography, fluoroscopy, general radiography, computed tomography, and brachytherapy treatment, amongst others. It has been demonstrated that most of the electrons produced by low photon energy beams have energies below 10 keV. However, the physical processes by which these low energy electrons interact with matter are not yet well understood. Besides, it is generally assumed that all the energy deposited within a dosimeter sensitive volume is transformed into a response. But such an assumption could be incorrect since part of the energy deposited might be used to create defects or damages at the molecular and atomic level. Consequently, the relationship between absorbed dose and dosimeter response can be mistaken. During the last few years, efforts have been made to identify models that allow to understand these interaction processes from a quantum mechanical point of view. Some approaches are based on electron-beam - solid-state-interaction models to calculate electron scattering cross-sections while others consider the density functional theory method to localize low energy electrons and evaluate the energy loss due to the creations of defects and damages in matter. The results obtained so far could be considered as a starting point. This paper presents some methodologies based on fundamental quantum mechanics which can be considered useful for dealing with low-energy interactions.

Track and dose-average LET dependence of Gafchromic EBT3 and MD-V3 films exposed to low-energy photons.

Gafchromic films are widely used in radiotherapy using photons, electrons and protons. Dosimetric characteristics of the films in terms of beam-quality is of great importance for a better evaluation of the absorbed-dose in the clinic. In proton-therapy, film's response has been reported in terms of track-average, LΔ,T, or dose-average, LΔ,D, linear energy transfer (LET), concluding that LΔ,D is a more reliable parameter than LΔ,T. Nonetheless, in photon-beams, the film's response is generally scrutinised in terms of photon-energy. This work aimed at investigating, the total (TEF) and secondary (SE) electron fluence produced in EBT3 and MD-V3 films exposed to 20 kV-160 kV x-ray and 60Co beams and their corresponding LΔ,T and LΔ,D to determine their influence on the film's relative-efficiency, REFilm. Regardless the film-model, at energies below 100 keV, LΔ,D for TEF are about 1.7 to 2.5 times those of LΔ,T while for SE they are relatively similar (8-29%). For 60Co-gamma, LΔ,D for TEF and SE are approximately 9 and 4 times LΔ,T, respectively, which implies that LΔ,D is more important for high-photon energies. Independent of the electron-fluence and film-model, REFilm is almost constant at low average-LET, rapidly increases and thereafter steadily rises with average-LET. The REFilm-LET curve indicated that LΔ,D is more sensitive to small change than LΔ,T and if it is evaluated for SE, it would even be more appropriate to better describing the dosimeter response induced by photons in terms of ionization-density instead of LΔ,T for TEF, as generally done. Based on these results, once can conclude that the effect of the average-LET on the film's response should be considered when use for clinical-dosimetry using photons and not only the energy.

Dose-average linear energy transfer of electrons released in liquid water and LiF:Mg,Ti by low-energy x-rays, 137Cs and 60Co gamma.

For many years, track-average linear energy transfer (LET), [Formula: see text] has been used to quantify the radiation-induced phenomena in biological and physical systems. However, due to the need for including into the radiotherapy treatment planning system, parameters that are clinical and biologically relevant, a precise knowledge of the dose-average LET, [Formula: see text] becomes essential. Besides, several dosimetric studies have revealed that [Formula: see text] is fundamental to describe the dosimeter's response induced by photons. The most important data sets publicly available for [Formula: see text] of electron generated by photons are those reported for measurements performed in methane-based tissue-equivalent gas. However, comparing to liquid water, the electron spectra generated by low photon energy might not be similar due to the photoelectric effect. Thus, this work aimed at investigating the [Formula: see text] of electron spectra generated in liquid water and LiF:Mg,Ti by ten x-ray beams from 20 kV to 300 kV, 137Cs and 60Co gamma. The results suggest that [Formula: see text] is more sensitive to the surrounding environment than [Formula: see text] and consequently, it might be a more appropriate parameter to quantify the radiation effect and damage in matter induced by photons. Besides, good agreement (6% to 12% differences versus 10% to 15% uncertainties in the experiments) was observed between the data obtained in this work for liquid water and the experimental values published for methane-based tissue-equivalent gas at energies above 60 keV. Whereas at lowest energies, the minimum difference is around 18% which can be associated to the difference between the two media.

Track-average LET of secondary electrons generated in LiF:Mg,Ti and liquid water by 20-300 kV x-ray, 137Cs and 60Co beams.

