|Year : 2019 | Volume
| Issue : 8 | Page : 115-122
Dosimetric evaluation of electron total skin irradiation using gafchromic film and thermoluminescent dosimetry
Leila Falahati1, Hassan Ali Nedaie2, Mahbod Esfahani3, Nooshin Banaee4
1 Department of Medical Physics and Biomedical Engineering, Radiation Oncology Research Centre, Faculty of Medicine, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran
2 Department of Radiotherapy Oncology, Cancer Research Center, Cancer Institute; Department of Medical Physics and Biomedical Engineering, Faculty of Medicine, Tehran University of Medical Sciences, Tehran, Iran
3 Department of Radiotherapy Oncology, Cancer Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran, Iran
4 Department of Medical Radiation, Engineering Faculty, Central Tehran Branch, Islamic Azad University, Tehran, Iran
|Date of Web Publication||22-Mar-2019|
Dr. Hassan Ali Nedaie
Department of Radiotherapy Oncology, Cancer Research Center, Cancer Institute, Tehran University of Medical Sciences, Tehran
Source of Support: None, Conflict of Interest: None
Aim of Study: The aim of this study is to evaluate some dosimetry parameters such as uniformity, surface dose, and max depth dose with thermoluminescent dosimetry (TLD) and EBT3 film in total skin electron beam therapy (TSEBT).
Methods: Stationary and rotary methods were set on Varian linear accelerator, Clinac 2100C. To create a radiation field large enough (168 cm × 60 cm) and uniform, the source skin distance was set 400 cm. Electron beam energy was 6 MeV. The skin dose values were obtained in 21 different points on the phantom surface.
Results: The results of dose uniformity in stationary technique were obtained as 10% and 2.6% by TLDs and 6% and 2.3% by films in longitudinal axis and transverse axis, respectively. The measurements at rotational technique by TLDs at the referred conditions showed a homogeneous total field with intensity variation of 10% in the longitudinal axis and 4% at horizontal axis.
Conclusion: Based on the results of this study, stationary techniques are preferred for TSEBT. The main advantage of rotational techniques is reducing the time of treatment. The results also demonstrate that TLD should be routinely used in TSEBT treatment. Due to the high sensitivity of radiochromic films, this type of film was suitable for a wide therapeutic field. Comprehensive treatment to Rando phantom showed that the uniformity is better at the trunk than in the mobile parts of the body; the soles of the feet, perineum region, and scalp vertex should be treated in boost.
Keywords: Cutaneous T-cell lymphoma, radiochromic films, thermoluminescent dosimetry, total skin electron beam therapy
|How to cite this article:|
Falahati L, Nedaie HA, Esfahani M, Banaee N. Dosimetric evaluation of electron total skin irradiation using gafchromic film and thermoluminescent dosimetry. J Can Res Ther 2019;15, Suppl S1:115-22
|How to cite this URL:|
Falahati L, Nedaie HA, Esfahani M, Banaee N. Dosimetric evaluation of electron total skin irradiation using gafchromic film and thermoluminescent dosimetry. J Can Res Ther [serial online] 2019 [cited 2021 Sep 27];15:115-22. Available from: https://www.cancerjournal.net/text.asp?2019/15/8/115/243461
| > Introduction|| |
Total skin electron beam therapy (TSEBT) is mainly used to treat mycosis fungoides (MF) and also in the treatment of the other superficial diseases, including lymphomas and leukemias involving the skin, Sézary syndrome, Kaposi sarcoma, and diffused inflammatory breast cancer.,
MF is a scarce cutaneous T-cell lymphoma (CTCL) that noticeably affects the first few millimeters of the skin. The incidence of MF is nearly three patients per a million in a year, and it will be increased by aging. Males are 2.2 times more likely to develop CTCL than females. The goal of TSEBT is to have a cure or to control the issue by delivering a uniform dose to all parts of the skin, both around the circumference of the patient and from head to foot with an equal penetration and also delivering the prescribed dose without harming any healthy tissues or organs. The therapeutic dose is prescribed for the total skin surface, but it is hard to gain a good homogeneity. Researches show that a homogeneity of around 10% can be obtained on the flattish surface of the body such as the anterior-posterior (AP) abdomen. Instead, there are small areas of the body where the given dose may vary remarkably depending on anatomy and positioning. TSEBT can be implemented in three different treatment techniques, regarding patient setup in respect to the large field electron static: (a) standing patient techniques in which patient takes special positions of treatment in each session, (b) rotary techniques – this treatment is accompanied with simultaneous rotation and irradiation of the skin, and (c) translational techniques in which the patient lies in a motor-driven flat area and is moved relative to a beam at a suitable velocity., The whole doses that will be applied to the skin are usually 30–36 Gy, over 8–10 weeks. The most important features of TSEBT are: using an extended source-skin distance (SSD), wide electron fields, and low energy electrons.
| > Methods|| |
6 MeV electron beam of Varian linear accelerator, Clinac 2100C (Varian Medical Systems, Palo Alto, USA) and a mounted total skin electron irradiation (TSEI) applicator were applied for irradiation.
