|Year : 2013 | Volume
| Issue : 2 | Page : 85-89
Additional imaging dose from kV-cone beam computed tomography
Heng Siew Ping1, Nor Fadilah1, Sivamany Kandaiya2
1 Radiotherapy Unit, Cancer Institute, Pantai Hospital Kuala Lumpur, Bukit Pantai, Kuala Lumpur, Malaysia
2 School of Physics, University Sains Malaysia, Jalan Sungai Dua, Penang, Malaysia
|Date of Web Publication||14-Mar-2014|
Heng Siew Ping
Radiotherapy Unit, Cancer Institute, Pantai Hospital Kuala Lumpur, 59100, Bukit Pantai, Kuala Lumpur
Source of Support: None, Conflict of Interest: None
Objectives: The objective of the following study is to provide the quantitative imaging dose dependency on treatment region and scanning settings concerning the additional imaging doses from kV-cone beam computed tomography (CBCT) scans. Materials and Methods: Patient imaging dose measurements were performed using head and body cylindrical poly methyl metha acrylate (PMMA) phantoms, 0.125 cm 3 ionization chamber and electrometer and metal oxide semiconductor field effect transistor dosimeter. Results: The dose values for the pelvis phantom was found vary from 8.6 mGy to 22.2 mGy for 120 kVp, 1040 mAs. For the head phantom, the dose values vary from 0.71 mGy to 1.37 mGy for 100 kVp, 36.1 mAs. For head scanning mode, the peripheral doses are greater than the central axis doses by a magnitude of 1.3-2 with the largest found consistently at 3 o'clock position of the PMMA head phantom. For the pelvis scan, the peripheral doses are greater than the central doses by a magnitude of 2.1-2.6 with the largest found consistently at 9 o'clock position of the PMMA pelvis phantom. Conclusion: One should be aware of the additional imaging dose with daily kV-CBCT when setting up CBCT scanning protocol and should try to reduce the high voltage usage since reduction of the tube voltage can reduce radiation dose to the patient, especially for pediatric patients.
Keywords: Cone beam computed tomography, imaging dose, pediatric
|How to cite this article:|
Ping HS, Fadilah N, Kandaiya S. Additional imaging dose from kV-cone beam computed tomography. Radiat Prot Environ 2013;36:85-9
|How to cite this URL:|
Ping HS, Fadilah N, Kandaiya S. Additional imaging dose from kV-cone beam computed tomography. Radiat Prot Environ [serial online] 2013 [cited 2020 Feb 27];36:85-9. Available from: http://www.rpe.org.in/text.asp?2013/36/2/85/128874
| Introduction|| |
Increased use of daily cone beam computed tomography (CBCT) for patient positioning correction has attracted significant concern due to its additional dose to patients. Although one might think that CBCT dose can be negligible if compared with the target prescriptions dose of radiation therapy; however, one should be aware that CBCT dose is distributed to the entire imaging region, which has a large amount of normal tissues.
The objective of this study is to provide quantitative imaging dose (CBCT dose index [CBDI]) dependency on treatment region and scanning settings concerning the additional imaging doses from kV-CBCT scans from daily Elekta Synergy kV-CBCT. Imaging doses from CBCT were measured with ionization chamber and the measured weighted doses were compared with those obtained from metal oxide semiconductor field effect transistor (MOSFET) dosimeter.
| Materials and Methods|| |
For CBCT image acquisition, there are a number of image acquisitions and reconstruction parameters that can be set that affect image quality and dose to the patient. These can be adjusted to suit the anatomic site, patient size and the image-guided task for which the images are being taken. The kV X-ray beams are directed in a circular cone. The un-collimated cone has a diameter of 425 mm incident on the flat panel detector. Image can be acquired with three different fields of view (FOV): small, medium and large.
In this study, 2-kV collimator cassettes were selected to measure CBCT imaging dose, namely S20 to yield small FOV for head and neck cases (27 cm diameter) and M20 to yield medium FOV for pelvis cases (41 cm) as shown in [Table 1]. Large FOV settings deliver dose to a larger volume of the patient and result in a poorer image quality due to scatter from the larger beam. All the CBCT images were acquired without additional filtration (denoted as F0).
