Dosimetric measurement of scattered radiation for simulated head and neck radiotherapy with homemade oral phantom

Abstract

During radiotherapy for head and neck tumours, the oral cavity and cheek area would be inevitably exposed to high energy radiation; thus, the material surface of the teeth, dental restorations with high atomic number, or alloy prosthodontics would generate backscatter electrons that cause the buccal mucosa adjacent to these materials to receive localized high dose enhancement, which primarily leads to side effects or oral mucositis. Based on the size of the adult oral cavity, this study aimed to use acrylic resin to create an oral phantom with two grooves on the left and right sides for placement of three molars. Moreover, the distance between the inner cheek and the side surface of the teeth could be accurately adjusted every 1 mm from 0 to 5 mm. This enhanced the dose in the buccal mucosa during head and neck radiotherapy and made the distribution measurement of the radiation dose simple and feasible at different depths (0–5 mm). Meanwhile, the study employed the film type optically stimulated luminescent dosimeter with a thickness of 0.3 mm to measure the absorbed dose inside the buccal mucosa to reduce the dose interference from radiotherapy. The study fixed three real molars in a row located at the left side of the phantom and employed 6 MV photons and intensity-modulated radiotherapy (IMRT) to treat and simulate oral cancer and measure the attenuation of the molar’s backscatter dose from 0 to 5 mm in an up beam direction. The result showed that, in every 3 mm, the phantom had attenuated the enhancement of backscatter dose <3%. The irradiation dose enhancement in a single direction was twice higher than that through IMRT 7 field treatment. These measurement results were consistent with the results of previous studies.

Keywords

Oral cavity backscatter dose dental restoration mucositis optically stimulated luminescent dosimeter

1 Introduction

In patients with head and neck tumour undergoing radiotherapy, the first side effect is oral mucositis [1 –3]. This is mainly because the backscatter electron generated by dental restorations of alloy teeth and teeth with higher atomic number than soft tissues with radiotherapy caused dose enhancement on the mucous membrane next to the teeth, except that the mucous membrane was more sensitive to radiation [4 –7]. Since the backscatter electron in the tissue has a short radiation range, the increased dose focused on the local buccal mucosa and eventually caused mucositis [8]. Regarding radiotherapy for head and neck tumours, especially in patients with nasopharyngeal carcinoma or oropharyngeal tumours, 83% of patients developed oral mucositis when they received a cumulative dose >5000 cGy. This side effect is also one of the factors for patients to suddenly suspend radiotherapy [9].

There are several studies on dose measurement in the buccal mucosa caused by backscatter electrons of the teeth or materials with different atomic numbers in the buccal mucosa. Farahani et al. installed a stack of Gafchromic films on both sides of the real teeth and some dental restoration materials. The films were irradiated by ⁶⁰Co gamma ray or 10 MV X-ray beam in a single direction [6]. Thilmann et al. used thermoluminesent dosimeters (TLDs) to measure the backscatter doses of the phantom along with different metallic dental materials on the oral cavity of in vivo patients [7]. Fuller et al. measured the backscatter doses by placing the TLD in the patient’s oral cavity and compared the difference between the obtained data and the intensity-modulated radiotherapy (IMRT) software’s estimates [10]. Wang et al. adopted the TLD and phantom to test the dose enhancement caused by the backscatter effect on the bone implant interface during simulated head and neck radiotherapy [11]. Reitemier et al. used alanine dosimeter to measure the backscatter dose of dental materials and studied the effectiveness of protecting the stents on the teeth [2]. Some measurement results were verified by the Monte Carlo calculation results. A detailed comparative study on Monte Carlo simulation and measurement results was also proposed [8]. Optically stimulated luminescence dosimeters (OSLDs) are widely used as personal dosimetry badges and for in vivo skin dose measurements in radiotherapy. OSL dosimeter has been recommended by many studies that can be a useful dosimeter for quality assurance in radiation therapy [12 –21]. Compared with a thermoluminescent dosimeter (TLD), the OSLD possesses the advantage of repeated readouts following single radiation exposure. Because of the size limitation of OSLD, the conventional OSLD (even a nanoDot OSLD) cannot be placed in the RANDO phantom for absorbed dose determination, thus restricting its use. Therefore, the OSLD material was cut into smaller disks, and dose determination tests were performed using a microStar reader [19].