Dosimeter selection and calibration
Lithium fluoride doped with magnesium, copper, and phosphorus (7-LiF: Mg, Cu, P) known as GR-207A thermoluminescent dosimetry (TLDs) (Fimel, Velizy, France) and EBT3 radiochromic film (International Specialty Products, NJ, USA) were used as dosimetry tools.
TLDs were annealed before they were used for dosimetry to get better stability of their sensitivity and lower fading. The annealing process was done at 240°C for 10 min that was followed by fast cooling. TLDs need to be calibrated before being used. Calibration is done in two processes which ascertain element correction coefficients (ECC) and plotting calibration curve.
Each TLD chip was placed in specific holes on a Perspex slab at the depth of maximum dose irradiated by 60Co beam. A dose of 50 cGy was delivered to TLDs to gain the ECC, which is calculated using Equation 1.
Where <TLR> is the average readings of the total TLDs and TLR j is individual reading. After acquiring ECCs, calibration curve which shows the variation of thermoluminescent response over various dose steps was obtained.
The calibration process was done by 10 dose steps including: 0, 2.5, 5, 7.5, 10, 12.5, 15, 20, 25, and 30 Gy delivered by 6 MeV linac electron beams. Two Perspex slab phantoms with thickness of 10 mm were used to create electron equilibrium conditions. After reading out the TLDs, calibration curve was plotted.
The next step was calibration of radiochromic films. 48 h after irradiation, films were scanned using a flatbed scanner (Microtek scanner; Scan maker 9800XL; Microtek, Carson, USA) in color mode (48 bits per red, green, or blue channel) with a resolution of 100 dpi. The maximum absorption was seen at a wavelength of 632 nm. The calibration curve was plotted with the same procedure used for TLDs. In order to reduce the dependence of the results to the scanner, EBT3 films were scanned in the central region of the scanner. Dimension of each piece of film that was used for irradiating was 1 cm × 1 cm.
A rotating plate with diameter of 60 cm and a height of 81 cm above floor level was fabricated [Figure 1]. The rotating speed is proportional to the input current. The minimum rotational rate was set 3 rpm. The lowest rotating speed was used for patient safety and comfort and also for the patient balance during the treatment.
The accelerator output factor at the depth of maximum dose in water at distance of 100 cm from the source on the central axis was equal to 1 cGy/MU. To measure the output of the accelerator at SSD = 400 cm, the parallel plates ionization chambers known as PPC (PTW, Freiburg, Germany), EBT3 films, and TLD dosimetry were applied.
Percentage depth dose curves
Stationary percent depth doses (PDDs) were measured with advance Markus PPC chamber (PTW, Freiburg, Germany). To measure output factor, PPC and EBT3 were applied, separately. EBT3 films were placed in a field with dimension of 34 cm × 34 cm, at distance of SSD = 400 cm, gantry angle was set 90°, and collimator angle was 45°. TSEBT applicator was mounted on gantry. A plexiglass filter (183 cm height, 122 cm width, and 5 mm thickness) was placed between the gantry and the surface of phantom, at distance of 20 cm from the phantom. The plexiglass filter works as an energy degrader, reducing average beam energy. Therefore, the maximum dose is in the superficial parts of the skin and also improves the dose uniformity. Rotational PDDs were measured with Gafchromic EBT3 films placed between two big polyethylene cylindrical phantoms with a diameter of 30 cm and thickness of 10 cm.
Electron beam energies at treatment plane
The most probable energy and the mean energy can be ascertained from Equations 2 and 3: The practical range (Rp) and the values of R50 can be obtained from the percentage of depth dose curve.
C1 =0.0022 MeV, C2 =1.98 MeV, C3 =0.00025 MeV
C2 =2.33 MeV/cm
Determination of the skin dose
[Figure 2] shows the patient positions during treatment. In this technique, 6 therapeutic fields were aimed in 2 days, and directions of the fields are depicted in [Figure 3].
|Figure 2: Patient positions for 6 filds. At the 1st day, right posterior oblique, left posterior oblique, and AP and at the 2nd day, PA, left anterior oblique, and right anterior oblique fields are treated|
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|Figure 3: The schematic view for direction of 6 filds. At the 1st day, right posterior oblique, left posterior oblique, and AP and at the 2nd day, PA, left anterior oblique, and right anterior oblique fields are treated|
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The electron dose rate of 600 MU/min was selected at SSD of 400 cm and field size of 34 cm × 34 cm at the same distance.