Additional imaging dose measurements were performed using head and body phantoms, cylindrical Farmer type 0.125 cm 3 ionization chamber (CC13, Scanditronix Wellhofer, IBA dosimetry and calibration factor 47.39 mGy/nC) and electrometer, as shown in [Figure 1]. CBDI represents the average absorbed dose from one rotation of gantry. This theoretically estimates the average dose within the central region of the dose volume.  The CBDI is measured both in cylindrical poly methyl metha acrylate (PMMA) phantom of 16- and 32-cm diameter simulating the head and body of a patient, respectively, with density P = 1.19 g/cm 3 . Both phantoms have the same axial length of 15.2 cm. In contrast to conventional slice-based computed tomography (CT) scanners, which irradiate only short lengths of the patient at any given instant,  the use of the standard CT PMMA phantoms does not cover the wide field CBCT and underestimate the CBDI due to missing scatter. Hence, the effective length of the CT PMMA phantom was increased by adding 15 cm of perspex to both ends of the body phantom and to one end of the head phantom. 
|Figure 1: The setup of kV-cone beam computed tomography (CBCT) dose measurements. The chamber was placed in the center of the phantom for CBDIc measurement|
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The center of the phantom was positioned at the center of rotation plane using the treatment wall alignment lasers and with the chamber axis parallel to the axis of rotation. CBDI c and CBDI p are defined, respectively, as the CBDI values measured with the ionization chamber positioned in the center and in periphery at four positions (12-, 3-, 6-, 9-o'clock) [Figure 2].
The weighted CBDI w is used for approximating the average dose over a single slice.  The measured weighted doses were then compared with MOSEFT measured doses at various locations in a PMMA phantom.
where CBDI (center) is a point dose at central axis and CBDI (peripheral) is an average point dose at peripheries.
Volume CBCT dose index (CBDI vol ) is the weighted average of CBDI measurements divided by the pitch. The pitch is the ratio between the length of the treatment couch moves during one 360° gantry rotation and the width of the radiation beam. For the CBCT scanning, the volume CBCT dose index CBDI vol takes into account the parameters that are related to specific scanning protocols and is defined by the following equation:
But for the CBCT, pitch is set to 1 as it is only one rotation so CBDI vol = CBDI w . 
In clinical practice, radiation measuring generally starts with the dose length product (DLP). The DLP reflects the total absorbed dose of a complete scan acquisition (the dose for a series of slices).  DLP takes into account the total length of the scan, the CBDI for the scanner and the table pitch. The more overlap there between gantry rotations; the more radiation is delivered in order to scan the same volume of the patient. 
High bias and high sensitivity MOSFET detectors (Model: TN-1002RD, Best Medical Canada) with mobile MOSFET reader (Model: TN-RD-16, Best Medical Canada) were employed to measure the absorbed doses. The MOSFETs used for patient dose measurements measure a voltage difference in mV proportional to the dose deposited. Therefore, measurement of dose with MOSFET requires linking the voltage shift, in units of mV to dose deposited, in units of mGy, through precise calibration. The responses of the MOSFET dosimeters are energy dependent. Thus, in this study I calibrated the sensors for 100 kVp and 120 kVp.
The MOSFET detectors were calibrated by simultaneous MOSFET and 0.13 cc Farmer type (IBA) exposure in-air on the Philips 16 slice CT scanner [Figure 3]. The electrometer used in this project was Dose1 from IBA. All calibration measurements were performed with stationary CT tube (from the system's service mode, no table movement or tube rotation was allowed) held at the 0° tube position (12 o' clock) with a small focal spot size 0.5 mm × 1.0 mm, large filter 16 mm × 0.625 mm detector configuration (for a total z-axis collimation ~ 40 mm at isocenter) and at 100 kV for head region calibration and 120 kVp for pelvis region calibration.  MOSFETs were secured on a foam pad with their sensitivity region facing the main axis of the X-ray tube; the ion chamber and MOSFETs were placed parallel opposed from each other along the same axis and equally distanced by 1 cm from the central X-ray beam as established using the collimator light field on top of a piece of radiochromic film (XRQA). The foam pad was used for its near air equivalent attenuation properties (CT number - 976 HU); while radiochromic film was used to provide a visual means of centering the ion chamber and MOSFETs within the 40 mm collimated CT beam.
|Figure 3: Metal oxide semiconductor field effect transistor calibration set-up|
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The MOSFET were raised with tissue paper to align the effective points of measurement of the two detectors; the three MOSFET detectors were grouped tightly (within a total of 3 cm of each other) to minimize signal inhomogeneity. CT table was raised and centered within the CT bore so that the detectors were located at the scanner's isocenter, as determined by room lasers (LAP laser, Lüneburg, Germany) for patient alignment. The CT X-ray source focal spot to isocenter distance was quoted as 64.5 cm (according to manufacturer documentation)
The MOSFETs and ion chamber were exposed 5 times with 5 min gap between each exposure for signal equilibration. In order to convert the exposure [X (R)] measurements from the ion chamber to absorbed dose, the American Association of Physicists in Medicine (AAPM) radiation therapy task group (TG) 61 protocol was adopted.  The following equation was used to calculate absorbed dose for a specific beam quality (Q) [D (Q)]:
where M (nC) was the free in air ionization reading that had been temperature and pressure corrected (the chamber wall should have sufficient thickness, 16.5 mg/cm 2 so that it does not need a buildup cap to eliminating electrons), Nk (42.62 mGy/nC for 120 kV; 42.87 mGy/nC for 100 kV) was the air kerma calibration factor from the Secondary Standard Dosimetry Laboratory (SSDL) for a given beam and the chamber stem correction factor was assumed to be unity since the calibration setup met AAPM TG-61 defined criteria. Then the MOSFET calibration coefficient was corrected by water kerma backscatter factor (Bw ) derived from the tabular data provided in AAPM TG-61 as a function of SSD, field diameter and radiation quality, HVL (mm AL) since calculations eventually were performed in phantom.
