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Year : 2016  |  Volume : 12  |  Issue : 2  |  Page : 858-863

Radiation dose measurements during kilovoltage-cone beam computed tomography imaging in radiotherapy

1 Department of Radiotherapy, Christian Medical College, Vellore, Tamil Nadu, India
2 Radiological Physics and Advisory Division, Bhabha Atomic Research Center, Mumbai, Maharashtra, India

Date of Web Publication25-Jul-2016

Correspondence Address:
B Paul Ravindran
Department of Radiotherapy, Christian Medical College, Vellore, Tamil Nadu
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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/0973-1482.164699

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 > Abstract 

Objective: The use of image guidance during radiotherapy for accurate localization and setup has become the standard care of practice in radiotherapy. This mostly involves the use of kilovoltage-cone beam computed tomography (kV-CBCT) for verification of patient setup on the first few days and on a weekly basis. Some protocols require this to be performed daily and also before and after the treatment. Though the radiation due to this kV-CBCT is small, the repeated use could deliver a dose that could increase the probability of the stochastic effect. The main purpose of this work is to measure radiation dose during image guidance with kV-CBCT.
Materials and Methods: In this work, we have attempted to measure the dose during kV-CBCT for different sites both on a humanoid phantom and on patients undergoing image-guided radiotherapy with MOSFETs calibrated against an ion chamber.
Results: The dose measurement on patients during kV-CBCT resulted in mean doses of 0.19 and 0.3 cGy to the ipsilateral and contralateral eyes, 0.625 and 1.097 cGy to the surface of the ipsilateral and contralateral breasts, and 3.01 cGy to the surface of the pelvis.
Conclusion: Radiation dose to the eye, breast, and the surface of the pelvis have been arrived at during CBCT. The doses measured on patients agreed closely with those measured on humanoid phantom and with published values.

Keywords: Cone beam computed tomography, image-guided radiotherapy, in vivo dose measurement, MOSFET

How to cite this article:
Kumar A S, Singh I R, Sharma SD, John S, Ravindran B P. Radiation dose measurements during kilovoltage-cone beam computed tomography imaging in radiotherapy. J Can Res Ther 2016;12:858-63

How to cite this URL:
Kumar A S, Singh I R, Sharma SD, John S, Ravindran B P. Radiation dose measurements during kilovoltage-cone beam computed tomography imaging in radiotherapy. J Can Res Ther [serial online] 2016 [cited 2020 Jul 12];12:858-63. Available from: http://www.cancerjournal.net/text.asp?2016/12/2/858/164699

 > Introduction Top

Technological advances in radiation oncology have enabled the delivery of high precision radiotherapy such as three-dimensional conformal radiotherapy (3D CRT), intensity modulated radiotherapy, and stereotactic body radiotherapy. These techniques aim to achieve therapeutic dose delivery to the planning target volume (PTV) while limiting the dose to organs at risk and normal tissues. Limiting dose to organs at risk and normal tissues requires that the margin on the PTV is reduced, planning risk volume with margin is provided and that the PTV is accurately targeted. Conventionally, external markers made on the patient surface are used to direct the radiation beam to the target volume. This could result in a geographical miss as the relationship between the external marker, and the clinical target volume could have been lost during the time between planning and the treatment and during the course of the treatment.[1] Hence, imaging during treatment enables accurate targeting of the lesion, and imaging using electronic portal imaging device (EPID) is commonly used for patient positioning in precision radiotherapy. Though EPID enables two-dimensional image guidance, it has its limitations due to poor image quality and low contrast of the images produced with megavoltage beams. The recent advancement in imaging systems with amorphous silicon flat panel devices has made image guidance with cone beam computed tomography (CBCT) during radiotherapy a possibility [2],[3] and the fact that it is possible to generate a complete 3D volume through a single rotation of a source and detector about the object has instilled interest in many in this field.[4],[5],[6] Although, both megavoltage and kilovoltage CBCT (kV-CBCT) systems are available, kV-CBCT is preferred because it offers significant performance advantages in terms of image contrast and SNR per unit dose for visualization of soft-tissue structures. The other advantage of the kV-CBCT system is that it offers the potential of combined volumetric and radiographic/fluoroscopic imaging using the same device.[7]

