|Year : 2017 | Volume
| Issue : 5 | Page : 159-166
A feasibility study of applying thermal imaging to assist quality assurance of high-dose rate brachytherapy
Xiaofeng Zhu1, Yu Lei1, Dandan Zheng1, Sicong Li1, Vivek Verma1, Mutian Zhang1, Qinghui Zhang1, Xiaoli Tang2, Jun Lian2, Sha X Chang2, Haijun Song3, Sumin Zhou1, Charles A Enke1
1 Department of Radiation Oncology, University of Nebraska Medical Center, Omaha, NE, USA
2 Department of Radiation Oncology, University of North Carolina at Chapel Hill, Chapel Hill, NC, USA
3 Department of Radiation Oncology, Duke University Medical Center, Durham, NC, USA
|Date of Submission||29-Jun-2017|
|Date of Acceptance||21-Sep-2017|
|Date of Web Publication||26-Oct-2017|
Department of Radiation Oncology, University of Nebraska Medical Center, S 42nd Street and Emile Street, Omaha, NE 68198
Source of Support: None, Conflict of Interest: None
Aim: High-dose rate (HDR) brachytherapy poses a special challenge to radiation safety and quality assurance (QA) due to its high radioactivity, and it is thus critical to verify the HDR source location and its radioactive strength. This study explores a new application for thermal imaging, to visualize/locate the HDR source and measure radioactivity using temperature information. A potential application would relate to HDR QA and safety improvement.
Methods: Heating effects by an HDR source were studied using finite element analysis (FEA). Thermal cameras were used to visualize an HDR source inside a plastic catheter made of polyvinylidene difluoride (PVDF). Using different source dwell times, relationships between the HDR source strength and heating effects were studied, thus establishing potential daily QA criteria using thermal imaging.
Results: For an Ir-192 source with a source radioactivity of 10 Ci, the decay-induced heating power inside the source was about 13.3 mW. After the HDR source was extended into the PVDF applicator and reached thermal equilibrium, thermal imaging visualized the temperature gradient of 10 K/cm along the PVDF catheter surface, which agreed with FEA modeling. For the Ir-192 source strengths ranging from 16.9 to 41.1 kU, thermal imaging could verify source activity with a relative error of 6.3% with a dwell time of 10 s, and a relative error of 2.5% with 100 s.
Conclusion: Thermal imaging could be a feasible tool to visualize HDR source dwell positions and verify source integrity. Potentially, patient safety and treatment quality may be improved by integrating thermal measurements into HDR QA procedures.
Keywords: Finite element analysis, high-dose rate brachytherapy, quality assurance, thermal imaging
|How to cite this article:|
Zhu X, Lei Y, Zheng D, Li S, Verma V, Zhang M, Zhang Q, Tang X, Lian J, Chang SX, Song H, Zhou S, Enke CA. A feasibility study of applying thermal imaging to assist quality assurance of high-dose rate brachytherapy. Cancer Transl Med 2017;3:159-66
|How to cite this URL:|
Zhu X, Lei Y, Zheng D, Li S, Verma V, Zhang M, Zhang Q, Tang X, Lian J, Chang SX, Song H, Zhou S, Enke CA. A feasibility study of applying thermal imaging to assist quality assurance of high-dose rate brachytherapy. Cancer Transl Med [serial online] 2017 [cited 2018 Dec 14];3:159-66. Available from: http://www.cancertm.com/text.asp?2017/3/5/159/217261
| Introduction|| |
Brachytherapy is a modality of radiation therapy whereby a radioactive source is placed close to a tumor or tumor bed. High-dose rate (HDR) brachytherapy using the Ir-192 isotope source is widely used to treat breast, prostate, cervical, and endometrial cancers.,,,, Despite advantages such as short treatment duration and remote after-loading, HDR brachytherapy imposes strict and critical safety requirements due to the very high radioactivity involved.,
