The use of EPID-measured leaf sequence files for IMRT dose reconstruction
in adaptive radiation therapy
Louis Lee, Weihua Mao, and Lei Xinga?
Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305
?Received 21 April 2008; revised 23 July 2008; accepted for publication 26 August 2008;
published 15 October 2008?
For intensity modulated radiation treatment ?IMRT? dose reconstruction, multileaf collimator
?MLC? log files have been shown applicable for deriving delivered fluence maps. However, MLC
log files are dependent on the accuracy of leaf calibration and only available from one linear
accelerator manufacturer. This paper presents a proof of feasibility and principles in ?1? using an
amorphous silicon electronic portal imaging device ?aSi-EPID? to capture the MLC segments dur-
ing an IMRT delivery and ?2? reconstituting a leaf sequence ?LS? file based on the leaf end positions
calculated from the MLC segments and their associated fractional monitor units. These EPID-
measured LS files are then used to derive delivered fluence maps for dose reconstruction. The
developed approach was tested on a pelvic phantom treated with a typical prostate IMRT plan. The
delivered fluence maps, which were derived from the EPID-measured LS files, showed slight
differences in the intensity levels compared with the corresponding planned ones. The dose distri-
bution calculated with the delivered fluence maps showed a discernible difference in the high dose
region when compared to that calculated with the planned fluence maps. The maximum dose in the
former distribution was also 2.5% less than that in the latter one. The EPID-measured LS file can
serve the same purpose as a MLC log files does for the derivation of the delivered fluence map and
yet is independent of the leaf calibration. The approach also allows users who do not have access
to MLC log files to probe the actual IMRT delivery and translate the information gained for dose
reconstruction in adaptive radiation therapy. © 2008 American Association of Physicists in Medi-
cine. ?DOI: 10.1118/1.2990782?
Key words: EPID, leaf sequence file, IMRT, dose reconstruction
An advantage of the fractionation scheme in radiation treat-
ment is that it offers room for adaptive radiation therapy
?ART?. ART is a radiation treatment strategy of which the
subsequent fractional delivery can be adaptively modified
based on a closed-loop control framework using systematic
feedback of geometric and dosimetric information.1–3The
ultimate goal of ART is to maintain adequate target coverage
with a desired dose and ensure doses received by normal
tissue are within tolerance at the conclusion of treatment.
The adaptive strategy comes into play at different levels of
complexity depending on the techniques and resources avail-
able. It ranges from the most accessible form of adapting
treatment margins based on daily portal images4to the most
sophisticated one of reoptimization or replanning of treat-
ment plans.1,2Common to all these strategies is the execution
of dose reconstruction at some stage during the ART process.
