Radiotherapy and Oncology 82 (2007) 174–178
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Head and neck tomotherapy
A dosimetric comparison of non-coplanar IMRT versus Helical
Tomotherapy for nasal cavity and paranasal sinus cancer
Ke Shenga,*, Janelle A. Molloyb, James M. Larnera, Paul W. Reada
a
Department of Radiation Oncology, University of Virginia, Charlottesville, VA, USA, bDepartment of Radiation Oncology, Mayo Clinics, USA
Abstract
Purposes: To determine if there are clinically significant differences between the dosimetry of sinus tumors delivered
by non-coplanar LINAC-based IMRT techniques and Helical Tomotherapy (HT). HT is capable of delivering highly
conformal and uniform target dosimetry. However, HT lacks non-coplanar capability, which is commonly used for linear
accelerator-based IMRT for nasal cavity and paranasal sinus tumors.
Methods and materials: We selected 10 patients with representative early and advanced nasal cavity and paranasal
sinus malignancies treated with a preoperative dose of 50 Gy/25 fractions without coverage of the cervical lymphatics for
dosimetric comparison. Each plan was independently optimized using either Corvus inverse treatment planning system,
commissioned for a Varian 2300 CD linear accelerator with 1 cm multileaf collimator (MLC) leaves, or the HT inverse
treatment planning system. A non-coplanar seven field technique was used in all Corvus plans with five mid-sagittal fields
and two anterior oblique fields as described by Claus et al. [F. Claus, W. De Gersem, C. De Wagter, et al., An
implementation strategy for IMRT of ethmoid sinus cancer and bilateral sparing of the optic pathways, Int J Radiat Oncol
Biol Phys 51 (2001) 318–331], whereas only coplanar beamlets were used in HT planning. Dose plans were compared
using DVHs, the minimum PTV dose to 1 cm3 of the PTV, a uniformity index of planned treatment volume (PTV), and a
comprehensive quality index (CQI) based on the maximum dose to optical structures, parotids and the brainstem which
were deemed as the most critical adjacent structures.
Results: Both planning systems showed comparable PTV dose coverage, but HT had significantly higher uniformity
(p < 0.01) inside the PTV. The CQI for all organs at risk were equivalent except ipsilateral lenses and eyes, which
received statistically lower dose from HT plans (p < 0.01).
Conclusions: Overall HT provided equivalent or slightly better normal structure avoidance with a more uniform PTV
dose for nasal cavity and paranasal sinus cancer treatment than non-coplanar LINAC-based IMRT. The disadvantage of
coplanar geometry in HT is apparently counterbalanced by the larger number of fields.
c 2007 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 82 (2007) 174–178.
Keywords: Dosimetric comparison; IMRT; Tomotherapy; Sinus
Malignant neoplasms of the nasal cavity and paranasal
sinuses are challenging to treat with radiotherapy (RT) due
to multiple adjacent sensitive normal structures including
the brain, optic nerves, optic chiasm, retinas, lenses,
parotid glands and brainstem. Compromising tumor dose is
sometimes necessary in order to avoid the optic structures
which when overdosed result in unilateral or bilateral
radiation-induced blindness [1]. For these reasons the
University of Virginia has a long standing history of treating
resectable patients preoperatively to 50 Gy/25 fractions
which is within the tolerance of the optic nerves and
chiasm. Conventional RT for nasal cavity and paranasal sinus
tumors is typically delivered with a heavily weighted
anterior field and half-beam blocked lateral fields to
compensate for dose fall off posterior to the eyes or with
an ipsilateral wedge pair. The RT of sinus tumors has
improved substantially with intensity modulated radiation
therapy (IMRT), which uses a large number of fields each
consisting of multiple beam segments to deliver a more conformal dose, thus reducing the dose to surrounding critical
organs [2–7].