The Ir-192 source used in HDR brachytherapy, with a half-life of about 74 days, exhibits high source strength (apparent radioactivity), approximately 40.3 kU (10 Ci) for a new source and 16.9 kU (4 Ci) after 3 months (at the end of its clinical life cycle). Owing to high radioactivity combined with various machine or operator errors, detrimental accidents have occurred during HDR. For instance, when an incorrect transfer tube was used, an excess radiation dose was given to normal tissue instead of the intended treatment site. Of even greater significance to public health, there is a risk of environmental leakage if the radioactive source breaks off the source wire and is lost. Indeed, most errors in HDR brachytherapy are related to missing the target or dosing an unintended site and this is largely caused by the use of incorrect transfer tubes or catheters of inaccurate lengths.,,,,,,
The treatment quality of HDR is determined by the accuracy of the radiation dose delivered. Radiation dose is computed using the distance from the radioactive source, its treatment and dwell time, and the source radioactivity; Task Group 43 provides the formalism used in treatment planning systems for dose computations.,, By following a general practice in clinic, the report of the American Association of Physics in Medicine (AAPM) Task Group 56, our current quality assurance (QA) of source positions and dwell times were performed using an optical camera, stopwatch, and radiographic and radiochromic films; air-kerma strength (AKS) was verified with a well-type reentrant ion chamber.
Although the ion chamber provides an absolute dose measurement that is traceable to a primary or secondary standard, an accident was reported involving incorrect chamber calibration coefficients of 2.6% for 2 years. While Ir-192 source activity is verified every 3 months during exchange of the source, source mechanical integrity is assumed without specific checks. Recently, daily source strength checks were proposed using a diode for radiation detection and a modified well-type reentrant chamber. However, source radioactivity checks were largely excluded from daily QA, mainly because of the extra time and efforts to set up films and the well-type reentrant ion chamber.
A byproduct of Ir-192 radioactive decay is heat, the generation of which has been studied previously using well-type ion chambers. With the source positioned in the central aluminum tube for 4 min, the temperature of the tube increased by 1.0 K above the ambient temperature. Therefore, a styrofoam insulator is inserted and wrapped around the central aluminum tube to prevent heat transfer from the Ir-192 source to the collecting volume of the unsealed ion chamber. While the mainstay of HDR QA is based on direct measurements of the ionizing radiation, the heat signal of the radiation source has been less characterized, and has thus largely been ignored.
The measurement of heat generated as a result of radioactive reactions necessitates a special reliability (i.e., redundancy) check in radiation safety and QA, as it uses a different mechanism from ionizing radiation detection. To the best of our knowledge, thermal imaging has heretofore never been reported in HDR QA application. To confirm the temperature differences from thermal imaging (heat effects generated by kinetic electrons from Ir-192 radioactive decay), we implemented finite element analysis (FEA) modeling to simulate temperature distributions, in efforts to interpret the measured thermal imaging herein.
This pilot study investigated the thermal characteristics of a radioactive HDR source, visualized the heat caused by radiation decay using thermal cameras, validated temperature differences with source radioactivity, and explored its potential application in integrating into an existing after-loader system and in improving QA and patient safety in HDR brachytherapy.
| Methods|| |
There was no involvement of human or animals in this study.
