Small photon field dosimetry using EBT2 Gafchromic film and Monte Carlo simulation

Medical Physics, 2011

Small field dosimetry is prone to uncertainties due to the lack of electronic equilibrium and the use of the correct detector size relative to the field size measured. It also exhibits higher sensitivity to setup errors as well as large variation in output with field size and shape. Radiochromic film is an attractive method for reference dosimetry in small fields due to its ability to provide 2D dose measurements while having minimal impact on the dose distribution. Gafchromic EBT2 has a dose range of up to 40 Gy; therefore, it could potentially be useful for high dose reference dosimetry with high spatial resolution. This is a requirement in stereotactic radiosurgery deliveries, which deliver high doses per fraction to small targets. Methods: Targets of 4 mm and 12 mm diameters were treated to a minimum peripheral dose of 21 Gy prescribed to 80% of the maximum dose in one fraction. Target doses were measured with EBT2 film (both targets) and an ion chamber (12 mm target only). Measured doses were compared with planned dose distributions using profiles through the target and minimum peripheral dose coverage. Results: The measured target doses and isodose coverage agreed with the planned dose within 61 standard deviation of three measurements, which were 2.13% and 2.5% for the 4 mm and 12 mm targets, respectively. Conclusions: EBT2 film is a feasible dosimeter for high dose per fraction reference 2D dosimetry.

Health and Technology, 2018

The relative output factor for small field photon beams with EBT3 (External beam therapy, Gafchromic ™) Radiochromic film were obtained using indigenously developed a program in MATLAB. MLC and JAWS created square small field sizes for dosimetry. To calibrate Radiochromic film, we have cut the film into several small segments of 6 × 6 cm 2. These small pieces of film were irradiated to known doses ranging from 25 to 300 cGy by SAD technique using 6 MV photon beam from Linear accelerator. A Program in MATLAB was written to analyze the scanned film data and all the images of the films were imported into this Program. To validate the accuracy of calibration curve, radiochromic films were irradiated with different field sizes of 10 × 10 cm 2 , 15 × 1 5 cm 2 , 20 × 20 cm 2 to known dose of 200 cGy each by SAD technique. Field sizes were defined using two methods, in the first method fields were defined using X-Y collimator Jaws with the MLC retracted to their maximum position. In the Second method, MLCs were used to define square field sizes of 1 × 1 cm 2 to 5 × 5 cm 2 , 10 × 10 cm 2 and 15 × 15 cm 2. Calibration curve created using gafchromic film show percentage variation between measured and delivered dose within 3% of standard field size. Output factors measured by Gafchromic EBT3 film showed close agreement with those measured using the other detector for field sizes of 2.0 × 2.0 cm 2 and above. EBT3 film has properties such as linear response, energy, and dose-rate independence. It makes it suitable detector for small field dosimetry. Our Indigenous developed MATLAB Program for film dosimetry gave the desired results. From this study, we can conclude that EBT3 film can be used for relative output factors ranging from small field to large fields.

Physica Medica, 2009

The aim of this study was to determine the surface doses using GafChromic EBT films and compare them with plane-parallel ionization chamber measurements for 6 and 18 MV high energy photon beams. The measurements were made in a water equivalent solid phantom in the build-up region of the 6 and 18 MV photon beams at 100 cm SSD for various field sizes. Markus type plane-parallel ion chamber with fixed-separation between collecting electrodes was used to measure the percent depth doses. GafChromic EBT film measurements were performed both on the phantom surface and maximum dose depth at the same geometry with ion chamber measurements. The surface doses found using GafChromic EBT film were 15%, 20%, 29%and 39% AE 2% (1SD) for 6 MV photons, 6%, 11%, 23% and 32% AE 2% (1SD) for 18 MV photons at 5, 10, 20 and 30 cm 2 field sizes, respectively. GafChromic EBT film provides precise measurements for surface dose in the high energy photons. Agreement between film and plane-parallel chamber measurements was found to be within AE3% for 18 MV photon beams. There was 5% overestimate on the surface doses when compared with the plane-parallel chamber measurements for all field sizes in the 6 MV photon beams. ª 2008 Published