Recent studies have evaluated the use of IMRT to treat
paranasal sinus tumors, and have concluded that IMRT provides a dosimetric advantage over conformal RT [1,2,8]. In
a 2005 study that evaluated IMRT in post-operative patients,
Duthoy found that IMRT resulted in good local control with
low acute toxicity and no blindness. When comparing IMRT
to a historic control, IMRT had slightly lower overall survival
and local control (although not statistically significant) which
the authors attributed to patient selection bias since the
implementation of IMRT, however in the sub-set of patients
without extension through the cribiform plate, IMRT demonstrated improved overall survival rates [9]. Other studies
have concluded that IMRT is not better than conformal RT
0167-8140/$ - see front matter c 2007 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2007.01.008
K. Sheng et al. / Radiotherapy and Oncology 82 (2007) 174–178
[10–12]. Pacholke et al. found that four field conformal RT
was as good as or better than IMRT (non-coplanar IMRT was
not included in the comparison) [10]. Tsien et al. also found
that IMRT can aid in sparing the optic tracts, however it usually requires some trade-off in dosing to the planned target
volume based upon clinical judgment [12].
Helical Tomotherapy (HT) is a novel radiation device
capable of delivering a highly conformal dose from a rotational gantry, which allows radiation delivery calculated
for approximately every seven degrees of rotation around
the patient or 51 fields per rotation [13]. The additional
freedom in inverse planning optimization of 51 beam angles
usually results in a more uniform target dose, and better
avoidance of organs at risk (OARs) compared to standard
IMRT using 7–9 coplanar fields (all fields in the same plane
that is perpendicular to the patient’s central axis) for
head and neck cancer patients due to the additional degrees
of freedom in the radiation entry and exit paths [14].
However, linear accelerator (LINAC)-based IMRT can deliver
non-coplanar beams that provide additional inverse
planning optimization freedom for OAR avoidance and
tumor coverage compared to coplanar LINAC techniques
[10,11,15,16]. Non-coplanar beams are delivered by a
LINAC by rotating the couch, an option not possible for HT
due to the closed gantry design. This limitation could pose
a significant geometric problem for certain tumor volumes
because adjacent critical structures lie mainly parallel to
the target in the axial plane.
In order to compare the dosimetry of HT versus noncoplanar LINAC-based IMRT for nasal cavity and paranasal
sinus tumors we planned 10 patients on a Corvus system that
is commissioned for a Varian 2300 linear accelerator (LINAC)
with 1 cm multi-leaf collimator (MLC) and HT. We compared
the respective calculated doses to the planning treatment
volumes (PTVs) and doses to the OARs to determine if there
was a clinically significant difference between these two
techniques.
Methods and materials
Ten patients with representative early and advanced
nasal cavity and paranasal sinus malignancies underwent
inverse treatment planning using a Corvus system and were
treated with non-coplanar LINAC-based IMRT on a Varian
2300 LINAC to a preoperative dose of 50 Gy and the planning
CT simulation scans were then re-planned on our HT inverse
planning system. Patients were simulated in aquaplast
masks and a CT simulation was obtained with 3 mm slice
thickness. The Gross Target Volume (GTV) was contoured
to cover all gross disease and a Clinical Target Volume
(CTV) was then contoured to cover microscopic spread of
disease at the skull base, surrounding sinuses and adjacent
soft tissues for all patients. The CTV was customized based
on the anatomic extent of the tumor, but included the ipsilateral medial orbital wall, the nasal cavity, cribiform plate,
bilateral ethmoid sinuses, and the ipsilateral maxillary and
frontal osteometal complexes at a minimum with larger
expansions depending on tumor location and extent. The
CTV was expanded symmetrically by 3 mm in all dimensions
to account for patient setup error and motion within the
175
aquaplast mask to obtain the Planning Treatment Volume
(PTV). Normal critical structures included the brainstem,
eyes, lens, optic nerves, optic chiasm, and parotid glands.
Patient CT images and contours were transferred from the
Corvus system to the HT system. The volumes were compared between two planning systems to ensure the accuracy
of contour transfer. Plans were optimized to minimize the
maximum dose to the optic structures, parotids, and brainstem to as low as possible while covering a minimum of 95%
of the PTV with the prescribed dose of 50 Gy. Normal tissue
constraints varied on a case by case basis depending on the
anatomic location of the PTV and the extent of intracranial
and orbital invasion. Six MV photons were used in all plans.
For Corvus optimization, five mid-sagittal fields equally
spaced by 30 and two anterior oblique fields with gantry angles of 75 and 285 as described by Claus et al. [1] were
used. In HT optimization, a field width of 2.5 cm, a pitch
of 0.3 and a modulation factor of 2.5 were used for all
cases. The volumes of OARs in HT overlapping with the
PTV are owned by both the PTV and the OARs but in Corvus
they are owned by the OARs. We reassigned the volumes in
HT to the OARs only to make the two systems identical in
how they handle the dosimetric statistics in overlapping regions. The number of iterations ranged from 100 to 200. A
complete block that restricts the optimization algorithm
from using beams that enter or exit through the structure
was used on all the lenses due to their minimal tolerance
of radiation.
