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MEDICAL SCIENCES
PHANTOM DOSIMETRY OF THE RADIATION DOSE
FROM THE DIAGNOSTIC AND RADIATION
THERAPY PLANNING 1 2 Vasilyev L.L. , Trofymov A.V. (Ukraine)
Email: Vasilyev431@scientifictext.ru
1Vasilyev Lеonid Leonidovich - Head of Group;
2Trofymov Artyom Vitalievich - Radiologist, DEPARTMENT OF CLINICAL DOSIMETRY AND CLINICAL TOPOMETRY, GRIGORIEVINSTITUTE FOR MEDICAL RADIOLOGY OF NATIONAL ACADEMY OF MEDICAL SCIENCES OF UKRAINE, KHARKOV, UKRAINE
Abstract: the purpose of this work were to evaluate the additional dose contribution from the diagnostic and topometric procedures into the total radiation dose in a commercial treatment planning system. By means of thermoluminescent dosimetric and ionization chambers-based methods the radiation doses from CT examination and the procedures used for topometric preparation of the patient to radiotherapy were measured in 17 organs of the anthropomorphic phantom. It was obtained that relative contribution of additional dose from CT examination and simulation is very small and of the order of 1%. However, including the imaging dose at the time of treatment planning would allow for a better prediction of the total dose to tumor and critical organs.
Keywords: treatment planning; image guided radiation therapy; patient dose; radiation safety.
ФАНТОМНАЯ ДОЗИМЕТРИЯ РАДИАЦИОННОЙ ДОЗЫ
ОБЛУЧЕНИЯ ОТ ДИАГНОСТИЧЕСКИХ И ПРОЦЕДУР
ПЛАНИРОВАНИЯ ЛУЧЕВОЙ ТЕРАПИИ 12 Васильев Л.Л. , Трофимов А.В. (Украина)
1 Васильев Леонид Леонидович - руководитель группы;
2Трофимов Артем Витальевич - врач-рентгенолог, группа клинической топометрии, Государственное учреждение Институт медицинской радиологии им. С.П. Григорьева Национальной академии медицинских наук Украины, г. Харьков, Украина
Аннотация: целью этой работы было оценить дозовый вклад от диагностических и топометрических процедур в общую дозу облучения в системе планирования лучевой терапии. С помощью термолюминесцентных методов дозиметрических и ионизационных камер, дозы облучения от КТ-исследования и процедуры, используемые для топометрической подготовки пациента к лучевой терапии, измерялись в 17 органах антропоморфного фантома. Было получено, что относительный вклад дополнительной дозы от КТ-исследования и моделирования очень мал и составляет порядка 1%. Однако, включая дозу визуализации во время планирования лечения, можно было бы лучше предсказать общую дозу на опухоль и критические органы.
Ключевые слова: планирование лечения, лучевая терапия с использованием изображения, доза пациента, радиационная безопасность.
UDC 615.849.5
I. Introduction
Surgery, radiation, and chemotherapy are the most common types of cancer treatment. Radiation therapy plays an important role in curing cancer on early, preventing metastatic spread to other areas, and treating symptoms of advanced cancer. External beam radiation therapy is one of the best options for cancer treatment for many patients. The main planning challenge of radiation treatment is to balance high levels of radiation dose to targets with sufficient sparing of organs-at risk and surrounding tissues.
For this purposes a new method of external beam radiotherapy called Image-Guided Radiation Therapy (IGRT) was introduce into clinical practice. IGRT allows to improve the precision and accuracy of the delivery of the radiation treatment and now involves multiple imaging procedures for planning, simulation, setup, and intrafraction monitoring [1].
A substantial contribution in the development of radiation therapy was made by 3D dosimetrical planning and conformal radiation therapy techniques [2-5]. Using of conformal radiation therapy allows reducing radiation exposure to target organs. Thus, the volume of rectum which receives dose of 66 Gy and higher equals 33,7% when using conformal radiation therapy and 62,7% when using conventional one, the bladder volume receives 22% and 50,5% respectively [6].
Currently therapy procedures involve taking a pre-treatment Computed Tomography (CT) scan of the patient, providing a geometrical model of the patient that is used to determine incident radiation beam directions and intensities. Image-guided patient setup requires simulation images in addition to the planning CT study. These images, which define the patient treatment position at the beginning of each fraction, can be done either with portal imaging, diagnostic x-ray imaging, or in-room CT (both conventional and cone-beam (CBCT), kilovoltage, and megavoltage (MVCT)) [7].
Improvement of lung cancer treatment results is associated with introduction of daily verification of radiation treatment conditions using different imaging techniques. This method of external beam radiation therapy is called image guided radiation therapy (IGRT) which allows improving the precision and accuracy of the radiation treatment delivery (precision of 1 -2 mm in reproduction of reference treatment conditions) and involves multiple imaging procedures for planning, simulation, setup, and intrafraction monitoring [7-9].
Many clinical trials showed the increase of disease-free survival rates in patients with lung cancer when using this technique. Using this method of radiation therapy allowed to increase the percentage of full tumor regression from 60-65% to 80-92% [10, 11].
