In teletherapy, the radiation issued by the applicable device is targeted at the body region to be treated, while patient is lying down, and sessions are performed daily. Cobalt therapy machines (⁶⁰Co) and linear accelerators are the equipment used.

The distribution of doses refers to the dose values at points surrounding and inside the irradiated volume. In radiotherapy, a detailed analysis of the dose distribution allows drawing conclusions with respect to the preservation of healthy tissues near the target volume. Thus, it is important to make sure that the doses prescribed by physicians focus on the target volumes, so that healthy tissues do not receive doses beyond the tolerance limits recommended in literature.

The result of measurement is a dosimetric amount expressed by a number associated with its unit. There are several types of radiation dosimeters, but not all meet the necessary requirements for the intended application. Thus, it is necessary to choose the most appropriate one, taking into account the requirements of the measurement situation. Among the types of dosimeters on the market, we shall highlight: ionization chamber, radiographic film, radiochromic film, thermoluminescent dosimeter (TLD) and semiconductors (diode) [6] .

The thermoluminescent dosimeter is a device that measures doses resulting from the thermoluminescence phenomena, which occurs when there is light emission after the excitation of a material medium by thermal energy. Light emission may result by fluorescence, when light emission occurs spontaneously, and by phosphorescence, when it is necessary to apply energy for it to be seen. TL dosimeters can be found in various shapes and compositions, as shown in Figure 1.

Among the main TL dosimeters, we shall highlight: LiF: Mg, Ti; CaF₂: Dy; CaSO4: Dy; CaF₂: Mn; LiF: Mg, P, Cu; and Li₂B₄O₇: Mn. The chip dosimeter, composed of lithium fluoride, is generally the most widely used in dosimetry, as it is easily manipulated, and presents good repeatability and reproducibility conditions.

After the initial heat treatment, these TLDs were inserted in coupelles to be read, organized in a predefined reading order and stacked with the assistance of a lift. A twelve hour interval to perform new radiations was taken into account. Once stacked, so that measures found in each TLD could be tracked, they were placed in a Fimel PCL3 reader. As such, the BG values of the 200 TLDs examined were determined. The mean reading of the “zero dose” was obtained through the mean of the readings of the 200 TLDs by Equation (1). The measurement uncertainty associated with this parameter was the standard deviation of the 200 readings.

The reader’s charger was able to stack up to 93 coupelles, or 93 chips (TLDs). The reader had a system which included a rotating disk and tweezers that removed every coupelle from the charger and took it to oven no. 1, to oven no. 2, to the photomultiplier valve and, finally, placed it in a disposal charger. In oven no. 1, the coupelle was submitted to a temperature of 170˚C for about 25 seconds and, in oven no. 2, to a temperature of 300˚C for about 25 seconds. At this point, the photomultiplier valve recorded the light photons resulting from the release of electrons that were caught up in the TLDs’ traps.

To determine the relative sensitivity of TLDs, the homogeneity of the field was ensured, by irradiating them at the same time, between solid water plates, as illustrated in Figure 3. A distance of 100 cm between the source and the plate surface was established, together with a 20 cm × 20 cm irradiation field and a reference dose of 1 Gy.

The relative sensitivity of each TLD is defined by Equation (2), where: is the number of evaluated TLDs; is the number of irradiations performed; and, is the reading of each TLD, after deducting the corresponding BG.

To evaluate the linearity, the 200 TLDs were divided into 10 batches initially identified by letters, and each batch received the following doses: (10, 50, 100, 150, 200, 250, 300, 350, 400 and 500) cGy. Each batch was irradiated three times and the dose-response relation (RDS) was calculated for each batch.

Using the Excel program, a graph of the Linearity Factor, , versus the dose received for each batch, was generated with the correction of the readings of TLDs, in relation to response linearity. After completion of the characterization stage, TLDs were distributed throughout the Alderson Rando phantom, where the male breast and critical organs could be found in slices 13 to 20.

Among the anthropomorphic phantoms used to reproduce the scattering and absorption characteristics of the body or body parts, when subjected to a radiation field, the best known is the Alderson Rando phantom (Figure 4) which consists of a skeleton covered with materials equivalent to tissues, lungs and soft tissues, and which follows the ICRU-44 standards [8] .

This phantom is 175 cm tall and weighs 73.5 kg, being transected-horizontally into 2.5 cm thick slices. Each slice has holes which are plugged with tissue equivalent pins which can be equivalent to: bones, soft-tissues or lungs. Such pins may be replaced by powder or chip TLD pins. TLDs were placed throughout the left side of the phantom between slices 13 and 20, as per Figure 5, where 5 mm-diameter holes with TLDs can be found in 3 cm × 3 cm matrix.

The planning system used to simulate the treatment of male breast cancer, following protocols for treatment of female breast cancer, was Prowess, Version 5.11, which is available at COI. Prowess has an algorithm that can manage information about doses, monitor units, image acquisition and fusion, as well as transport planning directly to the linear accelerator in which treatment shall occur.