In order to assess the uniformity of both plans, a uniformity index was used and defined as:
UI ¼
D5
;
D95
where D5 and D95 are the minimum doses delivered to 5%
and 95% of the PTV as previously described by Wang et al.
[17]. The greater UI indicates higher heterogeneity. We
chose to compare uniformity because of the close proximity
of the CTV and the normal structures of the optic pathways,
which frequently abut, making it critical that there are no
hot spots in the PTV that could expand into these adjacent
regions resulting in potential visual loss from optic structure
overdosing with slight variations in head positioning. In
addition, the minimal dose to 1 cm3 (Dmin 1cm3 ) of the PTV
was determined for both systems because significant tumor
underdosing can potentially lead to local failure.
Because of the individual difference between OARs and
PTV and the small volume of optical structures, substantially
different absolute doses between patients may affect the
statistic. Therefore, normalized quality indices (QI) and a
comprehensive quality index (CQI) of surrounding OARs were
used and defined as:
CQI ¼
N
N
1 X
1 X
ðDTomo
max Þi
QIi ¼
:
N i¼1
N i¼1 ðDCorvus
max Þi
In this equation, i is the index of the critical organs, which
are (1) ipsilateral eye, (2) contralateral eye, (3) ipsilateral
lens, (4) contralateral lens, (5) ipsilateral optical nerve,
(6) contralateral optical nerve, (7) optical chiasm and (8)
brainstem. CQI was designed to compare the ability of
avoiding these eight organs around the PTV given the same
176
Dosimetric comparison of IMRT and Tomotherapy
weighting to all organs. Although CQI may overweight certain organs that are below tolerance, we chose this index
as it represents a global measure of the capability of avoiding sensitive structures. Individual QIs are shown for direct
comparison of each OAR. A CQI less than one indicates that
HT provides a better plan for the surrounding OARs, and vice
versa. Statistical tests for all comparisons were performed
using t-test.
Results
The volume comparison of PTV, eyes, lens, optical nerves
and optical chiasms is presented in Table 1. Because of the
difference in calculating resolution, a slight difference is
observed. However, the difference in volume is generally
less than 1% of the volume (for PTV) or less than 0.1 cc
(for optical structures).
The proximity of sinus tumors to optic structures resulted
in considerable small cold spots in the PTV in both planning
systems (Fig. 1). The HT plans had a lower minimum point
dose as shown in Fig. 2a, however the mean dose to the
1 cm3 of the PTV that receives the lowest dose is comparable or slightly higher for the HT plans as seen in Fig. 2b.
All 20 plans meet the prescription criteria that 95% of the
PTV receives the minimum prescribed dose of 50 Gy. A composite plot is shown in Fig. 3a, where the difference between planning systems is evident: the HT DVH has a
steeper slope indicating a higher uniformity within the
PTV. The UI for each individual patient is plotted in
Fig. 3b. The UIs from HT plans are universally lower than
the non-coplanar Corvus plans for all patients, with an average UI for HT of 1.068 ± 0.026 and 1.115 ± 0.016 for Corvus
plans. The superiority of HT’s uniformity is statistically significant (p < 0.01). The average Dmin 1cm3 of HT plans is
47.7 ± 2.0 Gy, which is slightly higher than that of Corvus
plans 46.1 ± 1.6 Gy.
The average of the QIs and the standard deviations are
also summarized in Table 2. Of all the OARs, the lenses show
the most evident improvement with an average dose reduction of 31%, with 90% of lenses receiving a lower maximum
dose by HT plans. The mean maximum ipsilateral lens doses
for HT and Corvus plans are 9.8 ± 4.2 Gy and 18.6 ± 8.1 Gy;
the mean maximum contralateral lens doses for HT and Corvus plans are 7.5 ± 4.0 Gy and 9.3 ± 4.9 Gy, respectively. The
improvement of ipsilateral lens dose is statistically significant with p-value <0.01. However, the improvement of contralateral side is not significant. Similar results are observed
for eyes, which have significantly (p-value <0.01) improved
ipsilateral doses, but not for the contralateral side. For all
other organs, the difference is far less than the respective
standard deviations and not statistically significant.
