Medical Dosimetry
Volume 36, Issue 1 , Pages 109-112, Spring 2011

A Comparison Between Electronic Portal Imaging Device and Cone Beam CT in Radiotherapy Verification of Nasopharyngeal Carcinoma

Department of Health Technology and Informatics, Hong Kong Polytechnic University, Kowloon, Hong Kong

Received 14 July 2009; accepted 18 January 2010. published online 04 March 2010.

Article Outline

Abstract 

The demand of greater accuracy in intensity-modulated radiotherapy (IMRT) has driven the development of more advanced verification systems. The purpose of this study is to investigate the differences in verification accuracy in terms of the position error detected between cone-beam computed tomography (CBCT) and electronic portal imaging device (EPID) in the IMRT of nasopharyngeal carcinoma (NPC). Two groups of NPC patients (n = 22 and n = 28) verified by CBCT (G1-CB), EPID (G1-EP), and EPID (G2-EP) only, respectively, were recruited. The positional errors between the G1-CB group and the G2-EP group were compared. In addition, the magnitudes of the position errors of EPID taken in the same session of the CBCT, but after necessary corrections (G1-EP), were analyzed. In the CBCT group, 455 CBCT images (G1-CB) and 206 EPID images (G1-EP) were collected, whereas 319 EPID images (G2-EP) for the EPID group, were recorded. The median position errors detected in CBCT were between 0.80 and 0.90 mm in the antero-posterior (A-P), left-right (L-R), and supero-inferior (S-I) directions, whereas those of the EPID were all 0.50 mm. The magnitude of position deviation detected by the CBCT was higher than that of the EPID and their differences were extremely significant (p < 0.001). The frequencies in the G2-EP group with position errors greater than the tolerance (2 mm) were 32, 42, and 27 in the A-P, L-R, and S-I directions, respectively, which accounted for 16.5%, 21.6%, and 13.9% of the total number of EPID. There was difference in verification capability between the CBCT and EPID when applied to IMRT of NPC patients. Because an average of 1 of 6 verifications in EPID was inferior to that of the CBCT, verification by CBCT is recommended.

Key Words: Cone-beam CT, Electronic portal imaging device, Nasopharyngeal carcinoma, Positional error

 

Back to Article Outline

Introduction 

One of the key steps in radiotherapy to ensure treatment accuracy is to verify the patient position before the delivery of radiation. Conventionally, verification is performed by comparing portal images taken by the linear accelerator with images taken at the simulator. Recently, with the emergence of highly conformal radiation therapy techniques such as intensity-modulated radiotherapy (IMRT), treatment accuracy has been of increasing concern. It is because with its steeper dose gradient, a small deviation of the treatment field can result in geographical miss of the target or unnecessary irradiation of the organs at risk (OARs). This has driven the development of more advanced verification systems such as the electronic portal imaging device (EPID) and cone-beam computed tomography (CBCT).

EPID is used to take verification images from megavoltage (MV) x-ray source of the linear accelerator. It consists of a flat panel imager mounted on the linear accelerator opposite to the MV x-ray source. The flat panel imager receives the MV x-ray passed through patient's body by the large area pixilated array, which converts the x-ray into electronic signals and passes them to a host computer that processes and display the portal images. To verify the treatment position, two orthogonal images of the treatment region are taken and matched with the digitally reconstructed radiographs (DRR) generated from the planning CT data. CBCT helps to take CT images of the patient at the treatment region from a cone-shaped kV x-ray beam. It consists of a kV x-ray source with an opposite flat panel detector mounted perpendicular to the gantry of the linear accelerator. During imaging, the gantry with the x-ray source (40–130 kVp) is rotated once around the patient in the treatment position. Image projections are acquired by the flat panel detector and reconstructed using the Feldkamp filtered back projection algorithm to give the 3D volumetric CT image set. The CBCT images are then matched with the planning CT images to check for positional errors. With the provision of better resolution and 3D image information, many studies reported that CBCT provides better image quality than EPID.1, 2, 3, 4 This has been demonstrated in a study using a head and neck phantom, in which the volumetric setup verification using CBCT was able to detect translational error to within ± 0.5 mm, compared with the 2D planar registration of 1–2 mm5.