IFMBE Proceedings, 2009

Contemporary radiation therapy dose delivery methods like e.g. IMRT and RapidArc needs high-accurate quality assurance dose detection methods. For this purpose GafChromic EBT film is available. No developing process is required like for radiographic film, while the read out of the EBT films can be performed with cheap flatbed scanners. However, before clinical use the film -flatbed scanner (Epson 1680 Expression Pro as well as Epson 10000XL) combination should be tested on their characteristics and achievable overall dosimetric accuracy. Hereto comparison measurements in water with ionization chamber, diode (array) and EBT film were performed for standard and IMRT radiation fields. We observed that absolute dose measurement with EBT film can be performed with an uncertainty up to 1.8% (1 SD) for a single film and 1.4% (1 SD) applying per measurement 2 films simultaneously, and when precautions are taken. Precautions which should be considered are: 1) inhomogeneity effects in film thickness; 2) strictness in film orientation during calibration and film scanning; 3) variation in optical density read out over the scan window of the scanner which depends furthermore on the dose delivered to the EBT film. The first aspect is related to film production processes. The second aspect is related to the orientation of the polymer molecules in the film. The third aspect is related to light polarization capacity of the EBT film polymer molecules in combination with an inherent creation of polarized light in the scanner itself during transport of the light ray from each point in the light tube to the CCD chip.

Radiation Protection Dosimetry, 2018

Gafchromic EBT3 films are applied in proton radiotherapy for 2D dose mapping because they demonstrate spatial resolution well below 1 mm. However, the film response must be corrected in order to reach the accuracy of dose measurements required for the clinical use. The in-house developed AnalyseGafchromic software allows to analyze and correct the measured response using triple channel dose calibration, statistical scan-to-scan fluctuations as well as experimentally determined dose and LET dependence. Finally, the optimized protocol for evaluation of response of Gafchromic EBT3 films was applied to determine 30 × 40 cm 2 dose profiles of the scanning therapy unit at the Cyclotron Centre Bronowice, CCB in Krakow, Poland.

The use of small field sizes is increasingly becoming important in radiotherapy particularly since the introduction of stereotactic radiosurgery and intensity-modulated radiation therapy techniques. The reliable measurement of delivered dose from such fields with conventional dosimeters, such as ionization chambers, is a challenging task. In this work, methacrylic and ascorbic acid in gelatin initiated by copper polymer gel dosimeters are employed to measure dose in 3 dimensions. Field sizes of 5 Â 5 mm 2 , 10 Â 10 mm 2 , 20 Â 20 mm 2 , and 30 Â 30 mm 2 are investigated for a 6-MV x-rays. The results show an agreement with Gafchromic film, with some variation in measured doses near the edge of the fields, where the film data decrease more rapidly than the other methods. Dose penumbra widths obtained with gel dosimeters and Gafchormic film were generally in agreement with each other. The results of this work indicate that polymer gel dosimetry could be invaluable for the quantification of the 3-dimensional dose distribution in small field size.

Reports of practical oncology and radiotherapy : journal of Greatpoland Cancer Center in Poznań and Polish Society of Radiation Oncology, 2010

The aim of this work was to assess the suitability of the use of a Gafchromic EBT2 film for the measurement of anisotropy function for microSelectron HDR (192)Ir (classic) source with a comparative dosimetry method using a Gafchromic EBT2 film and thermoluminescence dosimeters (TLDs). Sealed linear radiation sources are commonly used for high dose rate (HDR) brachytherapy treatments. Due to self-absorption and oblique filtration of radiation in the source capsule material, an inherent anisotropy is present in the dose distribution around the source which can be described by a measurable two-dimensional anisotropy function, F(r, θ). Measurements were carried out in a specially designed and locally fabricated PMMA phantom with provisions to accommodate miniature LiF TLD rods and EBT2 film dosimeters at identical radial distances with respect to the (192)Ir source. The data of anisotropy function generated by the use of the Gafchromic EBT2 film method are in agreement with their TLD me...