In this study, an oral phantom was made from acrylic resin, having an average size as that of real oral cavities. The phantom was designed to measure the backscatter dose of the teeth or metal teeth substance. With or without the teeth, it was easy and feasible to simulate the buccal mucosa next to the teeth (0 mm) or at different depths (0–5 mm). To measure the dose of low energy backscatter electrons, this study used the film type OSLDs to replace the former chip based nanoDot OSLD, which was thick and interfered with the depth distribution of the dose or other radiation detectors.

2 Materials and methods

2.1 Oral phantom

Homemade oral phantom was self-made using acrylic resin, its shape and dimensions are shown in Fig. 1. The phantom is characterized by a simplified shape of the oral cavity, which was made to measure the backscatter doses caused by mandible molars and dose distribution in the facial tissues at different depths. The size of the phantom is the average value in 15 adults (random subjects with no specific age and sex) measured by the dental computed tomography. The interval between the inner margins of the left and the right molars was 44 mm; the thickness of the cheek was 10 mm; the thickness of the mandible was 30 mm. There is a clearance of 5.3 mm space between the molars and cheek; the inner width of the entire mouth was 74.8 mm. There were two grooves with a length of 36.4 mm and width of 10.1 mm on the left and right sides in three molars; the molar’s average width was 10 mm, and the depth of the grooves was 16.1 mm, which was approximately 5 mm deeper than the average value; thus, the experiment was designed with the molar embedded at different depths. In the meantime, these grooves could also provide different incident angles of the radiation beams and thus measure the backscatter doses formed by a particular molar and determine its location (left or right). Grooves could be placed on real human molars, various metallic alloys, dental restorations, or dentures. Molars and experimental materials could be attached to the groove using plaster or silicone.

Fig. 1

Homemade oral phantom.

An air gap of 5.3 mm was reserved between the molar groove and buccal mucosa; acrylic sheets with different thicknesses could be placed between the molar and mucosa according to the requirement of the experiment, which was to measure the dose distributions of the backscatter electron due to high energy X-ray radiation of different materials inside the mucosa phantom or simulate the oral structure and leave an air gap with different distances between the molars and mucosa. This would demonstrate the phenomenon that a low energy backscatter electron was attenuated by air. The takeout, imbedment, and fixation of acrylic sheets were realized by design of the latch, which made it easy to disassemble the acrylic sheet in the middle and maintain accuracy of the separation distance.

The shallow groove with a depth of 0.2 mm and diameter of 5.5 mm was formed on the inner sidewall of the mucosa phantom on the lateral side of the cheek along the three molars; film type OSLD could be placed to measure the doses caused by the backscatter electron.

2.2 Standard source

In this study, an Elekta Synergy linear accelerator (Elekta Oncology Systems Ltd, Crawley, UK) was used as the standard photon radiation source equipment. The source output dose of the medical linear accelerator was measured using a Farmer-type ion chamber that had been correctly adjusted in terms of temperature and pressure to obtain accurate radiation dose values, before each exposure. The standard source condition was that the dose at the axle centre of the linear accelerator was adjusted to 1 monitor unit (MU), equalling the 1 cGy absorbed dose requirement. According to the suggestion from AAPM TG-51 [22], for photon beams of ⁶⁰Co∼15 MV, the adjusted depth for absorbed dose in the water can be d = 5 cm or the maximum dose depth, which can be achieved by adjusting the linear accelerator, solid water phantom, and ion chamber. A megavoltage photon beam of 6 MV was selected as the energy of the radiation source in this experiment. The applied calibration conditions are outlined as follows: source-to-axis distance (SAD) was 100 cm; the field size was 10×10 cm²; the solid water phantom was placed under the gantry head; and a 0.6 cm³ Farmer type ion chamber (A19; Standard Imaging, USA) connected to a PTW electrometer (MAX4000, Standard Imaging, USA) was placed at the isocentre. The output dose (1 MU=1 cGy) was calibrated under these conditions with temperature and pressure corrections. The tolerance, flatness, and symmetry of the dose output were within 2%, 3%, and 2% (depth = 10 cm; field size = 80%), respectively. The Farmer type ion chamber with the PTW electrometer was used to measure the charge, which was further converted into dose.