Planning was done with the images of the Alderson Rando phantom taken by a Siemens Emotion Duo CT scanner installed at COI. The treatment consisted in the irradiation of two opposing parallel fields with the prescription of 200 cGy at 100% of the isodose line. During the first stage of irradiation, the head, also known as gantry was at 303˚, with a SSD (source-surface distance) of 88.1 cm, a field of 10 cm × 12 cm and a 240.7 MU monitor unit with 30˚ filter. In the second stage, the gantry rotated automatically to 123˚, with a SSD of 88.5 cm, a field of 10 cm × 11 cm and a 241.8 MU monitor unit, using the same filter.

For purposes of comparing the planned results with the ones obtained by TLDs, it was necessary to incorporate, in the planning system, some points of interest, i.e., specific TLD location, as shown in Figure 6. Thus, it was possible to generate a report where the doses planned were determined, at the points of interest.

After determining the relative sensitivity of each TLD, we found 64 TLDs which repeatability was of up to ±0.5%

of standard deviation of the mean, 155 TLDs with up to ±1% of standard deviation of the mean, 197 TLDs with up to ±2% of standard deviation of the mean and three dosimeters which average standard deviation was above 3%, within the specifications of the manufacturer.

The 155 dosimeters that showed standard deviations of the mean of up to ±1% were selected for measurements.

To determine the response of TLDs, as a function of linearity with respect to dose, a dose range between 50 cGy and 500 cGy was taken into account. The ratio between delivered dose and TLD response was calculated for each dose chosen within the range of creation of the calibration curve. This ratio was normalized in relation to the 1 Gy reference dose, then obtaining the factor, as to know, the correction factor for the non-linearity of reading presented by TLD irradiated with the unknown dose.

Figure 7 shows the calibration curve obtained with the Excel program for.

The R-squared value found was of 0.988, which expresses how much error can be assumed by using the adjustment equation. R2 values close to 1.00 show a good quality of point adjustment to the curve and values below 0.8 indicate poor quality. It is important to note that this adjustment often introduces a doubt in, which can be minimized with the determination of the uncertainty due to adjustment through Equation (3). Such uncertainty must be expanded and, then, combined with the measurement uncertainty. The calculation of adjustment uncertainty will be determined by Equation (3).

where:

is the uncertainty of the adjustment by the equation;

is the coverage factor, taken from the -student table for degrees of freedom;

is the number of points used to generate the equation;

is the value of the error in the -calibrated point, used to generate the equation of linearity;

is the value of the error in the -calibrated point, calculated by the linear equation.

The value found for, in Equation (3), considering a 68% reliability, referring to ±1 standard deviation, in a normal probability distribution, was 0.00015%.

This calibration curve has a considerable error for doses below 50 cGy, however, it was the best calibration curve generated in relation to the adjustment between actual and calculated points. This low adjustment uncertainty implies that such evaluated point, in relation to others, does not impact the final result of the use of the calibration curve. Nevertheless, it is necessary to observe the results found in doses of less than 50 cGy.

The uncertainty of the dose obtained by TLDs came from the analysis of input variables and resulted from the propagation of uncertainties, Equation (4):

The relative uncertainty for dose experimentally obtained with TLDs was of ±0.0114% or ±1.14%.

TLDs were scattered in the form of matrix on the right side of Rando Alderson on points presenting doses different from zero, within blue, yellow and red isodoses, from slice 15 to 18, as other slices showed no significant doses. TLDs were placed on pins that had a space for chip insertion.

Figures 8-11 show the distribution of TLDs in the phantom. Markings in red indicate TLDs inserted in the planning system. Points circled in white indicate the proper placement of TLDs, experimentally. In these figures, the area with higher reliability is the red one, followed by the yellow zone, and finally, by the blue zone, with the lowest reliability, based on the isodose curves assessed by the planning system.

Tables 1-4 show the comparison of absorbed and planned doses for each slice, where a relative standard deviation of up to 6% is observed.

Whereas the planning was done by placing the point of interest for the treatment of male breast cancer near TLD L01, located at position 10 of slice 16, the percentage error shown relative to the plan was of 0.3%. That is, the planning coincides with the measurements expressed by TLDs, in the dose distribution for the treatment performed.

In Table 1, there is a difference of approximately 25 cGy for TLD plan number 63, however, the dose planned for this TLD was of 42.4 cGy and the calibration curve of TLDs is not reliable for measurements inferior to 50 cGy. We can also see a difference of approximately −21 cGy for the TLD plan number 55, however, it was located in a transition area and this location may show results different than planned, since reliability was changed and a 1 mm difference in location, from actual to planned, may generate significantly discrepant results. TLD plan number 52, on the other hand, shows a difference of approximately −13 cGy between the actual dose and the planned dose, but if we evaluate it proportionally, when compared to the average dose, this difference does not exceed 6.3%.

As shown in Table 2, slice 16 is the one with smaller relative standard deviations and smaller differences between planned and actual doses, when compared to other slices, precisely because planning focused on this slice.

^*R—red; Y—yellow; B—blue.

*R—red; B—blue.

^*B—blue; R—red.

*B—blue; R—red.

^*Corresponding author.

In Table 4, there is a difference between the planned and the actual dose of up to 26 cGy, which may be associated with the location of the TLDs, which were in the blue zone, and because of the error existing in the calibration curve, for doses inferior to 50 cGy. We can also notice a difference of approximately −19 cGy for TLD plan number 37, between the actual and the planned dose; however, if we evaluate in terms of average dose, this difference means approximately −8.5% of the measured average dose.