Radiation heat within a high-dose rate source
Ir-192 decays into either Os-192 by electron conversion or Pt-192 by β− decay. The latter, releasing free electrons that deposit most of the kinetic energy within the source, is the greatest contributor to the heating power within the source. While the processes of electron conversion and γ-decay due to transition from excited states to the ground state generate γ-photons, those energetic photons deposit negligible heat inside the source as compared to the aforementioned free electron release. The heat loss of the electron energy due to bremsstrahlung radiation inside the source is of relatively small magnitude:
Wherein, KV is 380 and Z is 77 for Ir-192. Therefore, the heating power inside the Ir-192 source is a sum of all the electron kinetic energies, with three channels of β− transition weighted by the corresponding decay probabilities. For an Ir-192 source, the maximum kinetic energy and decay probability for the three decay modes are: E1(P1) = 240 keV (5.6%), E2(P2) = 540 keV (41.6%), and E3(P3) = 670 keV (47.9%), respectively. Because the average energy of the electron in β− decay is approximately one-third of the maximum energy, for an Ir-192 source with a radioactivity A = 10 Ci, the radiation heating power Q is given by:
A VariSource iX HDR remote after-loader was used in this experiment (Varian Medical Systems, Palo Alto, CA, USA). A schematic of the source wire together with a flexible plastic catheter is shown in [Figure 1]. The Ir-192 source wire uses a nickel-titanium alloy (Nitinol) and contains two tandem Ir-192 sources, each with the activity of 5.0 Ci. Each Ir-192 source measures 0.34 mm in diameter and 2.5 mm in length. They are encapsulated in a homogeneous Nitinol wire of 0.59 mm in diameter. The plastic catheter is made of braced polyvinylidene difluoride (PVDF), which has an outer diameter of 3.2 mm and an inner diameter of 1.0 mm. PVDF has a chemical formula of “-(C2H2F2)n-.” It is a special plastic material in the fluoropolymer family, commonly used as insulation on electrical wires owing to its combination of flexibility, low density (1.78 g/cm ), low thermal conductivity (0.2 W/mK), and heat resistance., The PVDF plastic catheter is also labeled the applicator probe by the manufacturer.
|Figure 1: The diagram of Ir-192 source wire used in the VariSource iX high-dose rate remote after-loader. Two Ir-192 sources are in tandem inside a nitinol source wire, and the applicator probe is a thermal insulator made of braced polyvinylidene difluoride|
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[Figure 2] presents the experimental setup of thermal imaging. The HDR after-loader and the PVDF catheter were connected using the same setup as performing daily HDR source dwell position QA. The thermal camera (Ti400 from Fluke Corporation; thermal sensitivity 0.04 K) was positioned 30.0 cm above the applicator probe, with the camera lens focused on the source dwell position. To avoid image artifacts from equipment or software dependent, the measurements were repeated with the same geometry setup, using a different thermal camera (MAG62 from Magnity Electronics; thermal sensitivity 0.04K). The measurements from both cameras were cross-checked to mutually confirm the signal integrity. In addition, temperature measurements using the thermal cameras were verified using a thermocouple (CNX T3000 from Fluke), before and after the HDR source measurements.
|Figure 2: Experimental setup of Ir-192 source radioactivity measurements using a thermal camera. The camera was positioned 30 cm above the polyvinylidene difluoride applicator probe|
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The detectors of thermal cameras use focal plane arrays, which are made of uncooled microbolometers. A bolometer measures the power of incident photons electromagnetic radiationvia the heating of a material with a temperature-dependent electric resistance. The thermal cameras are sensitive to infrared photons with long wavelength ranges (from 7.5 μm to 14 μm), and the temporal temperature variations during heating and cooling processes were recorded by the thermal camera using its video mode with a refresh rate of 9 Hz, analyzed using the software packages ThermoScope ® (Magnity Electronics) and SmartView ® Ver. 4.11 (Fluke Corporation).
The HDR source was then extended into a transfer tube 120 cm in length, shown in [Figure 2], made of a Varian GM11002560 flexible applicator probe (PVDF) and a GM11004160 stump applicator set. The VariSource iX remote after-loader was programmed to position the tip of the Nitinol wire at 119.8 cm from the turret entrance. The measurements were repeated for different source strengths, with AKS ranging from 41.1 to 16.8 kU. According to the source certificate, the apparent activity ranged from 10.2 to 4.18 Ci. Two sources were used in this study. Their source activities were provided by the vendor, and the accuracy of AKS (from the source certificates) is ± 5% for each source. During source changing, we also measured the source activity using our own well-type reentrant ion-chamber to ensure that the nominal AKS were within ± 2% agreement with the certificates. The relative differences between our measurements and the vendor's AKS were for the source ID 15–2216, and 0.2% for the source ID 15–1050, respectively. Heating effects were also observed using a Varian GM11005120 segment cylinder and a Varian AL123016000 tandem and ring applicator.