The mandible of the isocentre that simulated oral cancer in the phantom was 17.5 mm and found at the centre of the first left and right molars. Treatment planning was generated using Pinnacle3 version 8.0 and IMRT software. The prescribed dose of each fraction treatment was 200 cGy, which includes 7 fields: 150°, 100°, 50°, 0°, 310°, 260°, and 210°.

2.3 Optically stimulated luminescent dosimeter

The original OSLD is in the form of a long strip (Al₂O₃:C, Nagase Landauer Company, Japan). To insert the OSLD into the hole in the homemade oral phantom, we cut the strip OSLD into multiple smaller disks with identical size (diameter = 4 mm; thickness = 0.3 mm). The effective atomic number (Z = 11.2) of OSLD is not tissue-equivalent, and no correction was made considering the Z number in this study. Because Compton scattering dominates our irradiated energy level (6 MV photon beams), we consider that the sensitivity of energy response to the Z number is small [23, 24]. It is worth noting that the dose response and sensitivity of each OSLD varied because of the slight differences in the amount of impurities in each OSLD. Therefore, a repeatability test was performed for all OSLDs before dose measurement to eliminate the unsuitable dosimeters with larger errors. This test was performed by irradiating each of the 50 nanoDots in free air to the same dose of 1 Gy in 6 MV photon beam. The OSLD was placed 5 cm deep in the solid water phantom. The field size was 10 cm×10 cm and the SAD was 100 cm. The solid water phantom thickness was not considered for the radiation beam output conditions as it was merely 0.3 mm. Each dosimeter read out was performed prior to the irradiation to account for the background signal. This protocol could improve the measurement statistics of the Element correction factor (ECF) of individual dosimeters from the same batch [25]. The ECF of each dosimeter was defined as the ratio of average reading of all the dosimeters used for the individual reading. To prevent the fading phenomenon, the readout process was completed within 3 days after OSLD exposure to radiation. Such exposure and readout procedures were repeated four times to obtain the average values, standard deviations, and coefficient of variation. Note that the annealing process was performed for each OSLD after irradiation to ensure perfect sensitivity, although OSLDs can provide accumulated dose measurements.

The OSLDs were read using an InLight microStar reader system (Landauer Inc.), which included a reader, a laser scanner, and a PC running microStar software. The reader was operated in continuous wave (CW) mode using a low-intensity LED beam for irradiation measurement. To achieve excitation of trapped electrons, green light (530±10 nm) was incident on the nanoDot OSLD, and blue luminescent light (420 nm) was emitted when the excited electrons were de-excited. The photo-multiplier tube (PMT) installed in the device counted the number of photons of blue light, which is proportional to the irradiation dose [15]. OSLDs have the advantage of allowing repeated irradiation with a cumulative dose; in the present study, however, optical bleaching was performed after each readout session to ensure that the sensitivity of the OSLDs was not affected by the cumulative dose [26]. Before each readout, the dark count and LED count were checked to ensure consistency in the reader’s stability. After irradiation, all OSLDs were counted within the three following days at the same time in order to avoid any fading, which might affect dose reconstruction.

2.4 Backscatter dose in IMRT

Given the 7 field irradiation through IMRT, this study fixed three natural unprepared human molars in a row on the left side of the phantom; the three molars were of average sizes. The experiment on the relationship between backscatter dose enhancement and depths of buccal mucosa phantom was divided into two parts. The first part involved affixing three OSLDs that were directly affixed to the surface of the three molars in an up beam direction. Then, IMRT was provided with a graded 7 field irradiation, with a prescribed dose of 200 cGy. After measuring the space between the molars and 0 mm OSLD, five acrylic sheets with thickness of 1, 2, 3, 4, and 5 mm were employed to separate the OSLD and molar. The same radiation was applied each time to measure the attenuation caused by the backscatter dose inside the phantom with depth increments.

