Introduction
Preclinical radiation research with small animal models
is an indispensable step between in vitro experiment
and clinical implementation.[-] In the past
two decades, radiotherapy (RT) machines have undergone
huge technical development for the targeted
RT modalities such as intensity-modulated radiation therapy, volumetric-modulated arc therapy (VMAT),
and stereotactic radiosurgery/RT with sophisticated
treatment platforms (robotic, gyroscopic, ring gantry
system, etc.). Although, delivery of RT with targeted
beams led to a paradigm shift in cancer treatment,
there is no standardization to validate the feasibility
of highly conformal treatment modalities for radiobiological
experiment with small animal models.[-]
In fact, in many studies, conventional irradiation techniques
like whole body irradiation or using simple partial
treatment field have been still preferred as a standard
approach.[,,] Therefore, the data collected
from the radiobiological experiment for combination
therapies such as RT plus novel drugs or molecular targeted
agents does not accurately represent the effects
of highly non-uniform and conformal dose distribution
typically delivered to real patients. To overcome
this problem, recently, many studies have focused on
the dedicated methods for small animal radiation research
(e.g., the use of microcomputed tomography
(CT), dedicated treatment planning system (TPS), and
microirradiator).[] As an alternatively, although, dose
delivery in small animal scale is not generally verified
as a part of a quality assurance (QA) program in clinical
linear accelerator, these systems are routinely used
in many clinics to avoid large capital investments to the
dedicated small animal irradiation platforms. However,
preclinical radiation research with clinical linear accelerator
presents some challenges such as the use of very
small field size, wider penumbra, and build-up region
due to the higher beam energy. Therefore, the accuracy
of the delivered dose in small animal scale needs to be
verified with dedicated phantoms and detectors before
conducting radiobiological experiment in clinically
available treatment platforms.
3D-printed small animal phantoms have emerged
as one of the promising and cost-effective solution for
the dosimetric verification of the radiobiological experiment
in clinical linear accelerator.[-] In the
literature, mouse or rat phantoms were generally modified
to accommodate film dosimetry and small volume
detectors such as microionization chamber, SRS diode,
thermoluminescance or optically stimulated dosimetry
for commissioning or pretreatment verification of the
linear accelerator.[,] Although electronic portal imaging
device (EPID)-based in vivo dosimetry is defined
as one of the promising solution in clinical practice,[]
to the date, there is no study evaluating the feasibility of
EPID dosimetry for the verification of delivered dose
during radiobiological experiment. Therefore, in the
present study, it was focused to validate the feasibility
of clinically available EPID-based 3D in vivo dosimetry
system for small animal radiation research.
Methods
Modeling and 3D Printing
CT dataset of a real rat previously scanned for another
radiobiological experiment were transferred to RayStation TPS version 8.0 (RaySearch Lab., Stockholm, Sweden)
to create 3D model of external body (Fig. 1a). This
structure set was exported to 3DSlicer software version
4.3 (The Slicer Community, Harvard, MA, United States
of America [USA]) with SlicerRT extension in DICOM
format and external body was saved as. stl file format for
3D printing. Rat phantom was printed in Makerbot Replicator
Z18 3D-printer (MakerBot Industries, Brooklyn,
NY) using polylactic acid filament, a thermoplastic
polyester with a density of 1.25 g/cm3, and printing
parameters were set as 95% infill percentage, diamond
infill pattern, and vertical printing direction to create
tissue-equivalent phantom, as illustrated in Figure 1b.
Fig. 1. A representative view of (a) modelled and (b) 3D
printed rat phantom and coronal view of the CT
image for (c) real rat and (d) phantom model with
defined target volumes (whole brain and total lung).
Analysis of Printing Accuracy and Uniformity
Printing accuracy of the phantom was evaluated
through physical measurements at multiple position
along the phantom using a Vernier caliper with a resolution of 0.1 mm. In addition, the mean Hounsfield
unit (HU) value and line profile for HU in both superior-
inferior and left-right direction were analyzed in
TPS to evaluate the uniformity of the phantom.