As far as the radiation dose is concerned, CBCT is operating in the kV range and a mean skin dose of 15 mGy per scan in the head and neck region6 and a dose of up to 1.0 cGy adjacent to the imaged volume have been reported.7 For EPID that usually requires 2 exposures of MV x-ray for the verification imaging; the estimated dose at Dmax per verification ranges between 2.5–3.5 cGy.8 Because MV radiation is more penetrating, the integral dose delivered by EPID per verification is relatively higher than that of CBCT, despite it being suggested that kV radiation carried higher linear energy transfer than MV radiation.9 Nevertheless, the verification superiority of CBCT due to better image resolution may not be significant in all treatment conditions. For instance, in the head and neck cases, where the bony landmarks around the treatment region are reliable and accurate to verify the treatment position in 2D images, EPID may perform as well as the CBCT. If this is true, the use of EPID will be an advantage because it is relatively cheaper and the operation time is shorter than that of CBCT.

In Hong Kong, nasopharyngeal carcinoma (NPC) is one of most common head and neck cancers and is mainly treated by IMRT. However, if EPID can produce comparable verification results to that of CBCT, the adoption of EPID as the main verification method would have a positive impact to the clinical department, because more time and resources will be saved. Therefore, the purpose of this study was to investigate the differences in verification accuracy in terms of the position error detected between CBCT and EPID in the IMRT of NPC. The results of this study provide reference for the justification and optimization of the use of CBCT in NPC patients.

Back to Article Outline

Methods and Materials 

The first group of subjects consisted of 22 NPC patients (G1), who underwent verification with CBCT (G1-CB) and then EPID (G1-EP), after the corrections made from the CBCT in the same treatment session. The corrections were made only when the detected deviations were >2.0 mm, which was the tolerance set for IMRT of NPC cases. A second group of 28 NPC patients (G2) verified by EPID (G2-EP) alone were also recruited.

Verification by CBCT was conducted on the Elekta Synergy XVI system. After acquiring the images, the treatment position deviations in the 3 different directions—supero-inferior (S-I), left-right (L-R), and antero-posterior (A-P)—were determined automatically by the software through image registration of bony structures around the base of the skull and upper cervical regions with the reference planning CT images transferred to the VolumeView (Elekta, Crawley, UK) workstation. No manual adjustments were performed. Verification by EPID was done with the Elekta iViewGT system, which involved taking 2 orthogonal MV images of the A-P and lateral views with a flat panel detector. The position errors were assessed by manual matching of the bony anatomy identified in the EP images with the DRR generated from the reference planning CT. The A-P and S-I shifts were determined in the lateral projection images, in which the cervical spine and occipital bones were used as reference landmarks. The L-R shifts were determined by the anterior projection images, in which the landmarks were the nasal septum and spinous process of cervical spine.

To compare the verification capability between CBCT and EPID, the position errors between the CBCT data of the CBCT group (G1-CB) and the EPID data of the EPID group (G2-EP) were compared. The Mann-Whitney U test was used to test the significance of the differences of their medians (the data was not normally distributed). This comparison was aimed to demonstrate whether there was a difference between the magnitudes of position errors detected by the 2 different verification systems. Besides, to evaluate whether the EPID verification could replace CBCT in the verification of NPC patients, the magnitude of the EPID taken on the same session of the CBCT, but after necessary correction (G1-EP), was analyzed. A value of <2.0 mm would indicate that the EPID accuracy was comparable to the CBCT. It was because in CBCT, corrections were made only when the deviations exceeded 2.0 mm; a maximum deviation of 2.0 mm detected after the CBCT by the EPID would mean that it achieved the verification standard.