Subsequent to measuring the backscatter dose with the teeth present, the teeth were removed, leaving the area edentulous; the abovementioned experimental steps were repeated to obtain the dose value without the teeth present. The calculation method of the relative dose enhancement’s percentage (%) entails subtracting the backscatter dose without teeth from the dose with teeth and then dividing the value by the dose without teeth as arranged previously.

The second part of the experiment involved placing the acrylic sheet between the OSLD and molars during the first part of the experiment. The 1 mm acrylic sheet in front of the OSLD was left to fix the OSLD in the measurement position, and there was a remaining air gap. Regarding the irradiation and OSLD measurement found identical with those in the first part, we intended to observe the attenuation state of the low energy backscatter electron in the air gap (1, 2, 3, and 4 mm). The abovementioned experiments were repeated at least three times to obtain the average dose.

2.5 Backscatter dose in a single direction

To observe the enhancement and distribution of the backscatter dose in a single direction (non-vertical), the study utilized three groups of the same measurement arrangements as shown in Section 2.4. However, the spaces between the OSLD and three molars were 0 mm and 1 mm (the 1 mm acrylic sheet was inserted to separate them); the 1 mm acrylic sheet was employed to fix the OSLD and 4 mm air gap. Radiation of 26 cGy was provided singly at an incident angle of 50° with the same IMRT to measure the distribution of the backscatter dose. Similarly, the distribution of the backscatter doses was observed after 23.8 cGy was radiated singly at an incident angle of 100°.

3 Results

Figure 2 depicts the ECFs of 50 nanoDots irradiated under identical conditions. The ECF of individual dosimeter was determined as the ratio of average reading of all the dosimeters used to the individual dosimeter reading. The ECFs are discerned to be distributed in the range of 0.97 to 1.03. The OSLD with the least ECF of 0.97 revealed the largest deviation (3%) from the mean. The standard errors (represented by error bars) of the four repeated readings were occurred in the range of 0.3% to 1%. This result is in good agreement with other study [25], where ECFs of similar nanoDots were evaluated in ⁶⁰Co beam and found to occur in the range of 0.97 to 1.03. It was affirmed that the use of ECFs for individual dosimeter can improve the accuracy of dose measurement in general radiography by about 3%.

Fig. 2

ECFs of individual OSLD in the presence of uncertainties.

In the first part of the experiment, the teeth surface generated a backscatter electron with a higher atomic number than that in the soft tissue after a 6 MV photon beam was employed to irradiate the molar in the 7 field irradiation direction of IMRT. This backscatter electron allowed the buccal mucosa next to the teeth to receive higher radiation dose. Table 1 shows the molar of the buccal mucosa phantom receiving a higher backscatter dose (%) than under edentulous condition. The attenuation of the backscatter doses within the phantom is presented in Fig. 3, and the maximum dose enhancement in the up beam direction next to the teeth (0 mm) was 49.5% and minimum was 38.5%. With increasing depth of the buccal mucosa, the backscatter dose was quickly attenuated. The dose enhancement was maintained between 9.8% and 13.5%, roughly one-third or below the zero distance (0 mm) after passing through the 1 mm thick acrylic sheet, while the dose enhancement was <3% at 3 mm depth to the phantom. This result showed that the backscatter electrons were low energy electrons; therefore, the radiation range in the acrylic phantom (density of approximately 1.182 g/cm³, which was slightly higher than that of water [1.0 g/cm³]) was extremely short. However, when the surface distance in the presence of molars was 4 or 5 mm, the backscatter dose increased by 14.5%, 12.2%, and 7.7%.