This new online imaging approach, that is, the linac-integrated kV-CBCT is useful for patient positioning and determining inter-fractional organ movement.[8],[9],[10] The kV-imaging modality specifically aims to provide good soft-tissue contrast like diagnostic CT within the treatment room. Although, the radiation dose during a kV-CBCT is considerably low [11],[12] repeated use of this for daily treatment position verification could deliver considerable dose to normal tissues [13],[14] and organs at risk and has been shown to deliver more dose to critical structures in children than adults.[15] It is, therefore, necessary that the dose during a kV-CBCT session is known, and an attempt should be made to minimize these doses as per the ALARA principle.[16] Several authors have studied the dose during image guidance in radiotherapy both with phantom [13],[17] and on patients.[18] A few dose reduction and optimization methods have been suggested and the reduction of the scan exposure time or beam rotation range of the CBCT imaging significantly reduced the dose to certain organs.[19],[20] The beam characteristics and the organs doses have also been estimated by Monte Carlo simulation studies for various organs by a few investigators.[17],[21],[22]

In vivo dosimetry for CBCT could be performed with thermos-luminescent dosimeters, diodes, and MOSFETs, which are routinely used for in vivo dose measurement. In this study, we used high sensitive MOSFET dosimeters that are suitable for surface dose measurements [18],[23] for measurements during image guidance. The dose to critical organs such as eyes, contralateral breast, and on surface dose during image guidance of head and neck, breast, and pelvis treatments have been measured with MOSFETs on anthropomorphic phantoms and on patients undergoing CBCT based image guided radiotherapy (IGRT).

 > Materials and Methods Top

Description of materials


In this study, we used high sensitivity MOSFET dosimeters (TN 1002RD) along with dose verification system (TN RD 70W) shown in [Figure 1], supplied by the BEST Medicals, Canada. It consists of remote monitoring dose verification software, wall-mounted Bluetooth wireless transceiver, and a small reader module. The reader module is supplied with dual bias settings namely, standard bias, and high bias settings. The high sensitivity of the MOSFETs were used in this study were 10 mV/cGy for standard bias and 30 mV/cGy for high bias setting.
Figure 1: The MOSFET dosimeter with the reader unit

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External beam therapy units

Dose measurements were performed in Clinac 2100 C/D linear accelerator with integrated kV-CBCT for patient position verification. The on-board imaging system is provided with variable kV X-rays setting. Half bow tie and full bow tie filters provided by the manufacturers are used for half-fan full rotation imaging and full-fan, and half rotation imaging, respectively. The implementation of a bowtie filter in CBCT offers the method of addressing the issue of X-ray flux variation across the detector. These filters serve as a compensator for improved image quality through fluence modulation, reduce in X-ray scatter, and in patient dose.[24] The half-fan - full rotation was used for imaging of the breast, chest, and pelvis whereas the full-fan half rotation was used for imaging during head and neck treatments.


In this study, acrylic slab phantoms of physical density 1.18 were used for calibrating the MOSFET dosimeters against ion chambers. Phantom dosimetry for CBCT was performed with the Rando phantom (Alderson Rando phantom) which incorporates materials to replicate the body organs such as tissue, muscle, bone, lung, and air cavities. To validate the calibration of MOSFET, head water phantom was used that has a provision to place either the MOSFET or the ion chamber in the center of the phantom.

Ion chambers

Output measurements for the CBCT for different kV settings for calibration of the MOSFETs were carried out using parallel plate ion chamber and 0.6 cc PTW ion chamber with PTW Unidos electrometer. Ion chambers and electrometers were individually calibrated for variable kV energies, and the air kerma calibration factors were obtained from secondary standard calibration laboratory at Bhabha Atomic Research Centre, Mumbai, Maharashtra, India.

Description of measurements

Calibration of MOSFET

The MOSFETs were calibrated for both surface dose measurements and for dose measurements at depth. For the surface dose measurements, the MOSFETs were calibrated against parallel plate ion chamber, which has air kerma calibration factor (Nk) traceable to the primary standard. The [Figure 2] shows the calibration setup used for MOSFET calibration against the parallel plate chamber on the surface of the phantom. The calibration factors were calculated by taking the ratio of dose measured using ion chamber (cGy) to the MOSFETs response (mV) and individual calibration factors (cGy/mV) were determined for all the energies. Similarly, the MOSFETs were calibrated for measurements at depth by cross calibrating it against a 0.6 cc cylindrical ion chamber placed at 2 cm depth in an acrylic slab phantom.
Figure 2: Setup for calibration of MOSFET against parallel plate chamber (a) setup for measuring output using parallel plate ion chamber (b) setup to measure MOSFET response with the same condition used for parallel plate ion chamber

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Validation of MOSFET response with ion chamber measurements

