Measuring energy response for RTQA Radiochromic film to improve quality assurance procedures

Characterization of Gafchromic™ film response against radionuclide activity

PURPOSE

In this study, we used Gafchromic™ film XR-QA2 and RT-QA2 to characterize the film energy response against various radionuclides. We introduce a neutron depletion theoretical model that can describe film response as a function of cumulated activity. The film response was investigated with respect to different backscatter media such as polystyrene, perspex, lead and corrugated fibreboard carton (CFC). The sensitivity of the two types of film to different energies was also studied. Lastly, a film stack method was tested to allow the user to obtain sequential, cumulative doses at different time points.

METHODS

Pieces of Gafchromic™ film XR-QA2 and RT-QA2 were exposed to Am-241, Cs-137, Tc-99m, and I-131 to obtain various cumulative activities. After 24 h, each film piece was digitized by scanning it with an Epson Perfection V330 flatbed scanner to obtain 48-bit RGB TIFF images. Afterwards, each image was processed with the Image J software package. The film response was fitted to a theoretically derived function based on the neutron depletion model and the Beer-Lambert Law and compared with an existing fitting function. Layers of the film were also placed together and irradiated with the above-mentioned radionuclides to investigate the possibility of increasing the sensitivity of the film as a dosimeter. The energy response of the two types of film was investigated by irradiating pieces of film with different photon energies.

RESULTS

The theoretical response model fits OD vs cumulative activity accurately. XR-QA2 film shows good energy film response by using CFC as a backscatter material when using radionuclides. From the results, it is also evident that XR-QA2 is more sensitive to low energy gamma rays than RT-QA2. Its OD sensitivity can be increased by 2 ± 0.2 when using a double layer film and by 2.8 ± 0.3 when using a triple-layer film. By using a film stack, the experimental time can be decreased by using the second-order polynomial relationship obtained to relate the stacked film data to the single film data.

CONCLUSIONS

The neutron depletion theoretical model is accurate and contains less free parameters than higher-order polynomial fits. The Gafchromic™ XR-QA2 film is also better to use in nuclear medicine because of its higher sensitivity. The sensitivity of the film as a dosimeter can also be increased by using multiple layers of film. Experiment times can also be decreased by using the film stack method.

The relation between XR-QA2 and RT-QA2 GafchromicTMfilm optical density and absorbed dose in water produced by radionuclides

Purpose. In this study, Monte Carlo (MC) simulations were done to relate the dose-response of the film to that in water. The effect of backscattering materials (PMMA, lead, polystyrene, and air) was investigated on its influence on film density for radionuclides including Am-241, Tc-99m, I-131, Cs-137.Methods. A BEAMnrc MC simulation was designed to score a phase-space file (PSF) below the container of the radionuclide under consideration to use as an input file for the subsequent DOSXYZnrc MC simulation. The geometry of the container holding the radionuclide was built using the component modules available in BEAMnrc. BEAMDP was used to investigate the container effect on the radionuclide spectrum as well as the fluence. The DOSXYZnrc simulation produced the absorbed dose in XR-QA2 and RT-QA2 Gafchromic^TMfilms. The DOSXYZnrc simulations were repeated for the Gafchromic^TMfilm now replaced with water to get the absorbed dose in water. From these results, conversion factors for the dose in water to the film dose for the different radionuclides, Am-241, Tc-99m, I-131, and Cs-137 were obtained. The actual film dose was calculated using the specific gamma exposure constant (Γ) at a distance of 50 cm for a point source approximation. From the BEAMnrc simulations, the particle fluence was extracted from PSFs to correct for the fluence at 0.1 cm below the sources from the fluence 50 cm away since the inverse square law will not apply to finite-size sources. The absorbed dose profiles in the film were compared to the absorbed dose profiles from the MC simulations.Results. A fitting function based on the neutron depletion model fits the optical density versus absorbed film dose data well and can be used as a calibration tool to obtain the film dose from its optical density. Lead as a backscatter material results in a higher optical density change but a lower absorbed dose. The XR-QA2 Gafchromic^TMfilm is more sensitive than the RT-QA2 Gafchromic^TMfilm, showing a more responsive optical density (OD) change in the energy range of radionuclides used in this study. Conversion factors were determined to convert the dose in water to the dose in Gafchromic^TMfilm. The Am-241 and I-131 simulated absorbed dose in the film to dose in water does not fluctuate as much as the simulated absorbed dose in film and water when using Tc-99m and Cs-137. Validation was shown for the comparison of the film and MC simulation absorbed dose profiles.Conclusions. MC BEAMnrc simulations are useful to simulate radionuclides and their containers. BEAMDP extracted energy spectra showed that the radionuclide containers produced a Compton effect on the energy spectra and added filtration on the lower spectral photon components. Extracted fluence ratios from PSFs were used to calculate the absorbed dose value at 0.1 cm distance from the source. By using the fit function, the dose in the film can be determined for known optical density values. The effect of the backscatter materials showed that the XR-QA2 Gafchromic^TMfilm results in higher optical density values than the RT-QA2 Gafchromic^TMfilm. The absorbed dose in both the films is comparable but not for a radionuclide such as Am-241 with an activity of 74MBq. The lead backscatter material showed to be the most prominent in optical density enhancement, and the air equivalent material was the least prominent. The XR-QA2 Gafchromic^TMfilm is the most sensitive and will be the best option if working with low energies. The absorbed dose in the XR-QA2 Gafchromic^TMfilm also showed a good comparison to the absorbed dose in water for the Am-241 radionuclide with an activity of 74MBq. The absorbed dose in the films compares well to the MC simulated doses.