Table 1

Relative backscatter dose enhancements (%) in the buccal mucosa resulting from the IMRT of simulated oropharyngeal carcinoma

Thickness of the acrylic sheet between the tooth and OSLD	Relative backscatter dose enhancement (%)
	First molar	Second molar	Third molar
0 mm	40.3	38.5	49.5
1 mm	9.8	11.9	13.5
2 mm	6.7	6.1	5.4
3 mm	1.7	2.4	2.1
4 mm	11.4	10.7	9.7
5 mm	16.2	14.6	9.8

Fig. 3

The relative enhancement of the backscatter dose relative to the dose without tooth in the buccal mucosa after 6 MV photon irradiation of IMRT (7 fields). The 0 mm depth showed that the OSLD was next to the teeth, and the remaining buccal mucosa depth showed that the acrylic materials with different thicknesses were interposed between the teeth and OSLD.

The sources of radiation dose in the lateral buccal mucosa with teeth had three parts: radiation directions with 50° and 100° and backscatter electron and facing directions with 260° and 310°, respectively. This is based on the 7 field IMRT. However, the backscatter dose was extremely low, and the backscatter dose lost the shielding effect of the molars at a distance of 4 or 5 mm with molars. In contrast, the acrylic sheets with 4 and 5 mm thicknesses generated an additional buildup dose, thus increasing the dose enhancement in the up beam direction of 4 and 5 mm instead.

Table 2 shows the dose enhancement in the buccal mucosa when there is an air gap between the teeth and buccal mucosa. Figure 4 shows the change in dose enhancement. It is clear from the experimental results that the 1 mm acrylic sheet with a 1 mm air gap (the actual distance between the OSLD and teeth was 2 mm) caused the dose enhancement to remain between 10.5% and 16.4%. The dose enhancement remained at 5.4% to 6.7% (shown in Table 1) when the 2 mm acrylic sheet was used for separation; this indicated a low dose attenuation due to the existence of air. When the air gap was 2 mm, the dose enhancement reached a minimum range of 2.3% to 6.4%, which was slightly higher than that in the 3 mm acrylic sheet in Table 1.

Table 2

Relative backscatter dose enhancements (%) with varying air gaps between the teeth and buccal mucosa resulting from the IMRT of simulated oral carcinoma

Air gap between the tooth and OSLD	Relative backscatter dose enhancement (%)
	First molar	Second molar	Third molar
0 mm	40.3	38.5	49.5
1 mm	11.7	16.4	10.5
2 mm	2.3	6.4	3.1
3 mm	2.3	11.0	8.6
4 mm	2.3	1.7	7.7

Fig. 4

The relative enhancement of the backscatter dose with air gap after 6 MV photon irradiation of IMRT (7 fields). The 0 mm depth showed that the OSLD was next to the teeth, and each remaining air gap included 1 mm acrylic sheet for affixing the OSLD plus the air gap (mm) in the figure.

Results showed that the attenuation of the backscatter electron was low because the air density was lower than that of the acrylic sheet. Moreover, distance was one of the factors that caused the backscatter dose to attenuate, and the dose enhancement was lowest at a distance of 3 mm from the tooth surface. When the air gap increased to 3 or 4 mm, the change in dose enhancement was similar to the increase as shown in Table 1. However, the buildup dose formed by the 1 mm thick acrylic sheet (fixed OSLD) was indeed less than that formed by a 4 or 5 mm thick acrylic sheet.

Given the single direction radiation, the study adopted an irradiation with 50° and 100° radiation angles in a single direction based on the aforementioned IMRT planning indicated in Section 2.2. The data in Table 3 and Fig. 5 show that the dose enhancement was extremely different from the results of the IMRT in Tables 1 and 2. The backscatter dose enhancement radiated in a single direction was much larger than the one radiated in multiple directions. As for the configuration of 0 mm, the dose enhancement in a single direction was twice larger than the treatment radiation in multiple directions. The larger dose enhancement radiated in a single direction (vertical direction) in contrast to the one radiated in multiple directions that had been discussed in previously [6, 7].