Electrons generated in matter by photons could be a fundamental basis for an adequate analysis of radiation effects and damage. We have studied separately the 'primary electrons' generated directly by photons from the 'secondary electrons' (SE) produced by electron-electron interactions. In this work, track-average linear energy transfer, [Formula: see text], of SE in LiF:Mg,Ti and liquid water produced by twelve photon energy beams from 20 kV x-ray to 60Co gamma rays have been investigated using the EGSnrc Monte Carlo Code. The exposure of LiF:Mg,Ti in different phantom materials has been considered. Depending on the photon energy, SE represent 40%-90% of the total electron fluence (TEF) between 1 keV and 10 keV, being higher when the photon energy increases. Independent of the medium, [Formula: see text] versus mean photon energy displays a local minimum at around 40 keV, followed by a local maximum at ~80 keV-100 keV. The [Formula: see text] of SE generated by the x-ray beams are of order of 11 keV µm-1 to 19 keV µm-1 in LiF:Mg,Ti and 5 keV µm-1 to 9 keV µm-1 in liquid water which represent 3-5 times those produced by 60Co gamma rays in both media. These values were considerably greater than those of TEF, by factors of 3-8. Furthermore, [Formula: see text] of SE generated in liquid water by 20 kV-200 kV x-rays are similar to those of 76 MeV-120 MeV 3He ions. Contrary to the TEF, where [Formula: see text] were independent of the phantom material, at low photon energies [Formula: see text] of SE was found to be sensitive to the surrounding medium showing higher values within the phantom than in air. This result, which agrees with published experimental results, implies the importance of the SE ionization density for an understanding of dosimeter response induced by photon beams.

Measurement of the absorbed dose distribution near an 192Ir intravascular brachytherapy seed using a high-spatial-resolution gel dosimetry system.

The absorbed dose distribution at sub-millimeter distances from the Best single (192)Ir intravascular brachytherapy seed was measured using a high-spatial-resolution gel dosimetry system. Two gel phantoms from the same batch were used; one for the seed irradiation and one for calibration. Since the response of this gel is energy independent for photons between 20 and 1250 keV, the gel was calibrated using a narrowly collimated (60)Co gamma-ray beam (cross-sectional area ~1 cm(2)). A small format laser computed tomography scanner was used to acquire the data. The measurements were carried out with a spatial resolution of 100 µm in all dimensions. The seed was calibrated at NIST in terms of air-kerma strength. The absorbed dose rate as well as the radial dose function, g(L)(r), was measured for radial distances between 0.6 and 12.6 mm from the seed center. The dose rate constant was measured, yielding a value of Λ = (1.122 ± 0.032) cGy h(-1) U(-1), which agrees with published data within the measurement uncertainty. For distances between 0.6 and 1.5 mm, g(L)(r) decreases from a maximum value of 1.06 down to 1.00; between 1.5 and 6.7 mm, an enhancement is clearly observed with a maximum value around 1.24 and beyond 6.7 mm, g(L)(r) has an approximately constant value around 1.0, which suggests that this seed can be considered as a point source only at distances larger than 6.7 mm. This latter observation agrees with data for the same seed reported previously using Gafchromic film MD-55-2. Additionally, published Monte Carlo (MC) calculations have predicted the observed behavior of the radial dose function resulting from the absorbed dose contributions of beta particles and electrons emitted by the (192)Ir seed. Nonetheless, in the enhancement region, MC underestimates the dose by approximately 20%. This work suggests that beta particles and electrons emitted from the seed make a significant contribution to the total absorbed dose delivered at distances near the seed center (less than 6 mm) and therefore cannot be neglected, given the dimensions of blood vessel walls.

SU-E-T-96: Energy Dependence of the New GafChromic- EBT3 Film's Dose Response-Curve.