Dose distribution at the Bragg peak: Dose measurements using EBT and RTQA gafchromic film set at two positions to the central beam axis

AIM

To evaluate the impact of radiochromic film positioning relative to the central beam axis (CAX) in proton beam therapy. Secondarily, to compare the dosimetric measurements obtained by RTQA and EBT film and to compare these to the doses calculated by the treatment planning system (TPS).

METHODS

The EBT and RTQA dosimetric radiochromic films were immersed in a water phantom and irradiated with a proton beam. The films were placed parallel to the CAX and at a 5° angle on the horizontal plane to assess the effect of film inclination on Bragg peak profiles. Calibration was performed by irradiating small pieces of film at doses ranging from 0.0 Gy to 3.5 Gy in increments of 0.5 Gy. The TPS was used to create treatment plans for two different geometrical targets (cylindrical and cuboidal). After irradiation, all film pieces were scanned on a flatbed scanner and red channel data were extracted from the 48-bit RGB images using ImageJ, Photoshop, Origin8, and Excel software. The dose distributions from the irradiated films were compared to the dose obtained from the TPS. Bragg peak profiles were abstracted from the irradiated films and compared.

RESULTS

The dosimetric measurements obtained by both EBT and RTQA positioned at a 5° to the CAX closely matched the dose calculated by the TPS for the cylindrical target. In contrast, dose distributions measured in the cuboidal targets were less precise. Gamma index (GI) values (3%/3 mm acceptance criteria for isodose >90% of dose) were 99.8% and 93% for EBT film placed at a 5° angle versus 47.1% and 80.8% for EBT film parallel to the beam. The dosimetric measurements in RTQA film positioned parallel to the CAX showed GI values with <27% agreement with the TPS-calculated dose.

CONCLUSION

Our finding show that RTQA film can be used to accurately measure doses in the proton beam at the region of Bragg peak; however, to obtain the most accurate readings, the film should be positioned at a small angle to the CAX.

Acceptance, commissioning and clinical use of the WOmed T-200 kilovoltage X-ray therapy unit

OBJECTIVE

The objective of this work was to characterize the performance of the WOmed T-200-kilovoltage (kV) therapy machine.

METHODS

Mechanical functionality, radiation leakage, alignment and interlocks were investigated. Half-value layers (HVLs) (first and second HVLs) from X-ray beams generated from tube potentials between 30 and 200 kV were measured. Reference dose was determined in water. Beam start-up characteristics, dose linearity and reproducibility, beam flatness, and uniformity as well as deviations from inverse square law were assessed. Relative depth doses (RDDs) were determined in water and water-equivalent plastic. The quality assurance program included a dosimetry audit with thermoluminescent dosemeters.

RESULTS

All checks on machine performance were satisfactory. HVLs ranged between 0.45-4.52 mmAl and 0.69-1.78 mmCu. Dose rates varied between 0.2 and 3 Gy min(-1) with negligible time-end errors. There were differences in measured RDDs from published data. Beam outputs were confirmed with the dosimetry audit. The use of published backscatter factors was implemented to account for changes in phantom scatter for treatments with irregularly shaped fields.

CONCLUSION

Guidance on the determination of HVL and RDD in kV beams can be contradictory. RDDs were determined through measurement and curve fitting. These differed from published RDD data, and the differences observed were larger in the low-kV energy range.