Table 3

Relative backscatter dose enhancements (%) with different irradiation angles

Distance between The tooth and OSLD	50° of irradiation (%)			100° of irradiation (%)
	First molar	Second molar	Third molar	First molar	Second molar	Third molar
0 mm	96.5	93.0	90.3	98.1	97.6	106.6
1 mm interposed acrylic sheet	84.0	87.8	89.0	95.6	93.8	104.0
1 mm acrylic sheet and 4 mm air gap	81.2	84.1	83.8	92.2	85.9	95.4

Fig. 5

The relative enhancement of the backscatter dose after the irradiation with single incident angles (a) 50° and (b) 100°. The 0 mm depth showed that the OSLD was next to the teeth; 1 mm showed that the 1 mm acrylic sheet was interposed between the OSLD and teeth; the 4 mm air gap included the 1 mm acrylic sheet for affixing the OSLD plus the air gap with a 4 mm width.

The 100° radiation direction was closer to the vertical irradiation direction with the molar than the radiation direction of 50°. With 0 mm and the positions of each molar, the dose enhancement in the 100° radiation direction was higher than in the 50° radiation direction by 1.6% to 16.3%, with an average of 7.5%. If separated by a 1 mm acrylic sheet, the dose enhancement in the 50° radiation direction was attenuated by 12.5%, 5.2%, and 1.3% with an average of 6.3%, while the dose enhancement in the 100° radiation direction was attenuated by 2.5%, 3.8%, and 2.6% with an average of 3.0%. Thus, we inferred that the energy of the backscatter electron close to the 100° vertical irradiation direction was higher than the slant radiation of 50°; therefore, the attenuation degree of the backscatter dose of the 50° radiation was greater than that of the 100° irradiation after passing through a 1 mm acrylic sheet.

When a 4 mm air gap was added between the molars and OSLD, the enhancement of the backscatter dose was also attenuated by air. Regarding the 50° radiation direction, each molar was attenuated by 15.3%, 8.9%, and 6.5% with an average attenuation of 10.2%. It was attenuated by 3.9%, which was more than that without air gap. However, the attenuations in the 100° radiation direction were 5.9%, 11.7%, and 11.2% with an average attenuation of 9.6%; attenuation by 6.6% was more than that without air gap. This could explain the attenuation function of the air gap with a 4 mm width to a certain degree.

4 Conclusion

We created an oral phantom with an average size obtained by measuring dental CT images of randomly selected 15 adults. Two grooves were installed on the left and right sides to place three molars in a row, which could accommodate a variety of experimental materials, such as real teeth and dentures made of metal alloys or other materials. During head and neck radiotherapy, the phantom was made to measure local dose enhancement of the buccal mucosa caused by the backscatter electron that was released from the surface of the materials with high atomic number, such as real teeth, metal dentures, and dentures made of other materials and make the measurement of the backscatter dose in the buccal mucosa with different depths (0–5 mm) simple and feasible. Since the gaps between the grooves for placing the molars and buccal mucosa could be accurately adjusted every 1 mm from 0 to 5 mm, the acrylic sheet could be inserted to separate the distance between the side surface of the teeth and film type OSLD to facilitate the measurement of the attenuation of the backscatter dose in the up beam direction with different thicknesses of the buccal mucosa soft tissue. The acrylic sheet could also be drawn off to form an air gap between the teeth and soft tissue, with an attempt to observe the attenuation of the backscatter dose in the air.

To reduce the dose interference from radiotherapy, the study employed the film type OSLD with 0.1 mm thickness to measure the dose caused by the backscatter electron in the buccal mucosa rather than the nanoDot OSLD. In the actual measurement of the backscatter doses, the handmade oral phantom was used to fix three real molars in a row and 6 MV X ray and IMRT procedures were used to simulate the treatment of oral cancer. The results showed that the enhancement of the backscatter dose next to the teeth (with 0 mm spacing) was the highest. However, because the energy of the backscatter electrons was low, the dose of the buccal mucosa phantom was attenuated to a minimum with a depth of 3 mm from the surface of the molars. Either the soft tissue of the phantom or air was employed to separate them. The results were consistent with those of previous studies [8].

Conflict of interest

We declare that we have no conflict of interest.

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