Finite element analysis
When the PVDF applicator probe is heated by the source wire extended inside, the temperature distribution of the probe follows the heat conducting function:
Wherein, ρ is the density: 1.78 g/cc for PVDF; 2.7 g/cc for aluminum; 6.45 g/cc for nitinol. Cp is the specific heat capacity: for PVDF; 0.90 J/gK for Aluminum; for nitinol. k is the heat conduction constant: 0.2 W/mK for PVDF; 205 W/mK for Aluminum; 18 W/mK for Nitinol. q is the heat flux from the air. n is the boundary surface normal vector, of length unity. With air ambient temperature Text= 291.1 K. The air convective heat transfer coefficient is h = 10.0 W/m 2 K. The heat power at the radiation source surface is 13.3 mW, predicted in equation (2). To match the boundary condition, temperature (T) and heat input (Q) were solved for each spatial position, using the FEA software package, multiphysics.
In this study, we used FEA as a numerical model that offers an approximate solution for the partial differential equations (3) and (4) for heat transfer, and is implemented following the 'divide and conquer' steps: (i) objects are discretized/meshed, and each mesh is assigned its material properties; (ii) the boundary conditions are applied; (iii) linear functions are chosen as an approximation for easy software implementation and fast computing; and (iv) the original functions are formulated as a linear matrix and solved for evaluations.
FEA also allows a comparison of this experiment with previous thermal studies using the well-type reentrant ion-chamber. To rule out the possibility of image artifacts or device malfunction, we simulated two scenarios: Ir-192 catheter inside a PVDF plastic tube, and inside an aluminum tube, respectively. The FEA verification comprised of the following 4 steps. (1) Temperatures of the PVDF plastic catheter were modeled by FEA; (2) maximum surface temperature was verified with thermal imaging measurements; (3) the aluminum tube was modeled using the previous plastic FEA model, with the geometry and material properties of the plastic tube being replaced by those of the aluminum tube; and (4) the maximum surface temperature on the aluminum surface was verified using the results from the previous published report.
| Results|| |
Finite element analysis model
With a heating power of 13.3 mW from a 10 Ci HDR source, FEA simulated three-dimensional (3D) temperature distributions of an applicator probe with the HDR source inside. [Figure 3]a shows the mesh plot of the FEA applicator probe. [Figure 3]b shows the 3D temperature distribution within the probe at thermal equilibrium. [Figure 3]c shows the 2D temperature distribution in the axial plane. [Figure 3]d presents two axial temperature profiles along the surface of the applicator probe (solid line = PVDF applicator probe; dashed line = aluminum tube). FEA simulation showed the following: (a) the temperature of the aluminum surface rose above the ambient temperature by 1K, in agreement with a previous report; and (b) the surface temperature of the PVDF applicator probe would increase by about 10 K. This temperature difference between the PVDF and aluminum occurs mainly because of the differential heat conductivities of the two materials, specifically k = 0.2 W/mK for PVDF and k = 205 W/mK for aluminum. In [Figure 3]d, the heating effects in the PVDF probe were limited to a region from 116.5 to 120.5 cm, while the heating effects in the aluminum tube were spread over virtually the entire length. Of note, uncertainties existed in our FEA modeling. In addition to uncertainties regarding geometrical dimensions and material properties, uncertainties also came from each step of the simulation process and from the final convergence of the FEA solution. Specifically, the simulation results were significantly affected by the uncertainty of the air convective heat transfer coefficient, h. In the PVDF simulation, by setting h to 10, 20, 30, 40,…, 100 W/m 2 K, the surface temperature increase changed to 10, 4.9, 4.2, 3.4,… 1.5 K, respectively.