PURPOSE: To study and compare the dose response curves of the new GafChromic EBT3 film for megavoltage and kilovoltage x-ray beams, with different spatial resolution. METHODS: Two sets of EBT3 films (lot#A101711-02) were exposed to each x-ray beam (6MV, 15MV and 50kV) at 8 dose values (50-3200cGy). The megavoltage beams were calibrated per AAPM TG-51 protocol while the kilovoltage beam was calibrated following the TG-61 using an ionization chamber calibrated at NIST. Each film piece was scanned three consecutive times in the center of Epson 10000XL flatbed scanner in transmission mode, landscape orientation, 48-bit color at two separate spatial resolutions of 75 and 300 dpi. The data were analyzed using ImageJ and, for each scanned image, a region of interest (ROI) of 2×2cm2 at the field center was selected to obtain the mean pixel value with its standard deviation in the ROI. For each energy, dose value and spatial resolution, the average netOD and its associated uncertainty were determined. The Student's t-test was performed to evaluate the statistical differences between the netOD/dose values of the three energy modalities, with different color channels and spatial resolutions. RESULTS: The dose response curves for the three energy modalities were compared in three color channels with 75 and 300dpi. Weak energy dependence was found. For doses above 100cGy, no statistical differences were observed between 6 and 15MV beams, regardless of spatial resolution. However, statistical differences were observed between 50kV and the megavoltage beams. The degree of energy dependence (from MV to 50kV) was found to be function of color channel, dose level and spatial resolution. CONCLUSIONS: The dose response curves for GafChromic EBT3 films were found to be weakly dependent on the energy of the photon beams from 6MV to 50kV. The degree of energy dependence varies with color channel, dose and spatial resolution. GafChromic EBT3 films were supplied by Ashland Corp. This work was partially supported by DGAPA-UNAM grant IN102610 and Conacyt Mexico grant 127409.

SU-E-T-120: Minimum Absorbed Dose Limit for Gafchromic EBT2 Film Response after Exposure to Low-Energy Photons.

PURPOSE: To investigate the accuracy of the absorbed dose measured with Gafchromic EBT2 film in low-energy photon radiation fieldsMethods: Six EBT2 film (lot # F06110901) pieces (1cm2 ) per dose were exposed to x-rays of 50 kV, 80 kV, 120 kV and 60Co gamma rays from a Leksell Gamma Knife at dose values from 50 mGy to 100 Gy. The x-ray beams were calibrated following the AAPMTG-61 protocol using ionization chambers calibrated at NIST or Wisconsin University depending on the beam quality, while the 60Co gamma was calibrated in water using MD-V2-55 film. Each film piece was scanned once using a HP Scanjet 7650 document flatbed scanner in transmission mode, 48-bit color at 300 dpi spatial-resolution. The data analysis was made through the ImageJ. The measured light intensity for the red channel with its associate standard deviation was used to evaluate the netOD and its standard combined uncertainty. The absorbed dose as a function of the netOD was fitted using the logistic model and the relative combined uncertainties were evaluated for each energy photon beam. RESULTS: EBT2 film response curve depends on the low-energy photons and the degree of energy-dependence is a function of absorbed dose. The absorbed dose relative combined uncertainty as a function of the absorbed dose indicates that the minimum absorbed dose limit is also energy dependent. Lower is the energy photon; more accurate is the measurement at low dose value. This can be explain by the fact that comparing to high energy photons, low energy photons can produce locally enough ionization density to create more color centre in the same film area. CONCLUSIONS: Minimum absorbed dose limit of Gafchromic EBT2 films were found to be energy dependent. The response curve depends on the low-energy photons and the degree of energy-dependence is a function of absorbed dose This work is partially supported by DGAPA-UNAM grant IN102610 and Conacyt Mexico grant 127409.

The response of the new MD-V2-55 radiochromic film exposed to 60Co gamma rays.

The response of the new radiochromic MD-V2-55 film exposed to (60)Co gamma rays has been investigated. A HP Scanjet 7650 document flatbed scanner has been used to evaluate the response. Before studying the film response, the linearity and stability of the scanner were analysed using three calibrated neutral optical filters, finding that the scanner is linear up to an optical density equal to 1 and stable over a period of more than 6 months with a variation of about approximately 0.3% on the optical density for all filters. The warm-up effect of the scanner was also evaluated and a gradual increase of about 1% in the optical density was observed during the first 15 min of successive readings. Six 1 cm(2) pieces of film per absorbed dose were irradiated to doses ranging from 0.5 to 6500 Gy. The response of the film was evaluated at various spatial resolutions using the two scanning modes (transmission versus reflection) of the scanner. The data analysis was made by extracting all three colour channels from the film image. The results show that the spatial resolution does not have an effect on the absolute optical density, but strongly affects the relative uncertainty in the absorbed dose. A linear dependence of the optical density with absorbed dose is observed if the film is read in transmission mode, while in the reflection mode the optical density is described by a second-order polynomial function. The dose dynamic range of the dosimetry system depends significantly on the method used to evaluate the response of the film (about a factor of 3 on the maximum absorbed dose limit for the green and blue colour channels). Finally, a comparison between this new and an old dosimetry system was also made by scanning MD-55-2 in a Microtek ScanMaker E3 scanner under the same conditions as with the HP scanner. It is observed that the new film produces lower uncertainty in the measurement, which means that it is more uniform. Good agreement is obtained between the optical densities measured for both films read in different scanners. The optical density is statistically the same in the useful dose region using both digitizers, which can be attributed to the use of neutral optical filters with known optical density to calibrate the scanners. Based on these results, it is suggested to use calibrated neutral filters to standardize the systems when flat bed document scanners are used to evaluate the film response.