Treatment Planning and Measurements
Target volumes for various anatomical sites including
whole brain and total lung were delineated on the CT
dataset of rat phantom using fused real CT dataset,
as illustrated in Figure 1c and d. After that, four different
treatment plans; three-dimensional conformal
RT (3D-CRT) with lateral opposed fields and VMAT
technique with single arc (arc angle: From 175° to
185°) for whole brain, AP-PA treatment fields, and
VMAT technique with single arc (Arc angle: From
175° to 185°) for total lung irradiation, as shown in
Figure 2, were created using 6 MV photon energy in
Elekta Versa HD linear accelerator (Elekta AB, Stockholm,
Sweden) and dose was prescribed as 2 Gy/fr for
all scenarios. All measurements were performed with
EPID-based iViewDose v.1.0.1 software (Elekta AB,
Stockholm, Sweden) working in conjunction with
the existing EPID panel. Since the back-projection
algorithm used in the clinical version of iViewDose
software underestimate the dose values in field sizes
smaller than 3.0×3.0 cm. EPID was re-commissioned
for only small fields between the sizes of 1.0×1.0 cm
and 4.0×4.0 cm using the correction factor for the
cross calibration of the measured dose with respect to
calculated dose in TPS. The model created in the present
study was saved only for non-clinical use.
Fig. 2. A typical view of (a) phantom setup in treatment machine and (b) beam angle arrangement for VMAT technique
in total lung irradiation.
VMAT: Volumetric-modulated arc therapy.
γ Evaluation
EPID-reconstructed and calculated dose distribution
in TPS was analyzed using 3D γ analysis method. As
an evaluation criterion, 3 mm distance-to-agreement (DTA) and 3% dose differences (DD) were used. Passfail
criteria of the treatment planning are based on the
differences in mean γ value (γ mean), the maximum
1% γ value (γ 1%), and the percentage of points with
γ ≤1 within the 50% isodose surface of the planned
maximum dose or γ passing rate (% γ ≤ 1) and dose to
reference point (ΔDRP). The protocol (minor variation)
values for passing criteria used as a clinical protocol
were 0.5 (minor variation: 0.7), 3 (minor variation:
3.5), 90% (minor variation: 85%), and 3% (minor
variation: 5% at high dose gradient region) for γ mean,
γ 1%, % γ ≤ 1, and ΔDRP values, respectively.
Results
Analysis of Printing Accuracy and Uniformity
The measured differences between the modeled and
the printed dimensions of the rat phantom were within
0.5 mm (±0.1 mm resolution of the Vernier caliper).
Printed dimension of the external body was on average
0.3 mm (range: 0.1-0.5 mm) greater than modeled.
In addition, 3D-printed rat phantom had a uniformity
over the external body and there was no any region
containing unwanted air cavities or high-density areas
over than 1 mm in diameter. The mean HU value of the
phantom was found as -20.77 HU.
EPID-based Measurements
Reconstructed dose with re-commissioned beam model
for small field sizes was found to be compatible with TPS
data. As illustrated in Figure 3, in-plane beam profiles
measured at 1.5 cm depth for defined field sizes were
well matched with the calculated dose profiles in TPS. In
percentage depth dose (PDD) measurement, the relative
DD increased with depth, as shown in Figure 4. However, the maximum ΔDRPs at 5 cm and 10 cm were <3%
and 5% for all commissioned field sizes, respectively
(protocol: 3% and minor variation: 5% at high-dose gradient
region). In 3D γ analysis, % γ ≤ 1 value (γ passing
rate for evaluation criteria: 3 mm DTA/3% DD) for all
field size were >85% in defined phantom geometry.
Fig. 3. Measured (red) and calculated (blue) beam profile (in-plane direction) at 1.5 cm depth
for defined field sizes; (a) 1×1 cm, (b) 2×2 cm and (c) 4×4 cm using 6 MV photon energy.
Fig. 4. Measured (red) and calculated (blue) percentage depth dose graphics at defined
field sizes; (a) 1×1 cm, (b) 2×2 cm and (c) 4×4 cm using 6 MV photon energy.
According to measurement with rat phantom, the
results of 3D in vivo dosimetry for each technique and
treatment region met the protocol value except for %
γ ≤ 1 and γ mean values of lung treatment, as presented
in Table 1. Nevertheless, % γ ≤ 1 and γ mean values for
lung treatment met the proposed minor variation criteria
for both techniques. Dose line graphics for measured and
calculated dose distribution are illustrated in Figure 5 for
whole brain irradiation and in Figure 6 for total lung
irradiation. In point dose comparison, ΔDRP between
TPS and measured with EPID was 3.54% (3D-CRT),
2.50% (VMAT) for whole brain irradiation and 1.19%
(3D-CRT), 2.85% (VMAT) for total lung irradiation.
Fig. 5. A coronal view of the calculated (left), EPID reconstructed dose distribution (middle) and comparison of central
dose line profile (right) for whole brain irradiation using 3D-CRT (a-c) and VMAT (d-f) techniques.
EPID: Electronic portal imaging device; VMAT: Volumetric-modulated arc therapy.
Fig. 6. A coronal view of the calculated (left), EPID reconstructed dose distribution (middle) and comparison of central
dose line profile (right) for total lung irradiation using 3D-CRT (a-c) and VMAT (d-f) techniques.