Even through the magnitude of the imaging dose per CBCT scan is small, especially compared to that of therapeutic dose, the cumulative dose over a long treatment course may not be negligible. Moreover, the imaging dose spreads over many critical organs, as opposed to the therapeutic dose that conforms well to the tumor area [12].
This investigation focuses on measuring the absorbed dose from CT examination and topometric preparation procedures and evaluation of additional dose contribution into the total radiation dose in a commercial treatment planning system.
II. Materials and methods
Experimental studies have been conducted using anthropomorphic heterogeneous phantom of a patient "standard" (NNP "Atom" Riga), which is made of tissueequivalent materials. The phantom is transected at 2.5 cm intervals. An average male phantom corresponds to full body length of 173 cm and body weight 73 kg of soft tissue. The plastic used in the phantom has an effective atomic number of 7.47, 10.85, 7.54 and 1050 kg/m3, 1400 kg/m3, 260-500 kg/m3 for soft biological tissue, bone tissue, lung tissue, respectively. The anthropomorphic phantom has got some slots in these slices for putting the dosimeters
in. It contains a representation of 17 organs. The phantom consists of 39 separated slices, each slice has got slots for holding dosimeters (TLD-detectors) (fig. 1), unused slots are filled with tissue-equivalent plugs.
Phantom was undergone all stages of radiotherapy, including CT scanning, X-ray simulator and placing marks, treatment on the linear accelerator. Initially prior to treatment planning the phantom was scanned on "Toshiba AQUILION 64" CT scanner using full body scan range, with slice thickness of 1 mm. The irradiation conditions were as follows: time -24.1 s, voltage - 125 kV, rotation time - 50 ms, amperage - 80 mA. Information extracted from the detectors was registered on the automatic dose assessment.
Received images were imported to treatment planning system «Varian Eclipse». Selected planning tumor volume and critical organs were delineated. Afterwards four-field radiation therapy plan, which accounts for the potential risk of critical organs under treatment, was created. Secondary marking of isocenter projection compiled by irradiation plans was made using X-ray simulator «Varian Acuity». The irradiation conditions were as follows: time - 180 s, voltage - 125 kV, rotation time - 25 ms, amperage - 80 mA. A new set of TLD dosimeters was installed in the same volume of interest. Then, dose from detectors was evaluated for automatic evaluation unit doses. The phantom was setup on the treatment table in the same position as in the planning CT with the help of body markers and lasers. The phantom was treated in the supine position in accordance with the 3D plan calculated for the linear accelerator «Varian Clinac 600C». Ionization chamber "PTW 31010" was placed into projection of the irradiation field isocenter in order to register radiation dose on the linear accelerator.
Fig. 1. Picture of axial slice of the anthropomorphic phantom. Marks indicate TLD detectors positions
III. Results and Discussion
Doses in multiple organs were measured in this study: chest area (11 detectors) and pelvic area (9 detectors). Table 1 shows the results of our organ point dose CT i D, assessment with the TLD for the CT. Point index numbers i correspond to TLD locations indicated in fig. 1. Organ doses were found to be in the dose range 8.87 to 19.26 mGy. Results of dose measurements from kV simulator, sim i D, using new set of TLD dosimeters that were installed at the same slots i of the anthropomorphic phantom are tabulated in Table 2. Organ doses were found to be in the dose range 16.47 to 35.26 mGy.
Slice number of Hole number Organ Absorbed dose
the phantom of the slice i DCT,i > mGy
56 left lung 16,34
16 61 right lung 17,70
66 spine 12,99
17 78 left mammary gland 19,26
79 right mammary gland 19,13
80 left lung 17,62
81 left lung 16,91
18 85 right lung 18,18
88 right lung 16,30
91 left shoulder blade 15,90
92 right shoulder blade 15,76
234 pelvis 8,87
34 235 pelvis 17,30
236 pelvis 18,13
237 pelvis 9,63
245 urinary bladder 17,05
246 urinary bladder 13,95
35 247 urinary bladder 14,16
248 urinary bladder 12,87
249 bowels 14,22
According to the treatment plan total organ dose of 60 Gy is to be delivered in 30 daily fractions of 2.0 Gy each, which is 100% of the single exposure. Daily fraction is distributed equally between four 0.5-Gy fields in the region of the prostate. Indeed the reference point is shifted and receives 104.6%, as shown in fig. 2. Therefore, daily radiation dose to the prostate is equal to 2092 mGy. After the daily treatment delivery using the linear accelerator «Varian Clinac 600C» the ionization chamber registered the following values for each of four-field beam arrangements in the pelvic area: 567.7 mGy, 451.18 mGy, 484.6 mGy, 538.7 mGy. So total radiation dose to the prostate from the daily treatment procedure is equal to) (1 RTD =2042.18 mGy. The experimental data from the ionization chamber show good agreement with the calculated data from the 3D planning system. The use of a cross-sectional anatomical atlas enables us to identify the location of various organs of dosimetric interest throughout the anthropomorphic phantom. Within each numbered phantom slice, the detectors were placed within the boundaries of the selected organ region to determine average point organ dose as illustrated in fig. 3. In this study prostate was defined as a target organ. Average reading from four dosimeters (slot numbers are 245-248) placed in the bladder was adopted as organ dose for the prostate. It was assumed that radiation field was distributed uniformly over the entire volume of the phantom, so that radiation dose for the prostate can be calculated as the arithmetic mean of the doses for bladder.