Characteristics of a new polymer gel for high-dose gradient dosimetry using a micro optical CT scanner.

The properties of a new polymer gel with two sensitivities, made specifically for high-dose-gradient dosimetry, were investigated. The measurements were performed at NIST using a 1cmx1cm calibrated (60)Co field, and a 1cm active diameter (90)Sr/(90)Y beta particle source. A high-resolution laser CT scanner was used to quantify the response. The results show that the high-sensitivity gel responds linearly to the absorbed dose for doses from 0.5 up to 15Gy, while the low-sensitivity one is linear up to 225Gy. For both radiation types, the gel response remains stable in time up to a month after the irradiation. The response of the gel was found to have no dose rate dependence for dose rates ranging from 3.7 to 15mGy/s. Within the measurement uncertainty, the gel response is more sensitive for beta particles than high energy photons.

The use of gel dosimetry to measure the 3D dose distribution of a 90Sr/90Y intravascular brachytherapy seed.

Absorbed dose distributions in 3D imparted by a single (90)Sr/(90)Y beta particle seed source of the type used for intravascular brachytherapy were investigated. A polymer gel dosimetry medium was used as a dosemeter and phantom, while a special high-resolution laser CT scanner with a spatial resolution of 100 microm in all dimensions was used to quantify the data. We have measured the radial dose function, g(L)(r), observing that g(L)(r) increases to a maximum value and then decreases as the distance from the seed increases. This is in good agreement with previous data obtained with radiochromic film and thermoluminescent dosemeters (TLDs), even if the TLDs underestimate the dose at distances very close to the seed. Contrary to the measurements, g(L)(r) calculated through Monte Carlo simulations and reported previously steadily decreases without a local maximum as a function of the distance from the seed. At distances less than 1.5 mm, differences of more than 20% are observed between the measurements and the Monte Carlo calculations. This difference could be due to a possible underestimation of the energy absorbed into the seed core and encapsulation in the Monte Carlo simulation, as a consequence of the unknown precise chemical composition of the core and its respective density for this seed. The results suggest that g(L)(r) can be measured very close to the seed with a relative uncertainty of about 1% to 2%. The dose distribution is isotropic only at distances greater than or equal to 2 mm from the seed and is almost symmetric, independent of the depth. This study indicates that polymer gel coupled with the special small format laser CT scanner are valid and accurate methods for measuring the dose distribution at distances close to an intravascular brachytherapy seed.

LET and dose dependence of TLD-100 glow curve after exposure to intermediate-energy ions.

We present results from measurements performed with low fluences (10(5)-10(6) cm(-2)) of 15, 25 and 40 MeV u(-1) carbon, 25 MeV u(-1) oxygen and 40 MeV u(-1) neon ions incident on TLD-100 chips. Dosemeters were arranged individually or in stacks in front of the beam, allowing the study of various linear energy transfer (LET) values simultaneously. The thermoluminescence (TL) total signal is observed to be a linear function of deposited energy. To assess the contribution to the glow curve from the high-temperature peaks, two methods were studied: ratios of peak heights (peak 7 with respect to peak 5), and ratios of areas of the deconvoluted high-temperature peaks with respect to peak 5. The ratios were evaluated as a function of dose, showing in both methods a dependence on LET and ion identity. Some of the studied ions show these ratios to be independent of dose, up to 500 mGy, while for other ions, departures from linearity up to 4.5% +/- 2.5% per 100 mGy are observed at 500 mGy. These results show that, in general, the incident radiation LET is not a parameter that can be deduced from the glow curve.