|Figure 3: Finite element analysis simulation. (a) The mesh plot, simulating the polyvinylidene difluoride applicator probe with a high-dose rate source inside. (b) Three-dimensional temperature distribution of the polyvinylidene difluoride applicator probe. (c) Two-dimensional temperature distribution in the axial plane along the probe. (d) Axial temperature profiles along the surfaces of a polyvinylidene difluoride applicator probe (solid line) and an aluminum tube (dashed line), respectively. The high-dose rate source was centered at the nominal dwell position of 119.5 cm|
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High-dose rate source “visualization”
Thermal characteristics of the HDR Ir-192 source inside a PVDF applicator probe were visualized by a thermal camera. With an HDR Ir-192 source of 41.1 kU, high-temperature differences (about10 K due to radioactive decay heat) were measured at the PVDF probe surface. [Figure 4]a and [Figure 4]b shows the thermal images of the source wire dwelling at the 119.8 cm nominal position, with dwell times of 0.1 s and 200 s, respectively. In our clinic, 119.8 cm was the position selected to check the source dwell position, as part of conventional daily HDR QA. The ambient room temperature was 291.1 K. [Figure 4]a illustrates that the initial surface temperature of the PVDF applicator probe was also 291.1 K. After a 200 s interval dwell time in [Figure 4]b, the maximum temperature at the surface of the PVDF applicator probe was 301.4 K, and this temperature difference of 301.4 – 291.1 = 10.3 K agreed with the FEA simulation in [Figure 3]d and [Figure 4]c. Our thermal image results features a high signal-to-noise ratio: the uncertainty for the thermal imager itself is ~ 0.1 K, but the temperature difference measured with thermal imager was ~ 10 K.
|Figure 4: (a) A thermal image of the applicator probe with a source extended, dwell time = 0.1 s. The Ir-192 source activity was 41.1 kU. The tip temperature is 291.0 K. (b) A thermal image of the applicator probe with the source extended, dwell time = 200 s. The maximum temperature is 301.4 K. (c) The surface temperature simulated by finite element analysis. (d) An optic image shows the setup with a position measuring ruler|
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Heating effects within thermal imaging were further verified by positioning the source at various dwell positions. The source was programmed to dwell at the nominal 119.8 cm position for 20 s, followed by at the 115.8 cm position for another 20 s. Those position parameters were randomly selected only to demonstrate the thermal effects due to different dwell positions. Although the source was repositioned 4.0 cm away from the 119.8 cm nominal position, its previous heating effects still existed at 119.8 cm and were captured by the Fluke thermal camera (a video file attached). The thermal image in [Figure 5] was taken a right before the source was retracted into the after-loader. It displayed two hot spots, corresponding to the two dwell positions. The ambient temperature was 291 K; the surface temperature at the 115.8 cm nominal position reached its maximum of 294.3 K, and the temperature at the nominal 119.8 cm was 293.0 K. These thermal imaging results confirmed that our results were due to radioactive heat generation, and not from image artifacts.
|Figure 5: Heating effect verification using two dwell positions of 119.8 ± 0.1 cm and 115.8 ± 0.1 cm, respectively. The dwell time was 20 s at each position|
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Heating effects for a given source strength
We then studied thermal characteristics of the Ir-192 source using various source strengths. Using the setup illustrated in [Figure 2], HDR sources with strengths ranging from 41.1 to 16.8 kU were extended to the dwell position 119.8 cm at a time t = 30 s. After remaining stationary for 200 s, they were retracted back into the after-loader. Both the heating effects after the source was extended, and the cooling effects after the source was retracted, were recorded using the video modes of the thermal camera. Per the FEA simulation shown in [Figure 3]c, the surface points with the shortest distance to the source center had the maximum temperature. In [Figure 6], the maximum temperature variances due to different source activities are plotted as a function of time. The error bar of ± 0.1 K in [Figure 6] shows the uncertainty of temperature measurement using the thermal camera. During the experiment, the temperature measurements were affected by the power (on/off) of the room air conditioner, and the zero-point shift calibration of the thermal camera, both of which occurred randomly.