Fig. 2. Isodose distribution in the planning system
Radiotherapy treatment plan showing isodose lines and contoured volumes. Pelvic bones are contoured with the green lines, irradiated volume is represented by the red line, four overlapping irradiation fields are represented by the orange lines, dotted lines indicate the fields of CT simulator.
Results from kV simulator are tabulated in Table 2.
Table 2. Absorbed doses for selected organs from the treatment simulation on X-ray simulator
"Varian Acuity"
Slice number of the phantom Hole number of the slice i Organ Absorbed dose Dsim,i - mGy
16 56 left lung 22,64
61 right lung 21,73
66 spine 16,47
17 78 left mammary gland 23,38
79 right mammary gland 35,26
18 80 left lung 31,25
81 left lung 25,04
85 right lung 26,20
88 right lung 23,47
91 left shoulder blade 29,91
92 right shoulder blade 25,74
34 234 pelvis 21,98
235 pelvis 31,24
236 pelvis 33,63
237 pelvis 18,26
35 245 urinary bladder 23,64
246 urinary bladder 17,38
247 urinary bladder 22,28
248 urinary bladder 25,08
249 bowels 23,93
The ionization chamber has registered the following values to each field:1. 567,7 mGy, 2. 451,18 mGy, 3. 484,6 mGy, 4. 538,7 mGy.
According to the treatment plan radiation dose per fraction in the target organ should be equal to 2000 mGy, which is 100% of the single exposure. Indeed the reference point is shifted and receives 104, 7%, as shown in Fig. 2. So radiation dose to the prostate from the single treatment procedure is D®. = 2042.18 mGy.
The data from the ionization chamber were compared with the calculated data.
Data obtained from the TLD dosimetry were taken into account in calculating the tolerance dose to critical organs.
The use of a cross-sectional anatomical atlas enables us to identify the location of various organs of dosimetric interest throughout the anthropomorphic phantom. Within each numbered phantom slice, the detectors were placed within the boundaries of the selected organ region to determine average point organ dose as illustrated in Fig. 3. In this study prostate was defined as a target organ. Average reading from all dosimeters (hole numbers are 245-248) placed in the urinary bladder was adopted as organ dose for the prostate.
Therefore, radiation dose for the prostate from a single CT scan D (1) can be calculated as follows
p.(l) _ DCT,245 + DCT,246 + DCT,247 + DCT,248
Fig. 3. Scheme of 35th slice of the phantom. Marks indicate TLD detectors positions. The blue color shows the detectors in the volume of the prostate gland, red - pelvic bones and yellow-bowels
Analogous calculations were made for radiation dose from the topometric preparation procedures. Value of the radiation dose from the single treatment simulator procedure D^m can be calculated from data given in Table 2:
jj( 1) _ Dsim,245 + Dsim,246 + Dsim,247 + Dsim,248
sim ^ ^ '
Then, using previously calculated doses (1) and (2) and taking into account that CT examination is made two times during radiation treatment while simulation procedures are done prior to each treatment on the linear accelerator during whole treatment course (30 days), we can obtain contribution of additional dose from CT examination DCT and simulation Dsim into total dose Dtotal:
C = Dct + Dsim x 100% = Dct + Dsim— x 100% =
Dtotal DCT + Dsim + DRT
2 x D (1) + 30 x d£,
CT s'm x100% = 1.12% (3)
2 x D ® + 30 x D(m + 30 x dR1}
where CT D , sim D and RT D are the total radiation doses received by the patient from diagnostic CT investigation, simulation procedure and radiation therapy itself, respectively.
As for the clinical implications of the observed values, it is apparent from (3) that the doses are very small. The mean dose to the organ is of the order of mGy to cGy. However, whether or not these dose levels are of clinical concern will be subject to the specific context. If daily CBCT is repeatedly conducted during a long treatment course, the total dose may not be negligible and may require specific managements [13]. For example, Wen et al. [14] measured the cumulative kV CBCT dose in pelvic bones to be ~400 cGy during the treatment of prostate in a total of 42 fractions. Ding et al. reported the dose resulting from a single fraction kV CBCT acquisition being as high as 25 cGy in cranial bones [14].
There are several papers about measuring the dose from CT in phantom and on patient using various dosimeters or by Monte Carlo methods [14, 15]. Inclusion of this dose in the treatment planning process is the subject of additional investigations.
Including the imaging dose at the time of planning would allow for a better prediction of the total dose to tumor and critical organs since this dose can simply be added to the treatment plan in the case of conventional planning. Accounting for the dose to patient resulting from image guidance procedures in the stage of treatment planning can also provide choices for clinicians to make an informed decision regarding the risk and benefits of additional radiation exposure.
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