Through this, the dose deposited to a patient in a particular
fraction can be correlated or mapped to a reference set of
computed tomography, ideally by deformable registration,
contributing to an accumulated dose delivered so far, which
is a key parameter for the feedback mechanism in the ART
However, in most intensity modulated radiation treat-
ments ?IMRTs? employing ART strategies, the dose recon-
struction is tacitly based on an assumption that the delivery
of fluence maps is as planned.5–7This assumption might not
be necessarily valid. For instance, in IMRT using step-and-
shoot mode, the expected delivery of fluence maps might not
be realized due to intrinsic errors associated with the multi-
leaf collimator ?MLC? kinematics and beam control commu-
nication resulting in overshoot, undershoot segmental moni-
tor units, dropped segments, and beam delivery during leaf
motion.8–10In order to incorporate these errors in the dose
reconstruction, Lee et al.11and Litzenberg et al.10have dem-
onstrated a pragmatic approach of using MLC log files to
reconstruct the IMRT dose actually delivered. This is based
on the fact that the MLC log files have been validated to
faithfully reflect the actual delivery process of MLC-based
IMRT.12,13Because the MLC log file is only available from
one commercial linear accelerator ?linac? manufacturer
?Varian Medical Systems, Palo Alto, CA?, users with linacs
from other manufacturers are deprived of this straightfor-
ward approach to reconstruct the delivered IMRT dose. Fur-
thermore, the leaf position data recorded in a MLC log file is
taken from the same encoders used to position the leaves,
making the reported position dependent on the leaf position
calibration and by no means an absolute measure of the leaf
position. Any systematic error introduced in the MLC cali-
bration might lead to actual leaf positions different from the
expected ones, resulting in dose errors.14
In order to circumvent this dependence and provide a uni-
versal approach of probing the actual delivery of a fluence
50195019 Med. Phys. 35 „11…, November 2008 0094-2405/2008/35„11…/5019/11/$23.00 © 2008 Am. Assoc. Phys. Med.
map, we propose using an amorphous silicon electronic por-
tal imaging device ?aSi-EPID? to capture every segment of
the fluence map during the treatment. For each captured seg-
ment, the leaf positions for each pair of leaves are found by
an edge detection algorithm; the fractional monitor unit
?fMU? associated with this particular segment is also
sampled. After all the segments have been analyzed, a leaf
sequence ?LS? file is reconstituted using the segmental leaf
positions and their associated fMU based on the sequence the
segments are delivered. The EPID-measured LS files can
then be loaded to the treatment planning system ?TPS? to
derive the delivered fluence maps and reconstruct the deliv-
ered IMRT dose. aSi-EPIDs are geometrically and function-
ally stable, giving undistorted images of high resolution and
contrast.15,16The proposed approach is based on the fact that
the use of the aSi-EPID in measuring leaf end positions to a
high degree of accuracy has been proven, leading to its wide-
spread applications in MLC quality assurance,17,18leaf
calibrations,19,20and leaf motion tracking.14,21
The objective of this work is twofold: ?1? To present a
proof of feasibility and principles in reconstituting an EPID-
measured LS file which serves the same purpose for deriving
the delivered fluence map as a MLC log file does and ?2? to
demonstrate the dose reconstruction essential for adaptive
radiation therapy using the delivered fluence maps literally
calculated from the MLC segments captured by an EPID
during an IMRT delivery.
II. METHODS AND MATERIALS
II.A. Description of the MLC and EPID
All experiments were done on a Trilogy linac ?Varian
Medical Systems, Palo Alto, CA? equipped with a Millen-
nium 120-leaf MLC and kilovoltage/megavoltage EPIDs.
The Millennium 120-leaf MLC consists of two banks of 60
leaves. The leaf widths for the central 40 leaf pairs and the
outer 10 leaf pairs are 0.5 and 1.0 cm, respectively. The
leaves can travel a maximum of 16.5 cm across the beam
central axis, and the maximum leaf span between the two
leaves on the same carriage is 14.5 cm. All measurements
are referred to the isocentric plane. The leaf calibration pro-
cedure recommended by Graves et al.22was performed to
ensure that the MLC indicated field edge positions agreed
with the radiation field edges to within 0.3 mm before the
The megavoltage ?MV? EPID ?Varian aS1000 flat panel
detector? was used to acquire images for the experiments.
The EPID is mounted on retractable arms attached to the
gantry. It has a detector area of 40?30 cm2with a matrix of
1024 by 768 pixels, resulting in a physical pixel size of
0.392 mm. The EPID consists of: ?1? A 1.0-mm-thick copper
plate for build-up, ?2? a phosphor screen of gadolinium ox-
ysulphide doped with terbium ?Kodak Lanex Fast Screen? to
convert incident radiation to visible photons, ?3? a pixel array
implanted on an amorphous silicon substratum where each
pixel is made up of a photodiode and thin film transistor to
convert the light photons to electric charges, and ?4? elec-
tronics for readout. The electrical signals are digitized by a
14 bit analog-to-digital converter and processed into image
II.B. Geometric status of the EPID
Baker et al.19and Parent et al.14reported that a systematic
tilt of the EPIDs was observed in their studies and indicated
that it was likely to occur for all different EPIDs; Clarke and
Budgell20have also demonstrated the effect of the gantry
angle on the EPID sag. Therefore, it is expected that the
imaging geometry for the EPID at different gantry angles
might deviate from an ideal configuration that we use as the
basis for the measurement of the leaf end position. We need
to establish the geometric status of the EPID before we can
accurately measure the leaf end position from an EPID im-
age. Recently, our group has developed a geometric quality
assurance phantom and an automated analysis program ?gQA
tool? to study the geometric integrity of the on-board
imager.24This gQA tool was used in this work to study the
changes in the overall imaging geometry of the EPID, in-
cluding the source-to-imager distance ?SID?, the EPID cen-
ter, and the tilt of the EPID for every 10° of a full rotation of
the gantry. Based on the geometric information provided by
the gQA tool, the maximum discrepancies for the SID, EPID
center offset in either direction, and tilt were found to be
2.7 mm, 2.2 mm, and 1.4°, respectively. Positional correc-
tions were incorporated into the measurement of the leaf end
II.C. Measurement of the leaf end position
The coordinate systems used to describe the imaging ge-
ometry of the EPID are shown in Fig. 1. The ?x,y? plane
FIG. 1. Coordinate systems describing the imaging geometry of the EPID.
?a? Isocentric plane ?x,y? and gantry angle ? as viewed from the couch end.
?b? EPID ??,?? plane as viewed in the beam’s eye view. ?c? EPID’s ?-axis
tilt ? as viewed from the couch end. ?d? EPID’s ?-axis tilt ? as viewed from
the side. SAD: Source-to-axis distance; SID: Source-to-imager distance.
5020 Lee, Mao, and Xing: EPID-measured leaf sequence files for IMRT dose reconstruction5020
Medical Physics, Vol. 35, No. 11, November 2008
Wanlass and Bill Cheng, our linac engineers, for offering
technical support and advice on the control of the linac and
a?Author to whom correspondence should be addressed. Telephone: ?650?
498-7896; Fax: ?650? 498-4015. Electronic mail: email@example.com
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Medical Physics, Vol. 35, No. 11, November 2008