EPID: Electronic portal imaging device; VMAT: Volumetric-modulated arc therapy.
Table 1 The results of 3D ? analysis
Discussion
In the present study, EPID-based in vivo dosimetry
system which is already mounted to clinical linear accelerator was proved as a promising solution for independent
verification of the delivered dose in small
animal radiation research. According to our analysis,
all measurements for all defined scenarios including
whole brain and total lung irradiation with 3D-CRT
and VMAT techniques were within the clinically acceptable
tolerance levels in terms of 3D γ analysis. In
point dose comparison at beam isocenter, the maximum
point DD was found as 3.54% for whole brain irradiation
with 3D-CRT. Similarly, Perks et al.[] also
reported the accuracy of the measured point dose as
within 5% for different scenarios including lung tumors
(measurement with ionization chamber) and primary
subcutaneous or orthotopic tumors (measurement
with MOSFET detectors) irradiated in clinical linear
accelerator using small animal phantom. Recently, several
dedicated software (e.g., EPIgray (Dosisoft, Paris,
France), DISO (Università Cattolica S. Cuore, Rome,
Italy), Dosimetry Check (Math Resolutions, Columbia,
MD, USA), iViewDose (recently is not avaliable)
(Elekta AB, Stockholm, Sweden), etc., also provide a
3D γ analysis of calculated dose distribution in TPS
and measured transit EPID dose during treatment.
In this way, DD can be evaluated slice by slice on CT data and dose profile of the irradiated beam can also be
analyzed in any desired depth and axis.
The advance in 3D printing technology makes it
easy to create dedicated small animal phantoms or
QA tools with high precision and uniformity.[-]
In the present study, the maximum (mean) measured
differences at multiple points between the modeled
and the printed dimensions of the rat phantom were
found as 0.5 (0.3) mm and the phantom had good
homogeneity and uniformity over the external body.
Similar to our findings, Esplen et al.[] and Price et
al.[] reported that 3D-printed rat phantom revealed
an excellent uniformity over the printed product and
well matched with the designed model. In addition,
Price et al.[] provided their 3D-printed phantom
design and printing methodology as an open source
to encourage the pre-clinical researcher about QA
and to adopt a common QA standard using the dedicated
phantom geometry.
This study has still some limitations that have to
be pointed out. The first one is that since the backprojection
algorithm used in the clinical version of
iViewDose software underestimates the dose values
in field sizes smaller than 3.0×3.0 cm, we could not use the clinically commissioned model in the present
study. Therefore, we created new models for only
commissioned between the field sizes of 1.0×1.0 cm
and 4.0×4.0 cm and additional correction factor was
used to equalize calculated dose in TPS and measured
dose with EPID. Nevertheless, the differences
between the calculated and reconstructed dose for
PDD measurement were <3% and 5% at 5 cm and
10 cm depth, respectively. The second limitation is
that reconstruction algorithm used in iViewDose
software is commissioned in homogeneous conditions
and inhomogeneity correction is not applied
during the reconstruction of the dose distribution,
and so measurement was only performed with homogeneous rat phantom. Therefore, in real rat irradiation,
especially with heterogeneous medium like
lung irradiation, in-aqua vivo method can be used
to minimize the dose reconstruction errors during
calculation, but this approach was not validated for
small animal irradiation in the present study. Therefore,
this need to be further investigated with the
scope of another dosimetric study. The last one is
that despite the obvious advantages of EPID in vivo
dosimetry in preclinical studies, the use of0 EPID
as an in vivo tool is still limited over worldwide due
to the necessity of dedicated software and, recently,
this approach does not also seem as a useful tool for the verification of electron beams for both in clinical
practice and preclinical studies.
Conclusion
As an easily accessible in vivo dosimetry tool, EPID system
can provide standardization for the verification of
delivered dose in small animal radiation research. The
extension of this study would be the check of feasibility
of EPID dosimetry in real radiobiological experiment
for different irradiation scenarios. Nevertheless, there
is still need for the whole international community to come to a consensus about the standardization of QA
protocols for EPID dosimetry in small animal RT with
clinical linear accelerator before implementing this approach
in routine practice.
Peer-review: Externally peer-reviewed.
Conflict of Interest: All authors declared no conflict of interest.
Financial Support: None declared.
Authorship contributions: Concept - F.B.; Design - F.B.,
G.Ö.; Supervision - G.Ö.; Materials - F.B.; Data collection
and/or processing - F.B.; Data analysis and/or interpretation
- F.B., G.Ö.; Literature search - F.B.; Writing - F.B.; Critical
review - G.Ö.
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