Da. Jaffray et al., DUAL-BEAM IMAGING FOR ONLINE VERIFICATION OF RADIOTHERAPY FIELD PLACEMENT, International journal of radiation oncology, biology, physics, 33(5), 1995, pp. 1273-1280
Citations number
14
Categorie Soggetti
Oncology,"Radiology,Nuclear Medicine & Medical Imaging
Purpose: Due to the poor quality of megavoltage (MV) radiographs, dete
ction and assessment of discrepancies in radiation field placement are
difficult. Furthermore, the high imaging dose required to produce the
megavoltage radiograph prohibits frequent image acquisition, particul
arly for those fields that require the use of an ''open-field'' exposu
re. For these small, or conformal, radiation fields, an alternate meth
od of verifying field placement is required if the out-of-held dose is
to be minimized. An open-field image acquired with a kilovoltage (kV)
source would (a) deliver a very low patient dose, (b) increase the vi
sibility of bony landmarks, and (c) simplify intercomparison of portal
and prescription images. This article describes the development of a
dual-beam imaging system that produces diagnostic quality ''double-exp
osure'' portal images for verifying radiation field placement. Methods
and Materials: The dual-beam system consists of a kV x-ray tube mount
ed on the gantry of a medical linear accelerator. The kV beam shares t
he same isocenter (+/- 1 mm) as the treatment beam but is at 45 degree
s to the central axis. Both the kilovoltage and megavoltage images are
collected with a fluoroscopic imaging system that uses a low-noise CC
D camera to accumulate the light emitted from a phosphor screen. Two 4
5 degrees mirrors are used to remove the CCD camera from the x-ray bea
m. The light integration on the CCD array is controlled by a mechanica
l shutter, allowing easy synchronization with the radiation exposures.
The camera is shielded by a lead housing to reduce the number of x-ra
ys reaching the CCD array. A conventional thickness phosphor screen is
used for both the kV and MV exposures. In the dual-beam imaging proce
dure, an open-field kV radiograph is acquired with the patient in trea
tment position. Immediately following, a MV image is acquired with the
beam-defining blocks in position. Summation of the two images produce
s an online double-exposure image. The anatomical information in eithe
r the kV or MV image can be emphasized by weighting the images appropr
iately. This system was used to acquire MV and kV images of both a con
trast-detail phantom and a Rando head phantom. Dual-beam images were a
lso acquired for a pituitary treatment, demonstrating the feasibility
and usefulness of the dual-beam technique. Results: Analysis of the co
ntrast-detail images produced with the My and kV beams shows the expec
ted advantage of using the kV x-ray beam. Images of a Rando head phant
om confirm these results. A clinical demonstration of the dual-beam sy
stem for verifying the delivery of a pituitary held is shown. The qual
ity of the dual-beam image is similar to the prescription (simulation)
image, contains a larger anatomical region, and delivers a lower inte
gral dose to the patient. In addition, the kV beam also enhances the v
isibility of small markers implanted in the prostate. Conclusions: A d
ual-beam imaging system has been developed for the radiographic verifi
cation of small, conformal fields. This development demonstrates the a
dvantages and feasibility of using a kV x-ray beam in combination with
the treatment beam to improve the accuracy of detecting patient setup
errors.