Dosimetric evaluation employing TL and OSL techniques with different luminescent materials for clinical evaluation of extremity doses using electron beams applied to Total-Irradiation-of-Skin treatments

Total-skin electron beam (TSEB) irradiation is used to deliver a homogeneous dose distribution over the entire skin surface of a patient. TSEB dosimetry is quite complex as to the evaluation and me...


Introduction
One of the modalities of external radiation therapy is total-skin electron beam (TSEB) irradiation for delivering a homogeneous dose distribution over the entire skin surface of the patient.TSEB is the treatment of choice internationally for cutaneous T-cell lymphoma, both for curative and palliative purposes.In some anatomical regions, dosage may vary widely owing to the angle of treatment or the skin surface itself, which may be significantly curved and oblique to the plane of treatment. 1,2ecause common external-radiation therapy-planning software does not cover TSEB irradiation, the commissioning and quality assurance of this application must be handled another way.The Hospital Israelita Albert Einstein follows the six-dual-field technique (also known as the Stanford technique) for the commissioning of TSEB treatments, as reported in Ref. 2 by the American Association of Physics in Medicine (AAPM).In this method, dual fields are created by varying the gantry rotation of the linear accelerator ±17° over the horizontal plane with reference to the waistline of the patient, creating a very large field over distance.Patients are treated in a two-day cycle with three dual fields per day-on the first day, in the anterior position and posterior right and left obliques, and on the second day, in the posterior position and anterior right and left obliques.The dual fields minimize x-ray contamination of the central axis and nonuniformity owing to the inverse-square-of-the distance law. 2 Thermoluminescent dosimeters (TLDs), such as LiF:Mg,Ti TLD-100, CaSO 4 :Dy, and microdosimeters of LiF:Mg,Ti TLD-100 (with dimensions of 1 x 1 x 1 mm 3 ), have demonstrated great efficiency in clinical electron-beam dosimetry [3][4][5][6][7] and can be useful tools in detecting errors related to dose application.LiF:Mg,Ti is the TL material most used and studied in radiotherapy because of its near tissue equivalence and overall reliability. 8The Dosimetric Materials Laboratory of the Radiation Metrology Center/IPEN produces and markets CaSO 4 :Dy as a powder and pellets (see Refs. 9-11).CaSO 4 :Dy+Teflon pellets provide an extensive range of linearity to radiation, allowing a great variety of applications. 11,12]13 Another type of material that has gained importance is aluminum oxide (Al 2 O 3 :C) used in optically stimulated luminescence (OSL) dosimeters. 14,15][18][19] The advantages of these dosimeters over TLDs include high sensitivity, faster readout, the possibility of multiple re-readings, and the obviation of heating treatments. 15,16,20he commissioning of the TSEB six-dual-field technique is experimentally described by Platoni et al. (see Ref. 21), as applied at Attikon University General Hospital.The authors used a parallel-plate ionization chamber and LiF:Mg,Ti TLD-100 dosimeters to validate treatment dosimetry.This paper evaluates the performance of different dosimetric materials using TL and OSL dosimetry in the extremity-dose assessment of TSEB treatments, using the six-dual-field technique and an anthropomorphic phantom.

Materials
The dosimetric materials used in this study are shown in Fig. 1 and specified below: The dosimeters were divided into five groups to perform the measurements and one group for background dose control.

Reading systems and bleaching treatments
The TL measurements were performed using a Harshaw 4500 TLD reader in a nitrogen atmosphere.Both LiF:Mg,Ti dosimeters used a time-temperature profile (TTP) with preheating of 80°C and linear heating rate of 5°C/s with a maximum temperature of 400°C. 8 maximum temperature of 300°C was used. 8Each reading cycle was performed within ~40 s.The samples were thermally treated before and after irradiation.The LiF;Mg,Ti detectors were annealed in a Vulcan 3-550 PD furnace at 400 °C for one hour, followed by rapid cooling to ambient temperature, then placed in a preheated Fanen 315-IEA 11200 surgical heater for two hours at 100 °C.The CaSO4:Dy + Teflon dosimeters were annealed in a Vulcan 3-550 PD furnace at 300 °C for three hours. 8,10he OSL measurements of the TLD-500 were performed in a RISØ TL/OSL-DA-20 reader equipped with the standard bialkali EMI 9235QB photomultiplier tube, a 90% intensity blue-LED light source used for OSL stimulation, and a Hoya U-340 filter of 7.5 mm thickness and 45 mm diameter. 14Each reading cycle was performed in 50 s.Before and after each irradiation, the samples were optically annealed for 24 h using an Ourolux 1.3 W power lamp composed of 30 blue LEDs.The residual signal was evaluated after each bleaching cycle.

Irradiation systems
A 4π geometry 137 Cs gamma irradiator (activity = 38.11GBq on 17 April 2014) from the Dosimetric Materials Laboratory of IPEN-LMD/IPEN was used to test the repeatability of all dosimeters used.
Clinical measurements were carried out using the Varian Clinac 23EX linear accelerator (manufactured by Varian Medical Systems, Inc., Palo Alto) at the radiotherapy center of the Hospital Israelita Albert Einstein (HIAE).For TSEB therapy, high dose-rate (HDR) total-skin electron mode (HDTSe) was selected, as well as the monitor units (MU) for dose delivery.The nominal energy of the electron beam produced is 6 MeV.The collimator was opened to 36 x 36 cm after the insertion of a specific tray dedicated to TSEB practice.

