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Invasive frame-based: immobilization

l

Image-guidance

g

Quantification of intrafraction motion

g

Frame-based system

SBRT set up

A clinical comparison of patient setup and intra-fraction motion using

frame-based radiosurgery versus a frameless image-guided radiosurgery system

for intracranial lesions

Naren Ramakrishna

, Florin Rosca, Scott Friesen, Evrim Tezcanli, Piotr Zygmanszki, Fred Hacker

Department of Radiation Oncology, Brigham and Women’s Hospital and Dana Farber Cancer Institute, Boston, MA, USA

a r t i c l e i n f o

Article history:

Received 12 June 2009

Received in revised form 8 December 2009

Accepted 29 December 2009

Available online 28 January 2010

Keywords:

Frameless

Radiosurgery

Image-guided

Stereotactic

a b s t r a c t

Background and purpose:

A comparison of patient positioning and intra-fraction motion using invasive

frame-based radiosurgery with a frameless X-ray image-guided system utilizing a thermoplastic mask

for immobilization.

Materials and methods:

Overall system accuracy was determined using 57 hidden-target tests. Positioning

agreement between invasive frame-based setup and image-guided (IG) setup, and intra-fraction displace-

ment, was evaluated for 102 frame-based SRS treatments. Pre and post-treatment imaging was also

acquired for 7 patients (110 treatments) immobilized with an aquaplast mask receiving fractionated IG

treatment.

Results:

The hidden-target tests demonstrated a mean error magnitude of 0.7 mm (SD = 0.3 mm). For SRS

treatments, mean deviation between frame-based and image-guided initial positioning was 1.0 mm

(SD = 0.5 mm). Fusion failures were observed among 3 patients resulting in aberrant predicted shifts.

The image-guidance system detected frame slippage in one case. The mean intra-fraction shift magnitude

observed for the BRW frame was 0.4 mm (SD = 0.3 mm) compared to 0.7 mm (SD = 0.5 mm) for the frac-

tionated patients with the mask system.

Conclusions:

The overall system accuracy is similar to that reported for invasive frame-based SRS. The

intra-fraction motion was larger with mask-immobilization, but remains within a range appropriate

for stereotactic treatment. These results support clinical implementation of frameless radiosurgery using

the Novalis Body Exac-Trac system.

Radiosurgery has an important role in the treatment of primary

brain tumors, metastases, and functional disorders. Effective radi-

infection, and requires pre-medication. Furthermore, the care of

patients wearing head frames creates a clinical resource burden

Radiotherapy and Oncology 95 (2010) 109–115

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diosurgery has been the de facto gold

eatment. It provides reliable immobili-

efficacy is well established. However,

d frame is associated with substantial

tient comfort, safety, clinical and tech-

risk of errors related to frame slippage

of relocatable stereotactic immobiliza-

ill-Thomas Cosman frame were devel-

facilitating fractionated stereotactic

radiosurgery

[17]

The potential weak-

at the patient may shift relative to the

during relocation of the device or be-

ment

[5]

y of image-guided frameless radiosur-

veloped for clinical use which rely on

[13,18]

, X-ray image guidance

[19]

r patient localization

[14,20]

. X-ray im-

iffers from frame-based radiosurgery

ed frameless radiosurgery in that the

mobilization device and the skull anat-

ed from treatment planning to actual

at the time of treatment is used to di-

on of the patient in stereotactic space.

testing supports the technical capabil-

urately localize and deliver dose to the

idealized conditions

[19,21]

, clinical

e complex challenge to the use of these

-target testing in a cranial phantom to

verall system accuracy of the Novalis

have demonstrated a total error mag-

m standard deviation). This compares

ished IG systems such as Cyberknife

rame-based radiosurgery using a linac

sistent with other published studies of

f the Novalis Body Exac-Trac system

antom testing for intracranial targets

atomy and their interaction with the

guidance systems may in theory reduce

to idealized phantom testing results.

ong concordance between frame-based

ing. The clinical positioning coordinates

ance for our group of 69 patients and

1.0 mm (SD = 0.5) of the frame-based

ance is similar to that observed by Sol-

[23,24]

. A systematic anterior–poster-

= 0.3 mm) between frame-based and

bserved by Lamba et al. among their

anterior–posterior direction shift was

as were the predicted shifts. These results emphasize the impor-

tance of careful overview of the image-fusion step of the image-

guidance process and suggest that systematic exclusion from im-

age fusion of regions of skull prone to such imaging artifacts

may be prudent.