ADVANCES IN KNOWLEDGE

This article reports on the comprehensive and novel approach to the acceptance, commissioning and clinical use of a modern kV therapy machine. The challenges in the dosimetry of kV beams faced by the medical physicist in the clinic are highlighted.

Monte Carlo-based investigation of absorbed-dose energy dependence of radiochromic films in high energy brachytherapy dosimetry

Relative absorbed dose energy response correction, R, for various radiochromic films in water phantom is calculated by the use of the Monte Carlo‐based EGSnrc code system for high energy brachytherapy sources 60Co, 137Cs, 192Ir and 169Yb. The corrections are calculated along the transverse axis of the sources (1‐15 cm). The radiochromic films investigated are EBT, EBT2 (lot 020609 and lot 031109), RTQA, XRT, XRQA, and HS. For the 60Co source, the value of R is about unity and is independent of distance in the water phantom for films other than XRT and XRQA. The XRT and XRQA films showed distance‐dependent R values for this source (the values of R at 15 cm from the source in water are 1.845 and 2.495 for the films XRT and XRQA, respectively). In the case of 137Cs and 192Ir sources, XRT, XRQA, EBT2 (lot 031109), and HS films showed distant‐dependent R values. The rest of the films showed no energy dependence (HS film showed R values less than unity by about 5%, whereas the other films showed R values higher than unity). In the case of 169Yb source, the EBT film showed no energy dependence and EBT2 film (lot 031109) showed a distance‐independent R value of 1.041. The rest of the films showed distance‐dependent R values (increases with distance for the films other than HS). The solid phantoms PMMA and polystyrene enhance the R values for some films when compared the same in the water phantom.

PACS number: 87.53.Jw

Wax boluses and accuracy of EBT and RTQA radiochromic film detectors in radiotherapy with the JINR Phasotron proton beam

AIM

To present the results obtained using radiochromic films EBT and RTQA 1010P for the reconstruction the dose distributions for targets irradiated by proton beam and modified by wax boluses.

BACKGROUND

In Medico-Technical Complex at the Joint Institute for Nuclear Research in Dubna implemented technology of wax boluses.

MATERIALS AND METHODS

Wax boluses are easier to make and they give better dose distributions than boluses made from modeling clay previously used at our center. We irradiated two imaginary targets, one shaped as a cylinder and the other one as two cuboids. The evaluated calibration curve was used for calculation of the dose distributions measured by the EBT and RTQA radiochromic film. In both cases, the measured dose distributions were compared to the dose distributions calculated by the treatment planning system (TPS). We also compared dose distributions using three different conformity indices at a 95% isodose.

RESULTS

Better target coverage and better compliance of measurements (semiconductor detectors and radiochromic films) with calculated doses was obtained for cylindrical target than for cuboidal target. The 95% isodose covered well the tumor for both target shapes, while for cuboidal target larger volume around the target received therapeutic dose, due to the complicated target shape. The use wax boluses provided to be effective tool in modifying proton beam to achieve appropriate shape of isodose distribution.

CONCLUSION

EBT film yielded the best visual matching. Both EBT and RTQA films confirmed good conformity between calculated and measured doses, thus confirming that wax boluses used to modify the proton beam resulted in good dose distributions.

Variations in daily quality assurance dosimetry from device levelling, feet position and backscatter material

Daily quality assurance procedures are an essential part of radiotherapy medical physics. Devices such as the Sun Nuclear, DQA3 are effective tools for analysis of daily dosimetry including flatness, symmetry, energy, field size and central axis radiation dose measurement. The DQA3 can be used on the treatment couch of the linear accelerator or on a dedicated table/bed for superficial and orthovoltage x-ray machines. This device is levelled using its dedicated feet. This work has shown that depending on the quantity of backscatter material behind the DQA3 device, the position of the levelling feet can affect the measured central axis dose by up to 1.8 % (250 kVp and 6 MV) and that the introduction of more backscatter material behind the DQA3 can lead to up to 7.2 % (6 MV) variations in measured central axis dose. In conditions where no backscatter material is present, dose measurements can vary up to 1 %. As such this work has highlighted the need to keep the material behind the DQA3 device constant as well as maintaining the accuracy of the feet position on the device to effectively measure the most accurate daily constancy achievable. Results have also shown that variations in symmetry and energy calculations of up to 1 % can occur if the device is not levelled appropriately. As such, we recommend the position of the levelling feet on the device be as close as possible to the device so that a constant distance is kept between the DQA3 and the treatment couch and thus minimal levelling variations also occur. We would also recommend having no extra backscattering material behind the DQA3 device during use to minimise any variations which might occur from these backscattering effects.