Dosimetric characterization
The four types of dosimeters were characterized for the 6 MeV energy electron beam from the Clinac 23EX.For each individual pellet, the individual calibration factor (i.e., signal/dose) was determined by its sensitivity to this radiation.
Irradiation with 150 MU and 250 MU (147.6 cGy and 246.0 cGy, respectively) was performed by positioning all dosimeters between two 0.3-cm thick poly(methyl methacrylate) (PMMA) plates and at a depth of 1.30 cm obtained with a solid water bolus to put dosimeters under electronic equilibrium conditions.A field size of 20 x 20 cm 2 was used with a source-surface distance (SSD) of 100 cm and a 5-cm solid water bolus for electron backscatter.These doses were chosen because of the documented linearity of response of all the dosimetric materials 8,10,14 and the practical applicability to TSEB irradiation.The characterization setup is shown in Fig. 2.

Experimental setup and irradiation
The purpose of TSEB treatment is to distribute the dose as evenly as possible over the extent of the patient's skin surface.To obtain experimental data on absorbed doses in the extremities of patients, real conditions of TSEB treatment were simulated by using an Alderson Rando anthropomorphic phantom arranged on a turntable and a large 0.5 cm thick PMMA sheet to flatten the electron fields (Fig. 3).The TL and OSL dosimeters were placed where anatomical extremities would be located and over the abdomen as a reference point (z Ref ). 2,21The measurements were performed on alternate days, as reported by AAPM, 2 allowing greater study of sub-and over-dosage.The LiF:Mg,Ti TLD-100 dosimeters were used as reference dosimeters because of their composition properties, 8 results for 6 MeV electron-beam dosimetry, 6,7 and similar validated TSEB applications. 21he experimental results of the absorbed doses are presented as the average of three dosimeter measurements, and the error bars are the standard deviation of the mean.All calculations were performed using Microsoft Excel 2016 software.Graphics were plotted using OriginPro 8.1 and measurements were expressed in cGy, as is commonly used by medical physicists in clinical applications.The agreement between the four dosimetric materials in each point of measurement can be readily observed in Fig. 4.

Discussion
TSEB dosimetry is quite complex in the evaluation and measurement of absorbed dose in the cutaneous region.Solid-state luminescent (TL and OSL) dosimeters have demonstrated excellent results in assessing dose uniformity in the skin.The extremities are exposed in almost all treatment positions, resulting in a possibility of overdose that may vary up to 20% over the dose in the reference point. 2 Similar results were obtained with the experimental dosimetry presented.These variations may be explained by errors in positioning, movement, and the positions of the dosimeters, since the Stanford technique is a two-day cycle.
It is found that for the abdominal region (z Ref ), agreement with the prescribed 210 cGy dose was 97.62% for LiF, Mg, Ti TLD-100, 94.38% for µLiF, Mg, Ti, 97.61% for CaSO 4 :Dy, and 89.90% for Al 2 O 3 :C.The results that best matched those obtained by the TLD-100 were the CaSO 4 :Dy measurements, as indicated in Fig. 4. The results obtained with μLiF:Mg,Ti were slightly different from those with the TLD-100, even though they only differ by their dimensions.This can be explained by the lower TL intrinsic efficiency and reduced dimensions of μLiF:Mg,Ti. 7The Al 2 O 3 :C presented higher uncertainties; the percentage differences, however, remained acceptable.

Conclusions
The TL dosimeters showed good results in the assessment of extremity TSEB treatment doses, with percentage differences within the expected range.Variation in the results obtained for extremity doses can be explained by errors of positioning and movement of dosimeters.The Al 2 O 3 :C OSL dosimeters presented higher uncertainties, but results were close to those of TLDs.

Fig. 1 .
Fig. 1.The four luminescent dosimeters used in this study.

Fig. 2 .
Fig. 2. Positioning the dosimeters to perform characterization.(a) Dosimeter placement between two PMMA plates and (b) irradiation setup in the Varian Clinac 23EX at HIAE.

Fig. 3 .
Fig. 3. TSEB experimental setup of irradiation using the Alderson Rando anthropomorphic phantom.Distance a, between the phantom and field isocenter, is 3 m; distance b, between the phantom and the PMMA sheet, is 50 cm.

Fig. 4 .
Fig. 4. Agreement between TL and OSL dosimeters measurements for each point studied.

Table 4 .
Experimental results using Al 2 O 3 :C OSL dosimeters.Relation of each position with the results of the TLD-100.Int.J. Mod.Phys.Conf.Ser.2018.48.Downloaded from www.worldscientific.comby GERMAN ELECTRON SYNCHROTRON @ HAMBURG on 05/09/19.Re-use and distribution is strictly not permitted, except for Open Access articles. a