In this type of treatment, the patient is treated from 6 angles: 240°, 0°, 120°, 300°, 180°, and 60°. At the 1st day, the directions of right posterior oblique, left posterior oblique, and AP and at the 2nd day, PA, left anterior oblique, and right anterior oblique fields are treated. [Figure 4] shows the position of the tested points.
|Figure 4: Dosimetric points on the surface of the phantom at the stationary technique|
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Rotational total skin electron irradiation technique
The rotational TSEI (RTSEI) technique is a rotator frame with the patient standing on it. In order to obtain the largest accessible field size of 34 cm × 34 cm, the position of gantry and the collimator are set 90° and 45°, respectively. For this purpose, the TSEBT applicator was installed on the gantry. The rotating platform with a 60 cm diameter was designed. Usually, 3 rpm is selected during patient treatment. Low speed is required for the safety and comfort of the patient. [Figure 5] shows the position of the tested points at the RTSEI technique.
|Figure 5: Dosimetric points on the surface of the phantom and the schematic view of the tested points at the rotational total skin electron irradiation technique|
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Therefore, in both techniques, skin dose values are obtained in 21 different points, which are shown in [Figure 5]. For each point, two TLDs and two pieces of films were used, and the absorbed dose is reported as the mean value of doses obtained by two TLDs and films.
| > Results|| |
Thermoluminescent dosimetry calibration curve
By using ECCs, calibration curve (thermoluminescent response versus absorbed dose) was plotted. As it is shown in [Figure 6], the calibration curve shows a linear behavior in the studied dose ranges.
Film calibration curve
[Figure 7] shows the dose-response curve obtained for EBT3 films. Levenberg–Marquardt algorithm was utilized for optimizing the dose-response curve fits. The relative standard deviation associated with a solely mean pixel value (MPV) was 1.3%. The relative standard deviation for each net MPV was estimated to be within 1.6%, and the optimal fitting error was almost 3.01%.
Percentage depth dose curves
[Figure 8] shows the percentage depth dose curve for stationary technique with field dimensions of 34 cm × 34 cm at SSD of 400 cm. In other words, the output of the accelerator was about 142.2 cGy/3000 MU at the depth of maximum dose and SSD of 400 cm at stationary technique. The percentage of the skin surface dose is 89.5%, and the maximum dose point position is 0.6 cm under the skin.
|Figure 8: Percentage depth dose at stationary technique by PPC and EBT3 films|
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According to percentage depth dose curve, the practical range Rp and the R50 would be 2.05 g/cm2 and 1.52 g/cm2, respectively. According to Equations 2 and 3 and the data obtained from percentage depth dose curve, EP and E0 were obtained as 4.29 and 3.56 MeV, respectively.
[Figure 9] shows the percentage depth dose curve for rotational technique. The rotational PDD has a surface dose of 96.12%, R50 of 1.7 g/cm2, and Rp of 2.6 g/cm2. The most probable energy and the average energy on the surface were used to characterize the electron beam energy. EP and E0 were obtained as 5.38 and 3.96 MeV, respectively.
[Table 1] summarizes the dosimetric properties measured on the phantom.
Photon contamination in AP direction as expressed in Equation 4 is defined as the dose at 5 cm depth in RW3 slab phantom relative to the dose measured at umbilical level. The contaminating contribution of X-rays to absorbed dose was obtained by using percentage depth dose curve.
Therefore, photon contamination at stationary and rotational technique was 1.13 and 0.96, respectively.
Dose distribution measurements showed that relative dose values on phantom surface varied from 106% to 71% with a standard deviation of 2%. Mobile parts of the body – feet, perineum region, and scalp vertex which received the lower dose should be delivered boost dose.
In this way, uniformity does not exceed ±10% in the superior-inferior direction and ±2.6% in the left-right direction. The dosimetric results obtained on the Rando phantom are presented in [Table 2].
|Table 2: The corresponding results of relative dose in stationary technique|
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Uniformity in the midsternal, left midclavicular, and right midclavicular of the Rando phantom using EBT3 film and TLD are shown in [Figure 10].
|Figure 10: Uniformity in stationary technique for midsternal, right midclavicular, left midclavicular|
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Paired t-test results show that the average difference between the data obtained by the EBT3 film and TLD are <2%.