|Figure 6: The maximum temperature increased at the polyvinylidene difluoride surface due to different high-dose rate source strengths. The sources were programmed at the given nominal position (119.8 cm), starting at time = 30 s, and dwell time was 200 s. At 230 s, the source was retracted back, and temperature began to decrease|
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Verification of Ir-192 air-kerma strength
[Figure 6] shows that as the source strength increased, the same was true for the temperature differences and heating rates. The relationship between temperature differences and source strength using different dwell times could provide references for future HDR daily QA using thermal measurements. In [Figure 7], the star shows temperature differences for various HDR source strengths with dwell time sequaling 5, 10, 50, and 100 s, respectively. The red line in [Figure 7] shows the fitted curve of the temperature difference (ΔT), and its corresponding source AKS (a). The yellow band covers 95% fitted confidence. The fitted curve is a quadratic polynomial function:
|Figure 7: The relationship of the maximum temperature difference and the source strength, for dwell time at 5, 10, 50, and 100 s, respectively. The yellow band shows the range with 95% fitted confidence band|
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For dwell times equaling 5, 10, and 100 s, the source radioactivities, the fitted activities, and the relative errors are listed in [Table 1]. As the source radioactivity decayed and the dwell time decreased, the relative fitting error increased. When the dwell time was 100 s, the fitted radioactivity had a minimum relative error of 1.3% to 2.5%, with the source activity from 41.1–16.8 kU. If a 10 s dwell time were to be selected for a potential daily QA, the relative error of radioactivity prediction would increase from 3.4%–6.3% as the source decayed. Any measurements showing relative errors larger than 6.3%, or large deviations from the fitted 95% confidence band, should trigger further investigation, especially regarding source integrity. For the simple measurement setup shown in [Figure 2], the surface temperature of the applicator probe could be affected by free airflow controlled by an air conditioner in the HDR vault. We believe that, using a dedicated device with good thermal insulation, the accuracy of radioactivity could be even further improved, which might help to interpret the uncertainty in our measurements (for instance, the relative fitting errors for 41.1 kU were higher than those for 38.5 kU).
|Table 1: Temperature difference ΔT due to different source strengths after dwell times of 5, 10, and 100 s, together with fitted activity with its relative errors, respectively|
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Heating effects with the cylinder and Tandem and Ring applicators
The impact of heating using a Varian segmented cylinder and a Varian Tandem and Ring (TandR) applicator are shown in [Figure 8]. The thermal image of the cylinder in [Figure 8]a is without the source, and [Figure 8]b with the source inserted at the tip (nominal 119.8 cm position) after 200 s. Using subtraction between [Figure 8]a and [Figure 8]b, the temperature increases at the cylinder tip is displayed in [Figure 8]c. The maximum temperature difference was 0.3 K at the surface of the cylinder applicator. Heating effects of the TandR applicator are shown in [Figure 8]d,[Figure 8]e,[Figure 8]f, respectively. The maximum temperature difference was 0.4 K at the surface of the TandR applicator. It is worthwhile to point out that the temperature measurement at the bright metal surface using the thermal camera in [Figure 8]d,[Figure 8]e,[Figure 8]f is not reliable, owing to the low surface emissivity of the titanium metal. Such small temperature differences at the applicators' surfaces are difficult to measure accurately and would be even more challenging when the applicator is inserted into the patient. Given such small differences, we posit that during practical HDR treatment, this minimal tissue heating and its radiobiological effects would be negligible.