Back to Article Outline

Results 

In the 22 subjects of the CBCT group, 455 CBCT images (G1-CB) and 206 EPID images (G1-EP) taken after CBCT correction were collected. In the 28 subjects of the EPID group, 319 EPID images (G2-EP) were recorded. The magnitudes of position errors detected among the A-P, L-R, and S-I directions were close. CBCT detected median position errors between 0.80 and 0.90 mm in the 3 directions, whereas those of the EPID were 0.50 mm (Table 1). The magnitude of position deviation detected by the CBCT was higher than that of the EPID and their differences were extremely significant (p < 0.001).

Table 1. Comparison of treatment position deviations detected by CBCT (G1-CB) and EPID (G2-EP) in the A-P, L-R, and S-I directions
G1-CB (n = 22, CBCT = 455)G2-EP (n = 28, EPID = 319)
Median (mm)Range (mm)Median (mm)Range (mm)
A-P0.900.00–4.500.500.00–4.00
L-R0.800.00–5.600.500.00–3.00
S-I0.900.00–4.100.500.00–4.00

On the other hand, the mean position errors detected by EPID in the CBCT group after CBCT correction were 0.93 mm, 1.06 mm, and 0.83 mm in the A-P, L-R, and S-I directions, respectively. The frequencies of the EPID group (G2-EP) with position errors greater than the tolerance (2 mm) were 32, 42, and 27 in the A-P, L-R, and S-I directions, respectively, which accounted for 16.5%, 21.6%, and 13.9% of the total number of EPID.

Back to Article Outline

Discussion 

Because IMRT of NPC uses a rigid immobilization system, as well as the fact that there is little body movement in the treatment region, the position error due to patient movement is expected to be small. This is the reason that a more stringent tolerance of 2 mm is set for such treatment. The results of this study revealed that >70% of the deviations detected including CBCT and EPID were within the tolerance. CBCT verification in NPC has gained increasing attention, and some studies reported that CBCT improved the set-up precision of IMRT for NPC.10, 11 When comparing between CBCT and EPID, significantly larger position errors were detected by the CBCT (G1-CB) relative to the EPID (G2-EP). This did not mean CBCT was less accurate in detecting positional deviations; on the contrary, it could detect the actual magnitude of the deviation, which the EPID might not be able to achieve. Therefore, the result was not surprising because compared with EPID, CBCT detected errors in 3D, in which 2 more rotational axes were taken into account. The inclusion of these 2 extra rotational axes in CBCT could affect the translational error, which was perpendicular to these extra axes of rotation and subsequently increased the overall position error values. On the contrary, the position errors detected by EPID that did not take into account these extra errors would give a smaller error magnitude. Besides, the time used in obtaining images by CBCT was usually longer than that in EPID. Patient movement during the acquisition of images might occur and could contribute to larger measured position errors. Although treatment of NPC patients used a thermoplastic cast for immobilization, position errors up to 2.0 mm might still exist. This could further be exaggerated if the patient had lost weight and the thermoplastic shell did not fit the patient as well.

As stated earlier, a mean deviation of <2.0 mm detected in GI-EP would indicate that EPID had comparable verification capability as that of the CBCT. However, the incidence of the G1-EP that produced position errors exceeding 2.0 mm ranged from 13.9%–21.6% (mean 17.3%) for the 3 directions. This implied that an average of about 1 of 6 verifications taken in the EPID were inferior to that of the CBCT.

The results of this study demonstrated a difference in verification capability between the CBCT and EPID, which was reflected in the significant difference in their magnitudes of error detection. This was in line with the study by Li et al.,12 who compared 2D kV and CBCT alignments of head and neck cases and reported that the differences between the 2 verification capabilities were mainly a result of the relative flexibility of certain head and neck structures and head rotation, which was difficult to detect with EPID.5 Taking CBCT as the standard in verification, the performance of EPID was slightly inferior because it was found to happen in an average of 17.3% of the verifications. Logically based on this result, CBCT would be the choice of verification method and recommended for the NPC patients. The average percentage error of EPID could be high if it is not taken frequently during a radiotherapy course (i.e., 17.3% in 6 verifications). However, increasing the frequency of EPID sessions will deliver greater extra dose to the patient and therefore may not be desirable.

