Assessment of dose heterogeneity in TBI using the thorax of the anthropomorphic Alderson-Rando phantom and TLDs in two different setups

Total body irradiation (TBI) is a treatment modality of radiotherapy. It can be used for immunosuppression of transplanted patients or for metastatic protocols. In this study, TBI was performed using the anthropomorphic Alderson-Rando phantom filled with thermoluminescent dosimeters (TLDs) and irradiated with a 6 MV photon beam from the Elekta linear accelerator in two different setups, one at the hospital São Francisco, BH, MG and second at the hospital Santa Casa in Lavras, MG. The dose distribution in the left and right lungs was estimated, analyzed, and compared with results from the literature. Our results showed that dose homogeneity is more adequate with dual-field irradiation.


INTRODUCTION
Total body irradiation (TBI) is a method of radiotherapy that consists in the total body irradiation of a patient in case of cancer. This technique is used in disseminated tumors or in cases of immunosuppression in patients with leukemia to minimize rejection non-hodgkin's lymphoma. Generally, the total body radiation dose is divided into six fractions at six-hour intervals, twice a day. This procedure is based on the redistribution of the individual cell cycle, taking into account the phase in which cellular radiosensitivity is greatest, i.e. the mitotic phase of the cell. In this way, it is possible to inactivate the patient's tumor cells grow or, in the case of immunosuppression, to perform a satisfactory bone marrow transplant [1][2].

The International Commission on Radiation Units and Measurements recommendations in
Publications 50 and 62 establish guidelines a maximum uncertainty of 5% and a gradient between -5% and +7% in the Planning Target Volume (PTV). Other organizations give values that differ from the limits set by the ICRU, such as the American Association of physicists in Medicine Guidelines (AAPM-report 17) and the Netherlands Commission on Radiation Dosimetry (NCSreport 34), which discuss the difficulties in setting deviations within 10% of the PTV in TBI and also believe that it is consistent to set the dose to the lung between (60-80)% of the planned dose.
One of the main difficulties in TBI is to achieve a homogeneous dose in all irradiated organs [3].
These difficulties in the dose delivered in the organs is due mainly to the different experimental irradiation arrangements that are possible and proposed in TBI in different facilities [1][2][3]. A wide variation of parameters, e.g., positioning, gantry angle, source-to-surface distance (SSD), body mass, available space, and equipment technology, are specific factors of each institute and hinder the reproducibility of this modality.
Zarghani et al show a bibliographic review on the TBI technique taking into account the position, distances, equipment, etc. However, the authors pointed out that the evidence for choosing the most appropriate therapy is indeed the limit to the technological and instrumental availability of each center, which makes it very particular. In particular, the lungs have a low tolerance dose limit, so dosimetry studies are needed to optimize dose distribution in TBI for radiosensitive organs such as the lung [1][2][3][4]. Treating the entire body is a difficult task for many reasons, including the size of the patient compared with the size of the conventional radiation field and the heterogeneous tissue composition of the body, among many other parameters. Usually, in a TBI procedure a new set-up is required due to the reference protocol used in the different radiotherapy centers. Therefore, TBI is a technique that can be adapted to any facility with nonstandard clinics, equipment, and treatment rooms [5]. According to Studinski et al, a variety of strategies have been studied, proposed and reported in literature in order to improving and keeping the uniformity of practice. They reported a survey from different radiotherapy centers in Canada and several inadequacies of the TBI treatment. Another factor considered by the authors, were the field size used and the differences in the dose delivered for same modality between different radiotherapy centers, all these may be resulting in a high dose variation for the same treatment.
Due to the dose heterogeneity, because of the large irradiation fields set, a more detailed study of the dosimetric parameters, using physical phantoms and radiation detectors are needed to promote and/or ensure the quality of TBI treatments. Several papers published in literature used physical phantoms to study the variation of absorbed dose in different organs [7][8][9][10]. The thoracic region of the phantom has always been the subject of various scientific investigations because of its sensitive to ionizing radiation, and in TBI protocol it is important to accurately determine the dose homogeneity in the lungs to minimize the occurrence of adverse events such as pneumonitis [8,9,16].
Thermoluminescence detectors (TLDs) made of magnesium-and titanium-doped lithium material (LiF:Mg, Ti) with a thickness of 0.9 mm and a diameter of 4.5 mm and when exposed to a radiation field, it can detected the absorbed dose. Due to their good response for dosimetric studies, TLDs and phantoms are often used in experiments to estimate and study the dose distribution in several types of treatments [11][12][13][14]. The detectors must be calibrated before its use.
This means that they are exposed to a reference energy beam quality to evaluate the homogeneity response and linearity, among other parameters. They are qualified and quantified for use in a wide range of dosimetric tests, minimizing the uncertainties that arise [15].
In this present study, the Alderson-Rando physical phantom was used to mimic the absorbed dose delivered to the target organs according to two specific protocols. The phantom consists of tissue similar to human tissue in terms of density, elemental composition and anthropomorphic geometry. The phantom contains representative cavities that house radiation detectors and allow dosimetric studies of various diagnostic procedures and therapies using ionizing radiation. Here, the TLDs were calibrated at the calibration laboratory of the Nuclear Technology Development   Table 1 shows the parameters set in the irradiation protocols of each radiotherapy centers.