|Figure 8: (a) Thermal image of the cylinder without the Ir-192 source. (b) Thermal image of the cylinder with the source at the tip (nominal 119.8 cm position) for 200 s. Heating increased the cylinder surface temperature by 0.3 K, (c) Difference between images a and b, (d) Thermal image of the Tandem and Ring applicator without the Ir-192 source, (e) Thermal image of the Tandem and Ring applicator with source at the tip (nominal 119.8 cm position) for 200 s, without emissivity corrections applied on the metal titanium surface. (f) Difference between images d and e|
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| Discussion|| |
Patient safety and treatment efficacy of HDR brachytherapy depend not only on effective safety interlock systems and QA of radioactivity calibration but also on the temporal and positional accuracy of the source. Layers of redundancy have been devised to address potential risks from various errors in HDR.,, However, events with incorrect dwell positions owing to operator errors still occur from time to time. For example, an event was reported in “First Quarter 2016 Ro-ILS Education” regarding the confusion over dummy markers in a new HDR system causing incorrect dwell position input (https://www.astro.org/RO-ILS-Education.aspx). Herein, we demonstrate that the radioactive HDR source can be visualized using thermal imaging. Such confusion about dummy markers and the source position could be easily resolved using the video of thermal imaging overlaid on, and compared with, optical images (https://youtu.be/p6VnPZRu6X4) [Video 1]. It can also be installed 2 meters away from the patient to verify that the temperature of the transfer tube should remain unchanged during treatment, thereby preventing the human error of using the wrong transfer tubes, when the source could be positioned within the transfer tube. Using the source strength measurement with an error threshold of 6.3%, this new technique could be employed as a redundant safety check to verify source integrity.
AAPM Task Group 56 provides QA guidance to practical HDR in the clinic. Currently, HDR daily QA does not include thermal imaging. It is a hope that thermal imaging can be integrated into future HDR daily QA procedures. For instance, the position verification tool inside the Varian HDR after-loader can be potentially modified for QA using thermal imaging. A thermal camera can be installed next to the optical camera inside the PTV system, focusing on the mechanical ruler. To improve thermal measurement accuracy, the camera and mechanical ruler could be thermally insulated using styrofoam. Had we used the styrofoam insulator, the measurement repeated in [Figure 6] may have yielded more consistent results. Next, contactless infrared thermal sensors could be installed as independent detectors. These thermal sensors measuring 1.6 mm × 1.6 mm (for instance, TMP006 from Texas Instruments, http://www.ti.com/product/TMP006), could be positioned near the mechanical ruler and carry out daily QA checks of source radioactivity. For radiation safety, any modification of the HDR after-loader must only be carried out by the vendors.
The introduction of many new HDR devices and techniques, such as Multi-helix rotating shield brachytherapy,, increases the complexity of radiotherapy delivery and QA. Because many clinics no longer have darkrooms and film processing equipment, it is desirable to have real-time visualization and QA tools to verify treatment positions and source integrity. Having the combined benefits of continuous monitoring and data recording (such as that in an optical camera), and additional dosimetric verification, thermal imaging may be uniquely advantageous for integration into HDR brachytherapy procedures to complement current safety measures. Moreover, recent technological advances make thermal imaging inexpensive to implement, costing roughly $1,000 for a thermal camera with reasonable technical specifications for the proposed HDR use. Since thermal cameras use the same electronic readout systems as optical cameras, radiation side effects on electronic devices should be considered in future applications. In this study, we did not observe any visual artifacts or radiation damages such as “dead pixels” through the experiments that lasted for 3 months.
Heating effects by the HDR source at the surface of a PVDF applicator probe were visualized using thermal imaging. Using a dwell time of 10 s, source strength can be measured with a relative error of 6.3%. This technique would provide an independent and valid safety and QA tool in HDR brachytherapy, with several applications. For daily HDR machine QA, it helps verify source mechanical integrity and radioactivity independently. For patient safety, it can visualize the source dwelling inside the transfer tube in case of accidents owing to wrong transfer tubes or a stuck source. Regarding manufacturing, additional thermal measurement due to HDR source could be utilized for future product improvement.
We would like to thank Dr. Stavros D. Prionas and Dr. Mike Sitter from Varian Medical Systems, Inc., for providing the design information of the source wire and the applicator probes. Our thanks also go to Mr. Rick Kleinau from Fluke Corporation and Dr. Chongfei Shen from Magnity Electronics for technical consultation on thermal imaging.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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