The calibration of the MOSFET dosimeters was verified by performing isocenter dose measurements for CBCT. The measurement was performed by placing the MOSFETs at the center of a water filled head phantom. The MOSFETs were replaced with calibrated 0.6 cc PTW ion chamber in the head water phantom, and the dose measurements were repeated for CBCT with the same kV settings. The setup used for this measurement with the water filled head phantom is shown in [Figure 3].
Figure 3: Verification of MOSFET dose measurement in water filled head shaped phantom against 0.6 cc ion chamber

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Dose measurement in phantom for kilovoltage-cone beam computed tomography

Radiation dose measurements for kV-CBCT were performed initially with Rando phantom using MOSFETs. The doses to ipsilateral and contralateral eyes, the surface of the contralateral breast, and that of the pelvis were measured with Rando phantom for CBCT imaging of the head and neck, breast, and pelvis, respectively. The MOSFETs were placed at the level of the eye during head and neck imaging and at the level of the rectum in the Rando phantom during pelvic imaging. The setups used for these measurements are shown in [Figure 4]a,[Figure 4]b,[Figure 4]c. The site-specific CBCT setting used during these imaging are given in [Table 1].
Figure 4:(a) Setup for dose measurement with MOSFET on Rando phantom (a) near the eye for head and neck scan with MOSFETs fixed (insert), (b) on the surface of breast, and (c) on the surface of pelvis

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Table 1: Site specific default settings used for CBCT imaging of phantom and patients

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In vivo dose measurements during kilovoltage-cone beam computed tomography

In vivo dose measurements during kV-CBCT for patients undergoing IGRT treatment for head and neck, breast, and pelvis were performed. The doses were measured for 15 patients undergoing treatment for head and neck cancers by placing the MOSFET on the eyelid during CBCT imaging. Similarly, doses to ipsilateral and contralateral breasts were measured on the surface with MOSFET for 10 patients during CBCT. The radiation dose on the surface of the pelvis was also measured during the kV-CBCT for 10 patients treated for carcinoma prostate. Site-specific default kV settings used for these imaging and are tabulated in [Table 1].

 > Results and Discussion Top

Calibration of MOSFET

The calibration factors for surface dose measurements for the MOSFETs in cGy/mV were obtained with the setup as discussed in methods for 100, 110, and 125 kV beams used for CBCT of head and neck, thorax, and pelvis imaging, respectively. Since, the MOSFETs were nearly energy independent for these kV settings, an average calibration factor of 0.11 cGy/mV could be obtained. The calibration factor obtained for measurements at depth in the acrylic slab phantom was 0.101 cGy/mV. These calibration factors were used for all the phantom measurements and the in vivo measurement with MOSFETs.

Validation of MOSFET dosimeters with Ion chamber measurements

The calibration of the MOSFETs was verified by comparing the measurements at isocenter for CBCT with the head water phantom. The measurements were performed for half rotation full cone scan and the mean dose to the isocenter measured with MOSFET, and the ion chamber were 0.356 cGy and 0.363 cGy, respectively for three trials with a standard deviation of ±0.00843 for the MOSFET reading. The deviation of the MOSFET measurement in the phantom compared to the ion chamber was −1.93% and agreed well with measurements made with the ion chamber.

Phantom and patient dose measurement

Phantom measurements

The doses to ipsilateral and contralateral eyes were measured for both clockwise and anti-clockwise direction. A small variation in dose was observed between the clockwise, and anti-clockwise direction of imaging and the anti-clockwise rotation was slightly lower and this could be attributed to the X-ray beam stability at the beginning of the scan.

[Table 2] shows the doses measured with MOSFET dosimeters at the eye, breast, and pelvis in Rando phantom for the CBCT for clockwise direction and anti-clockwise direction rotation of the gantry. The maximum doses to the contralateral and ipsilateral eyes were 0.28 and 0.35 cGy respective, the maximum doses to the contralateral and ipsilateral breasts were 0.69 and 1.91 cGy, respectively and the average dose to the surface of the pelvis was 3.86 cGy. Dose measurements were also performed on the phantom at the point of the eye, by placing the MOSFET in the cavities of the Rando phantom during the head scan, and at the point of the rectum during the pelvic scan. The average doses measured in the eye in Rando phantom is 0.56 cGy for standard imaging settings provided in linac and 1.43 cGy for high-quality imaging setting. The average dose to the rectum in the Rando phantom during CBCT is 4.71 cGy.
Table 2: Dose measured in Rando phantom for eye, breast and pelvis during kV-CBCT imaging

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In vivo dose measurements during cone beam computed tomography