Back to Article Outline

Conclusion 

There was a difference in verification capability between the CBCT and EPID when applied to IMRT of NPC patients. An average of 1 of 6 verifications in EPID was inferior to that of the CBCD. Added to the fact that there is no significant advantage in reducing patient dose in EPID, CBCT is recommended as the verification method for IMRT of NPC patients.

Back to Article Outline

Acknowledgments 

This study was supported by the Clinical Oncology Department, Tuen Mun Hospital, Hong Kong.

Back to Article Outline

References 

  1. Borst GR, Sonke JJ, Betgen A, et al. Kilo-voltage cone-beam computed tomography setup measurements for lung cancer patients: First clinical results and comparison with electronic portal-imaging device. Int. J. Radiat. Oncol. Biol. Phys. 2007;68:555–561
  2. Xing L, Thorndyke B, Schreibmann E, et al. Overview of image-guided radiation therapy. Med. Dosim. 2006;31:91–112
  3. Sykes JR, Amer A, Czajka J, et al. A feasibility study for image guided radiotherapy using low dose, high speed, cone beam x-ray volumetric imaging. Radiother. Oncol. 2005;77:45–52
  4. Zeidan OA, Langen KM, Meeks SL, et al. Evaluation of image-guidance protocols in the treatment of head and neck cancers. Int. J. Radiat. Oncol. Biol. Phys. 2007;67:555–561
  5. Kim GY, Pawlicki T, Le QT, et al. Linac-based on-board imaging feasibility and the dosimetric consequences of head roll in head-and-neck IMRT plans. Med. Dosim. 2008;33:93–99
  6. McBain CA, Henry AM, Sykes J, et al. X-ray volumetric imaging in image-guided radiotherapy: The new standard in on-treatment imaging. Int. J. Radiat. Oncol. Biol. Phys. 2006;64:625–634
  7. Perks JR, Lehmann J, Chen AM, et al. Comparison of peripheral dose from image-guided radiation therapy (IGRT) using kV cone beam CT to intensity-modulated radiation therapy (IMRT). Radiother. Oncol. 2008;89:305–311
  8. McGarry CK, Grattan MWD, Cosgrove VP. Optimization of image and dose for Varian aS500 electronic portal imaging devices (EPIDs). Phys. Med. Biol. 2007;52:6865–6877
  9. Frankenberg D, Kelnhofer K, Bar K, et al. Enhanced neoplastic transformation by mammography x-ray relative to 200 kVp x-rays: Indication for a strong dependence on photo energy of the RBEm for various endpoints. Radiat. Res. 2002;157:99–105
  10. Wang J, Xu F, Bai S, et al. Preliminary application of kilo-volt cone-beam computed tomography to intensity-modulated radiotherapy of nasopharyngeal carcinoma. Ai. Zheng. 2008;27:761–765
  11. Wang J, Bai S, Chen N, et al. The clinical feasibility and effect of online cone beam computer tomography-guided intensity-modulated radiotherapy for nasopharyngeal cancer. Radiother. Oncol. 2009;90:221–227
  12. Li H, Zhu XR, Zhang L, et al. Comparison of 2D radiographic images and 3D cone beam computed tomography for positioning head-and-neck radiotherapy patients. Int. J. Radiat. Oncol. Biol. Phys. 2008;71:916–925

PII: S0958-3947(10)00016-6

doi:10.1016/j.meddos.2010.01.005

Medical Dosimetry
Volume 36, Issue 1 , Pages 109-112, Spring 2011

Article Tools