Medical Dosimetry, 2013

The electron benefit transfer (EBT) GAFCHROMIC films possess a number of features making them appropriate for high-quality dosimetry in intensity-modulated radiation therapy (IMRT). Compensators to deliver IMRT are known to change the beam-energy spectrum as well as to produce scattered photons and to contaminate electrons; therefore, the accuracy and validity of EBT-film dosimetry in compensator-based IMRT should be investigated. Percentage-depth doses and lateral-beam profiles were measured using EBT films in perpendicular orientation with respect to 6 and 18 MV photon beam energies for: (1) different thicknesses of cerrobend slab (open, 1.0, 2.0, 4.0, and 6.0 cm), field sizes (5 Â 5, 10 Â 10, and 20 Â 20 cm 2), and measurement depths (D max , 5.0 and 10.0 cm); and (2) stepwedged compensator in a solid phantom. To verify results, same measurements were implemented using a 0.125 cm 3 ionization chamber in a water phantom and also in Monte Carlo simulations using the Monte Carlo N-particle radiation transport computer code. The mean energy of photons was increased due to beam hardening in comparison with open fields at both 6 and 18 MV energies. For a 20 Â 20 cm 2 field size of a 6 MV photon beam and a 6.0 cm thick block, the surface dose decreased by about 12% and percentage-depth doses increased up to 3% at 30.0 cm depth, due to the beam-hardening effect induced by the block. In contrast, at 18 MV, the surface dose increased by about 8% and depth dose reduced by 3% at 30.0 cm depth. The penumbral widths (80% to 20%) increase with block thickness, field size, and beam energy. The EBT film results were in good agreement with the ionization chamber dose profiles and Monte Carlo N-particle radiation transport computer code simulation behind the stepwedged compensator. Also, there was a good agreement between the EBT-film and the treatmentplanning results on the anthropomorphic phantom. The EBT films can be accurately used as a 2D dosimeter for dose verification and quality assurance of compensator-based C-IMRT.

Therapeutic Radiology and Oncology, 2019

Background: It is essential for the physicist to quantify the dose of the radiation prior and during cancer treatment to ensure proper execution of the treatment plan. The most common way to measure the dose is utilizing Farmer chamber. The limitation of Farmer chamber is that it only provides dose of a specific point, while the beam dose profile is needed for the planning system. The Gafchromic EBT3 film can provide a quantitative dose profile in a 2D space. The EBT3 film has low energy dependence, similar effective atomic number to water, and self-developing; making it an ideal candidate for dose profile measurement. Methods: Dose profiles of EBT3 was measured, analyzed and compared to semiflex chamber and EDGE detector under same conditions. The EBT3 films were placed at the center of RW3 water phantom (30×30× 30 cm 3) under flattening filter free (FFF) Mode of Linac. The EBT3 films were calibrated by additional RW3 with a Farmer chamber under the stacked RW3 water phantom. Percentage depth dose (PDD) and Matlab were used to fit the dose curve of EBT3 and the results are presented. Results: The penumbra of the measured dose profile through EBT3 film demonstrated little differences when comparing with that of the EDGE detector and noticeable differences when compared to that of the semiflex chamber. For a field size of 2×2 cm 2 , (b=0.08, depth of 5 cm, beam off central axis 0.8 cm), the measured difference of dose profile between EBT3 and EDGE detector is-2.24%. Under the same conditions, the dose profile difference between EBT3 and semiflex chamber is 10.59%. Conclusions: This study shows that EDGE detector has the highest spatial resolution to the dose profile. EBT3 film in combination with solid phantom can quickly sample dose profile in 2D. EBT3 film can be considered to be an excellent quality assurance (QA) tool with accurate dosimetry, a good spatial resolution and tolerable dose uncertainty.

2009

my major advisor for her support, instruction, care and remedial English language in this research. A special thanks goes to my co-advisor, Mr. Taweap Sanghangthum and Mr. Sornjarod Oonsiri, who is most responsible for helping me complete this thesis. He