The MOSFET calibration coefficient [CF (mGy/mV)] was calculated using the following equation:
where the MOSFET reading (mV) was measured with the aid of the MOBILEMOSFET software calibration tool and D (Q) air is the calculated absorbed dose, from Eq. 5.
The absorbed doses were measured in the cylindrical phantoms as in section "Chamber measurements". The MOSEFTs were positioned at 3, 6, 9, 12 o'clock and at the center. The experimental setup for the phantoms with MOSFET dosimeters is shown in [Figure 4]. MOSFET reader was connected to personal computer via RS-232 cable and data were read immediately after the scan.
|Figure 4: Poly methyl metha acrylate body phantom and metal oxide semiconductor field effect transistor (MOSFET) detectors set-up. MOSFET detectors were placed at the midpoint of five clockwise cylindrical cavities centers, 3-, 6-, 9-, 12-o'clock|
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Weighted CBDI (CBDI w ) for the MOSEFT measurements were calculated from the point dose measurements. The CBDI w can be expressed by the same equation as (1).
The point doses and CBDI w of the MOSFET measurement were compared with those of the chamber measurements at the four peripheries and in the center for the head and the body phantom.
| Results and Discussion|| |
CBCT dose chamber and MOSFET measurements
[Table 2] shows the MOSFET calibration coefficients measured with ionization chamber and MOSFETs for two different beam qualities.
[Table 3] shows the CBDI vol values measured using chamber and MOSFETs for the CBCT scanning setting adopted at Pantai Hospital Kuala Lumpur.
The dose values for the pelvis phantom varied from 8.6 mGy to 22.2 mGy with ionization chamber measurements for 120 kVp, 1040 mAs. For the head phantom, the dose values varied from 0.71 mGy to 1.37 mGy for 100 kVp, 36.1 mAs. Amer et al. in their study have reported the CBDI w of 1.6 mGy and 25 mGy for head (100 kVp, 38 mAs) and pelvis (140 kVp, 456 mAs) respectively. 
For head scanning mode, the peripheral doses are greater than the central axis doses by a magnitude of 1.3-2 with the largest found consistently at 3 o'clock position of the PMMA head phantom. For the pelvis scan, the peripheral doses are greater than the central doses by a magnitude of 2.1-2.6 with the largest found consistently at 9 o'clock position of the PMMA head phantom. The dose to the patient on the opposite side of the source rotation is lower, as found by Islam, et al. 
For the Elekta CBCT unit, the CBDI vol doses range between 0.092 cGy and 1.6 cGy for the head-and-neck and pelvis protocol, respectively and agree well with the nominal scan dose supplied by the manufacturer.
As you can be seen in [Table 3], a similar absorbed dose was found in MOSFET measurements. The highest absorbed doses were recorded at 3'-position (1.36 mGy) for the head scanning protocol and 9'- position (22.86 mGy) for the pelvis scanning protocol.
The differences of CBDI vol between chamber and MOSFET measurements were 13% for the head phantom, 3.7% for the body phantom. The CBDI vol for head scanning acquired from chamber showed higher reading compared with MOSFET measurement while MOSFET CBDI vol pelvis measurements had higher reading compared with chamber CBDI vol measurement. These percentage differences can be explained by two things-SSD dependency (Best Medical Canada Technical Note, 2008) and angular dependency. 
Head and neck cancer patients receiving 33-35 sessions of radiation therapy and if daily kV-CBCT were performed, the accumulated dose from kV-CBCT will be around 5 cGy; the maximum accumulated dose for pelvis is determined to be 100 cGy for 43 sessions of daily image guided radiotherapy.
| Conclusion|| |
Based on the measured absorbed dose attained for patients undergoing daily CBCT, one should be aware of the relative risks associated with daily kV-CBCT when setting up CBCT scanning protocol and should try to reduce the high voltage usage and reduce mAs setting since reduction in both parameters can reduce radiation dose to the patient, especially for pediatric patients. Care should be given to the long-term follow-up of patients under image guided radiation therapy, whereas the indications for its use in certain cases should be reconsidered.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4]
[Table 1], [Table 2], [Table 3]