In vivo dose measurements were carried out for 15 head and neck patients treated for carcinoma maxilla, carcinoma larynx, and carcinoma buccal mucosa, etc., and for 10 patients having carcinoma breast/lung and 10 patients with carcinoma prostate. The measurements were performed for 3 days for each patient. The eye dose measured for the head and neck patients varied slightly due to varying distance of the position of MOSFET from the isocenter for each patient. The average value of the dose to the eye measured for three consecutive days for each of the 15 patients are plotted in [Figure 5]. The average of doses measured for three consecutive days of kV-CBCT imaging on the surface of the ipsilateral and contralateral breasts are given in [Figure 6] and the doses measured on the surface of the pelvis for 10 patients undergoing kV-CBCT imaging were also obtained for three consecutive days and the average dose is plotted in [Figure 7].
Figure 5: Average doses measured at the contralateral and ipsilateral eyes for 15 patients during kilovoltage-cone beam computed tomography

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Figure 6: Average doses measured on the surface of the contralateral and ipsilateral breasts for 10 patients during kilovoltage-cone beam computed tomography

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Figure 7: Average doses measured on the surface of the pelvis for 10 patients during kilovoltage-cone beam computed tomography

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As shown in [Figure 5], the maximum dose measured to the contralateral and ipsilateral eyes are 0.29, 0.42 cGy, respectively, and the minimum to the contralateral and ipsilateral eyes are 0.06 and 0.19 cGy, respectively, the mean dose to contralateral and ipsilateral eyes from the measurement performed on the 15 patients are 0.19 and 0.3 cGy, respectively. Similarly from [Figure 6], the maximum dose to the contralateral and ipsilateral breasts are 0.91 and 1.63 cGy, respectively, and minimum dose to the contralateral and ipsilateral breast are 0.43 and 0.88 cGy, and the mean dose to the contralateral and ipsilateral breast from the 10 patients measured are 0.625 and 1.097 cGy, respectively. The maximum and minimum doses measured on the surface of the pelvis are 3.33 cGy and 2.95 cGy and the mean dose to the surface of the pelvis from the 10 patients measured is 3.01 cGy. These values are also tabulated along with the scan parameters in [Table 3] for reference.
Table 3: Maximum and mean doses measured on patient during kV-CBCT imaging

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Our measurements performed on phantom and the in vivo measurements on patients agree closely for the doses measured during imaging of head and neck, thorax, and pelvis. The results also agree well with the in vivo dose measurements performed by Amer et al. studies with TLD [14] and the Monte Carlo measurements of Nelson and Ding.[25] The dose measured is compared with Amer et al. in the [Table 4], for the measurements in Rando phantom and the patients. For head and neck scans Ding et al.[17] have reported much higher eye dose (6–12 cGy per scan) with half-fan and full scan settings.
Table 4: Dose comparison with literature values

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The doses measured for pelvic patients on the surface also agrees closely with the results of Wen et al.[26] which is about 3 cGy per scan and that of Ding et al.[17] (3–6 cGy per scan) obtained with Monte Carlo calculations. The higher pelvic dose is due to the higher kV and mAs setting used for pelvic imaging [Table 1]. As some of the pelvic protocols require daily CBCT, this would results in delivering excess dose of about 100–120 cGy for 30–35 fractions over the prescribed dose and a repeat scan can double the dose.

 > Conclusion Top

The dose management process of IGRT involves (i) assessment, (ii) reduction, and (iii) optimization.[16] The radiation dose during a kV-CBCT varies depending on the site being imaged, and the imaging protocol used. The MOSFET dosimeters have been found to be very suitable for assessing the dose during CBCT and the dosimetric data helps to optimize the imaging procedure and the dose during IGRT and also helps to document the patient's dose due to imaging. The dose analysis for the measurement performed in Rando phantom and patients measurements closely agrees with the earlier studies carried out by Amer et al.[14] Reference doses to the eye, the surface of the breast, and pelvis for site-specific dosimetry protocols have been generated and provided. Dose reduction strategies should be employed to reduce dose during IGRT and protocol for CBCT decided should take into account the imaging dose.


This work was funded by Board of Research in Nuclear Sciences, Department of Atomic Energy, India.

Financial support and sponsorship

Board of Research in Nuclear Sciences, DAE, Government of India.

Conflicts of interest

There are no conflicts of interest.

 > References Top

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Nelson AP, Ding GX. An alternative approach to account for patient organ doses from imaging guidance procedures. Radiother Oncol 2014;112:112-8.  Back to cited text no. 25
Wen N, Guan H, Hammoud R, Pradhan D, Nurushev T, Li S, et al. Dose delivered from Varian's CBCT to patients receiving IMRT for prostate cancer. Phys Med Biol 2007;52:2267-76.  Back to cited text no. 26


  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7]

  [Table 1], [Table 2], [Table 3], [Table 4]


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