Frame-based radiosurgery depends critically on maintenance of

the spatial relationship of the frame to the skull. Any slippage or

deformation of the fra e between planning and treatment will re-

sult in a displacement of the stereotactic space relative to the tar-

get and is important to exclude carefully at the time of treatment

[8]

. A depth helmet has been routinely employed to monitor for

frame slippage between treatment planning and treatment

[2]

The depth-helmet technique relies on potentially imprecise skin

markings and depth measurements which are particularly difficult

in patients with certain hair types or loose skin. The passive use of

X-ray IG allowed us an offline method to accurately detect frame

slippage in one patient in our series for whom the X-ray IG system

determined a predicted shift of 4.81 mm in the vertical direction

(

Fig. 4

). Based on this, we propose routinely combining X-ray IG

with frame-based radiosurgery as a replacement for depth-helmet

verification. This combination would also provide a useful offline

method to verify patient setup.

Another major concern regarding frameless radiosurgery treat-

ment is of intra-fraction motion. As an accurate assessment of real-

time motion was not possible using our system, we utilized intra-

fraction displacement as a proxy to estimate intra-fract on motion

and to compare immobilization properties of the BRW head frame

age-guided radiosurgery differs from fra e-based radiosurgery

and non X-ray image-guided frameless radiosurgery in that the

relationship between the immobilization device and the skull anat-

omy need not be preserved from treatment planning to actual

treatment. Instead, imaging at the time of treatment is used to di-

rectly de ermine the position of the patient in st reotactic space.

While en -t -end phantom testing supports the tech ical capabil-

ity of the e system to accura ely localiz and deliver dose to he

treatment isocenter under idealized conditions

[19,21]

, clinical

application presents a mor compl x challe ge to the use of these

systems.

We have utilized hidden-target testing in a cranial phantom to

evaluate the e d-to-end overall system accuracy of the Novalis

Body Exac-Trac system and have demonstrated a total error mag-

nitude of 0.7 mm (±0.3 mm standard deviation). This compares

favorably with other published IG systems such as Cyberknife

[19]

and with traditional frame-based radiosurgery using a linac

[2,22]

. Our results are consistent with other published studies of

overall system accuracy of the Novalis Body Exac-Trac system

using anthropomorphic phantom testing for intracranial targets

[23,24]

Variations in patient anatomy and their interaction with the

immobilization and image-guidance systems may in theory reduce

overall accuracy compared to idealized phantom testing results.

Our results demonstrate strong concordance between frame-based

and image-guided positioning. The clinical positioning coordinates

determined by image-guidance for our group of 69 patients and

102 isocenters were within 1.0 mm (SD = 0.5) of the frame-based

patient setup. This concordance is similar to that observed by Sol-

berg et al., and Lamba et al.

[23,24]

. A systematic terior–poster-

ior shift of 0.5 mm (SD = 0.3 mm) between frame-based and

im ge-guided setup was observed by Lamba et al. among their

group of 19 patients. The anterior–posterior direction shift was

also largest in the report by Solberg et al. for their group of 35 pa-

ti nts

[23,24]

. This difference was not observed in our study with

the mean shift distributed essentially equally among the three

axes. Our radiosurgery setup procedure utilizes a Radionics localiz-

er which, in contrast to the BrainLAB localizer te plate box used

by Lamba et al., does not cause flex of the ring and frame mount

resulting in a systematic deviation in the anterior–posterior

direction.

We found that for certain patient setups, the kV X-ray images

did not initially fuse accurately to the planning DRR resulting in

aberrant predicted shifts (

Fig. 3

). In each of these cases, the kV

X-ray images demonstrated a graded density through the skull

at the vertex distinct from the CT-DRR images, resulting in aber-

determined a predicted shift of 4.81 mm in the vertical direction

(

Fig. 4

). Based on this, we propose routinely combining X-ray IG

with frame-based radiosurgery as a replacement for depth-helmet

verification. This combination would also provide a useful offline

method to verify patient setup.

Another major concern regarding frameless radiosurgery treat-

ment is of intra-fraction motion. As an accurate assessment of real-

time motion was not possible using our system, we utilized intra-

fraction displacement as a proxy to estimate intra-fraction motion

and to compare immobilization properties of the BRW head frame

Fig. 4.

CT confirmation of fr me slipp g detected by X- ay image-guidance: X-ray

image guidance suggested a 4.81 mm VRT shift in isocenter position relative to

frame-bas d positioning. The p tient was re-imaged by CT. The CT was relocalized

revealing that the stereotactic space had shifted by approximately 4.5 mm relative

to the target isocenter, confirming frame slippage.

4.5 mm frame slippage detected in one patient

Radiotherapy and Oncology 2010;95:109–115