Experimental Setup Hospital São Francisco
Ten

Experimental Setup Hospital Santa Casa
Three TLDs were inserted in each cavity of the correspondent lungs, given a total of 135 TLDs for both right and left lungs. The phantom was in the supine position under a 115 cm high bed and at 354 cm from the gantry. The ELEKTA 6MV accelerator was used with a maximum field size and with a beam quality toward direct to the middle (slice number 16) of the phantom. The gantry was rotated to an angle of 90º and a total dose of 3228 MU was given in two irradiation fields for 24.7 min in total (i.e., 24.7 min for the right and left sides). Figure 2-b shows the sketch and a picture of the setup arranged to irradiate. It is noteworthy that the TLDs were not removed after the first irradiation field (right side), but the phantom was rotated on the stretcher and then the second irradiation field was performed with an incidence on the left side of the Alderson-Rando.
After irradiation in double field, the TLDs were removed and read with the RISØ reader. The energy deposited in the lungs were analyzed, and the results were compared to the dose measured in the left and right lungs. This deviation was to be expected, since the phantom was irradiated only from the right side and then the higher dose for the right lung was seen.
The experimental arrangement proposed in the Hospital Santa Casa in Lavras, MG seems to be more suitable for the TBI protocol. The results showed a ~2.55% difference between left and right lung, which is within the expected result.  when compare the measured and computed data. The authors found less than 2% variation between the planning calculation and the measured dose. They also reported the best combination of the patient setup, for the lung inhomogeneity being the arms closed to the patient torso. This may compensate the energy deposited in the lungs while irradiate the patient. However, in [1,18,19], it is shown that 10% deviation is acceptable for TBI, and furthermore, in [18] even more significant values are observed. It is extremely difficult to achieve deviations of less than 10% without the use of shielding or protections mechanisms [1].
The results obtained for the experiment executed at Hospital São Francisco yielded difference of approximately ~26% in the right lung, whereas at Hospital Santa Casa, a difference of ~31% was obtained in the right when compared to the prescribed dose. Although no protective device was used in both experimental scenarios, these results are consistent with the recommendations of the TBI guidelines [18,19]. In this context, supine positioning seems to naturally promote better lung protection, which is also reported by [2,17]. It was found that the dose heterogeneity in the left and right lungs of the Hospital São Francisco phantom is directly related to the proposed single-field irradiation method. Contrary to the variation found in the experiment carried out at Hospital Santa Casa, due to the irradiation being performed in a double field.  The central axis of incidence proposed in the setup of Santa Casa, is different due to the angle rotation of the gantry, as shown in Figure 2. Therefore, the observed differences between the doses deposited along the phantom torso can be associated with the positioning at different distances from the beam source, the geometrical incidence of the beam and the anthropometry of the phantom, in this context, uniformities are to be expected.
A trend of the total dose deposited per slices, showed in Figure 3, may contribute to better understand the setups proposed for each facility and even to be objective of chosen a more efficient protocols of TBI treatments. The use of protection devices may be another strategy to be used in which may adjust the total dose along the lung, mainly for the Hospital Santa Casa in which was observed higher homogeneity dose in the different slices. On the other hand, the results estimated for the experiment executed at São Francisco Hospital showed more homogeneity between the slices. However, the differences shown in Table 2 cannot be neglected because of the heterogeneity between the dose deposited on the left and right side of the phantom's lungs. In addition, studies found in the literature presented results for different configurations and confirming the difficulty of commissioning the single protocol for the application of TBI [6,8,10,20].

CONCLUSION
A study of the heterogeneity of the absorbed dose using the TLD insert in the lung slice of the male Alderson-Rando phantom subjected to the TBI treatment modality was performed in two different institutions, the Radiotherapy Treatment Service of the Hospital São Francisco in Belo Horizonte (MG) and Hospital Santa Casa of Lavras (MG), Brazil. Establishing the homogeneity of the dose deposited in the organs is one of the most recommended parameters in the reference documents to carry out an adequate treatment of the patient. Therefore, it is important to emphasize that a single field irradiation does not meet the homogeneity required and is therefore strongly discouraged from a clinical point of view. The results showed that it is possible to obtain a homogeneous dose distribution when the irradiation is performed in two fields. However, it is important to also consider the use of shielding devices for the lungs to minimize the dose to the lungs.

ACKNOWLEDGMENT
The