Dose distribution measurements demonstrated that dose values on the phantom surface varied from 105% to 76% with a standard deviation of 3%. In the rotational technique, acceptable validation variation of dose distributions (±10% vertically and ±4% horizontally) are acquired. Uniformity of the rotational technique in the midsternal, left midclavicular, and right midclavicular of the Rando phantom by use of EBT3 film and TLD are shown in [Figure 11].
|Figure 11: Uniformity in rotational technique for midsternal, right midclavicular, left midclavicular|
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Paired t-test results demonstrated that the mean difference between the data measured by the film and TLD was <3%.
[Table 3] shows the measured absorbed dose in rotational technique.
|Table 3: The corresponding results of relative dose in rotational technique|
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| > Discussion|| |
The results of dosimetry [Figure 10] and [Figure 11] show a uniform dose distribution on skin surface of the patient. [Table 4] shows the maximum and minimum relative dose obtained in stationary and rotational technique.
|Table 4: The maximum and minimum relative dose in rotational and stationary technique|
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In stationary technique according to the percentage depth dose curve, surface dose was 91.95%. While in the rotational technique, surface dose was 96.12%. In stationary technique, uniformity did not exceed ±10% vertically and ±2.6% horizontally within the central 168 cm × 60 cm field area. Uniformity results of film dosimetry on the Rando anthropomorphic phantom using stationary technique in previous studies have shown that whole-body dose uniformity in the longitudinal direction is within the range of ±4%–±10%,, and in this study, the best uniformity of film dosimetry is achieved ±6%. For better uniformity, it is recommended to install the TSEBT applicator on the gantry and use a plexiglass spoiler for scattering electrons and beam energy degradation near the phantom surface. An overview of the literature is found in [Table 5].
According to the results of the stationary technique in previous studies with 6 MeV electron beam energy on the RW3 phantom, maximum dose position point was 0.6 cm under skin. The results of this study showed that the maximum dose depth is consistent with other studies.,
According to the results of the rotational technique on polyethylene cylindrical phantom (30 cm in diameter, 10 cm thick) and with 6 MeV electron beam energy, the maximum dose point was 0.3 cm under skin. Previous studies on paraffin cylindrical phantom (30 cm in diameter, 10 cm thick) and with electron energy 6 MeV, the maximum dose was at the skin surface.,
The results of uniformity with film and TLD dosimetry in the Rando anthropomorphic phantom midsternal using the rotational technique were within ±7% and ±10%, respectively. An overview of the literature is found in [Table 6].
Patient positioning and rotation during irradiation in the rotational techniques permitted a reduction in the number of fields and reducing the treatment time.
The results show that the nonuniformity of dose distribution often occurred at mobile body parts such as head, feet, and hands. The dose distribution in some parts of the body such as chest, pelvis, and abdomen is much more homogeneous. Mobile parts of the body – hands, feet, perineum region, and scalp vertex which received the lower dose should be treated in boost. These boosts are delivered with the standard 6 MeV (not high dose rate mode) electron beam at stationary setup, 100 cm distance from the source with an electron applicator installed on the head of accelerator.
The effective atomic number of lithium fluoride (8.5) is very close to the soft tissue (7.1). Therefore, the absorption of energy in the lithium fluoride is roughly equivalent to the energy absorption in soft tissue. Eventually, it is hoped that the measurement technique provided here will also be useful for quality assurance for uniformity of dose distribution in the TSEBT.
According to the results and comparing the results of plane-parallel plate chamber and TLD, the accuracy of TLD was higher than film and the precision was lower. Film is the most suitable dosimeter for relative dosimetry and measuring the dose distribution.
For all 21 measured points, the standard deviation was about 4%. Dose uniformity is achieved by placing a plexiglass filter near the phantom and a proper position during exposure.
| > Conclusion|| |
Based on the results of this study and problems in setup of rotational technique such as patient comfort and safety, the probability of nonuniform rotation of the rotating platform, stationary technique is preferred to TSEBT. The main advantage of rotational techniques is reducing the time of treatment. The results also demonstrate that TLD is a useful tool to monitor actual skin doses and therefore should be routinely used in TSEBT treatment. TLD and quality assurance should be a main part of treatment planning for TSEBT. Due to the high sensitivity of radiochromic films, this type of film was suitable for a wide therapeutic field. Comprehensive treatment to Rando phantom showed that the uniformity is better at the trunk than in the mobile parts of the body; the soles of the feet, perineum region, and scalp vertex should be treated in boost.
This research has been supported by Cancer Research Centre of Tehran University of Medical Sciences and Health Services with grant number 93-02-51-25664.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]