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Cranial stereotactic radiotherapy

ExacTrac X-ray 6 degree-of-freedom image-guidance for intracranial non-invasive

stereotactic radiotherapy: Comparison with kilo-voltage cone-beam CT

Jinli Ma

a,b,

, Zheng Chang

, Zhiheng Wang

, Q. Jackie Wu

, John P. Kirkpatrick

, Fang-Fang Yin

Department of Radiation Oncology, Fudan University Cancer Hospital, Shanghai, China;

Department of Radiation Oncology, Duke University Medical Center, Durham, NC, USA

a r t i c l e i n f o

Article history:

Received 23 May 2009

Received in revised form 25 August 2009

Accepted 27 September 2009

Available online 19 October 2009

Keywords:

Intracranial non-invasive stereotactic

radiotherapy

IGRT

ExacTrac X-ray 6D

Cone-beam CT

a b s t r a c t

Background and purpose:

To compare the residual setup errors measured with ExacTrac X-ray 6 degree-

of-freedom (6D) and cone-beam computed tomography (CBCT) for a head phantom and patients receiv-

ing intracranial non-invasive fractionated stereotactic radiotherapy (SRT).

Materials and methods:

Setup data were collected on a Novalis Tx treatment unit for an anthropomorphic

head phantom and 18 patients with intracranial tumors. Initial corrections were determined and cor-

rected with the ExacTrac system only, and then the residual setup error was determined by means of

three different procedures. These procedures included registrations of ExacTrac X-ray images with the

corresponding digitally reconstructed radiographs (DRRs) using the ExacTrac 6D fusion, and registrations

of CBCT images with the planning CT using both online 3D fusion and offline 6D fusion. The difference in

residual setup errors between ExacTrac system and CBCT was computed. The impact of rotations on the

difference was evaluated.

Results:

A modest difference in residual setup errors was found between ExacTrac system and CBCT. The

root-mean-square (RMS) of the differences observed for translations was typically <0.5 mm for phantom,

and <1.5 mm for patients, respectively. The RMS of the differences for rotation(s) was however <0.2

degree for phantom, and <1.0 degree for patients, respectively. The impact of rotation on the setup dif-

ference was minor but not negligible.

Conclusions:

This study indicates that there is a general agreement between ExacTrac system and CBCT.

Recent advances in non-invasive immobilization and image-

guided localization have aroused increased interest in the use of

fractionated stereotactic radiotherapy (SRT) as an alternative to sin-

gle fraction stereotactic radiosurgery (SRS)

[1]

. The potential benefit

of utilizing SRT is that normal tissues undergo repair between treat-

ments, while tumor cell kill may be increased by delivering a higher

total dose, thereby improving the therapeutic ratio

[2,3]

. Key to the

high accuracy of SRS is the use of invasive immobilization devices to

accurately localize the tumor. Non-invasive masks used for SRT can

has been developed in a novel dedicated platform for non-invasive

SRS and conventional RT (Novalis Tx, Varian, CA, USA and BrainLAB,

Heimstetten, Germany). The implementation of kV-CBCT is partic-

ularly favorable because anatomical structures and soft tissues are

often better visualized on transaxial images than planar images

[10,12]

. In contrast, ExacTrac

X-ray 6D offers several advantages,

including faster setup, motion tracking, 3D quantitative rotational

errors, real-time imaging during treatment, and less radiation to

the patient

[13]

. However, it is important to determine if localiza-

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lly, no shifts were applied in between CBCT

images to ensure comparability of positions.

re then determined by registering each pair

the corresponding DRRs using online Exac-

re, or planning CT using online CBCT 3D fu-

tively. All registrations were done using

hm. If the offsets measured by CBCT were

determined by the treating physician, shifts

re treatment, as CBCT is favored for the rea-

lity and image contrast exceeds that of p a-

parison, registration of CBCT images with

of offline 6D fusion in Eclipse (version 8.5)

ll to evaluate the impact of the rotations in

ns (i.e., SI and LR) which are not available

of Varian online review software (version

are (RMS) and standard deviation (SD) of the

easured with ExacTrac

X-ray 6D and CBCT,

ferences between them were calculated. The

was used to test the setup differences, and

es in setup differences caused by retrospec-

ata with offline 6D fusion in Eclipse. Statis-

considered at

< 0.05.

r phantom

he residual setup errors for translation in

ther with the rotation(s) for the phantom

residual setup errors measured with Exac-

und 0.30 mm for translations and 0.30 de-

respectively. The residual errors for

with online 3D CBCT were within +/

less than 0.50 mm. The RMS of the residual

ed with CBCT was <0.20 degree.

nalyses of CBCT scans using offline 6D fusion

se in the RMS of residual errors for transla-

as well as a slight decrease in the RMS of

, in comparison with online 3D CBCT setup

however, were statistically not significant

.642 for translations in LR, SI and AP direc-

0.629 for yaw).

Fig. 2

. The RMS and SD of residual setup errors were calculated

for all patients (

Table 3

). As can be seen from

Table 3

, the residual

setup errors measured with online CBCT were generally larger than

that determined by the ExacTrac system. In general, the RMS of

residual setup errors measured with 3D CBCT was <1.50 mm for

translations, and <1.00 degre for rotation; whereas the RMS of

residual setup errors measured with ExacTrac system was no more

than 1.00 mm for translations, and 0.50 degree for rotations,

respectively.

Setup error for ExacTrac

X-Ray 6D and CBCT in SRT

Statistical analysis

The root-mean-square (RMS) and standard deviation (SD) of the

residual setup errors measured with ExacTrac

X-ray 6D and CBCT,

as well as the setup differences between them were calculated. The

paired Student’s

-test was used to test the setup differences, and

to evaluate the changes in setup differences caused by retrospec-

tive analysis of CBCT data wi h offline 6D fusion in Eclipse. Statis-

tical significance was considered at

< 0.05.

Results

Setup measurements for phantom

Tabl 2

pre nts the esidual s tup errors for translation i

three directions, togethe with the rotation(s) for the phantom

study. The RMS of the residual setup errors measured with Exac-

Trac X-ray 6D was around 0.30 mm for translations and 0.30 de-

gree for rotations, respectively. The residual errors for

translations measured with online 3D CBCT were within +/

1.00 mm, with RMS of less than 0.50 mm. The RMS of the residual

errors for yaw measured with CBCT was <0.20 degree.

The retrospective analyses of CBCT scans using offline 6D fusion

showed a slight increase in the RMS of residual errors for transla-

tions in all directions, as well as a slight decrease in the RMS of

residual errors for yaw, in comparison with online 3D CBCT setup

data. These changes, however, were statistically not significant

(

= 0.463, 0.637 and 0.642 for translations in LR, SI and AP direc-

tions, respectively;

= 0.629 for yaw).

The residual setup difference between ExacTrac

X-ray 6D and

3D CBCT was plotted f r each phantom measurement (

Fig. 1

). The

RMS of the residual setup difference between each pair of mea-

urements with ExacTrac system and CBCT is also presented in

Ta-

ble 2

. For the 3D CBCT data, none of the differences in residual

errors were statistically significant. The difference for translations

and yaw was generally within 0.50 mm and 0.20 degree, respec-

tively. By comparison, the retrospective analysis using offline 6D

fusion showed an increased difference. However, none of these dif-

ferences reached statistical significance (see

-values in

Table 2

). In

general, the RMS difference between ExacTra

X-ray 6D and off-

line 6D CBCT was <0.40 mm for translations, and <0.3 degree for

rotations.

Setup measurements for patients

A total of 50 pairs of X-ray and CBCT scans w re collected, with

9 patient having >one pa r of sc ns. A represe t ive example of a

p tient’s planning CT ( r DRRs), CBCT, and orthogonal X-ray

images, as seen during one se sion of image registration, is shown

Fig. 1.

A plot of the residual setup discrepancy between ExacTrac

X-ray 6D and 3D

CBCT for each phantom measurement for (a) tr nslation in the le t– ight (LR), (b)

translation in the superior–inferior (SI), (c) translation in the anterior–posterior

(AP) directions, and (d) rotation around the AP direction (yaw). The X-ray images

were registered with the corresponding digitally reconstructed radiographs (DRRs)

sing the ExacTrac 6D fusion software, while the CBCT was r gistered with planning

CT using online CBCT 3D fusion software.

Radiotherapy and Oncology 2009;93:602–608

Modest difference in localiz t on accura y f r ph n om

and patient measurements between 2D and 3D