Discussion
In a recent study, van Vulpen reported a dosimetric comparison of HT and coplanar LINAC-based IMRT for oropharyngeal carcinoma and concluded that HT provided improved
dose homogeneity and reduced dose to certain normal
structures [16]. However, these conclusions cannot be
extrapolated to nasal cavity and paranasal sinus tumors because these tumors are generally treated with a non-coplanar technique. We therefore investigated the dosimetric
differences between non-coplanar IMRT vs. HT to determine
if either technique was clinically superior.
In the present study, we found that the non-coplanar LINAC-based plans and HT plans provided adequate PTV coverage, similar PTV Dmin 1cm3 , and roughly equivalent clinically
significant OAR avoidance. There are many organs involved
in treatment planning of nasal cavity and paranasal sinus tumors that have very small volumes (<2 cm3), such as the
lens, optic nerves and chiasm. The accuracy of dose calculation is affected by resolution at this scale. HT offers a
dose voxel size of 1.88 · 1.88 · 3 mm versus 4 · 4 · 3 mm
for Corvus and the higher spatial resolution of the HT dose
computation enables a higher precision for small organ
Table 1
The volume (cc) comparison between HT and Corvus
Patient number
1
2
3
4
5
6
7
8
9
10
Helical Tomotherapy
PTV
Eye (ipsilateral)
Eye (contralateral)
Lens (ipsilateral)
Lens (contralateral)
Optical nerve (ipsilateral)
Optical nerve (contralateral)
Optical chiasm
81.82
8.52
8.16
0.16
0.17
1.14
0.92
1.2
252.79
7.87
8.18
0.28
0.21
0.83
0.8
1.26
658.37
7.85
8.16
0.15
0.12
1.08
1.14
0.82
633.01
7.8
7.5
2.14
2.07
0.98
1.07
0.38
552.05
8.96
9.82
0.73
NA
1.42
1.41
1.28
223.27
8.69
8.35
0.16
0.29
0.97
0.91
1.39
160.88
6.64
7.44
0.85
1.17
1.56
1.45
1.72
408.8
7.11
8.14
0.09
0.12
1.36
0.51
1.89
555.43
7.12
6.42
0.59
1.42
1.55
1.41
0.37
851.37
9.32
9.22
0.18
0.21
1.18
1.3
1.89
Corvus
PTV
Eye (ipsilateral)
Eye (contralateral)
Lens (ipsilateral)
Lens (contralateral)
Optical nerve (ipsilateral)
Optical nerve (contralateral)
Optical chiasm
80.3
8.47
8.09
0.18
0.16
1.21
0.88
1.12
251.2
7.82
8.02
0.25
0.25
0.8
0.86
1.17
657.55
7.92
8.11
0.14
0.1
1.07
1.04
0.74
633.07
7.82
7.44
2.08
2.13
0.91
1.01
0.36
550.25
8.93
9.84
0.76
NA
1.4
1.46
1.33
224.18
8.54
8.23
0.21
0.29
0.93
0.97
1.49
161.05
6.61
7.38
0.9
1.17
1.55
1.19
1.76
410.03
7.15
7.98
0.08
0.12
1.37
0.57
1.78
552.33
7.12
6.54
0.6
1.45
1.57
1.39
0.35
850.64
9.31
9.23
0.19
0.21
1.27
1.12
1.78
K. Sheng et al. / Radiotherapy and Oncology 82 (2007) 174–178
177
Fig. 1. Treatment plans show the difficulty of treating sinus tumors because the tumor is often adjacent to and enclosed by optical structures.
Both the Corvus plan (a) and the HT plan (b) exhibit cold spots inside the PTV to limit dose to a optical nerve. Colorbars show isodose levels in
Gy for the Corvus plan (in solid lines) and HT (in colorwash), respectively. (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this paper.)
a
120
Volume (%)
100
80
60
40
PTV Covus
PTV Tomo
20
0
0
20
40
60
80
Dose (Gy)
b
1.16
Uniformity Index
1.14
1.12
1.1
1.08
1.06
Corvus plan
Tomo plan
1.04
1.02
0
2
4
6
8
10
12
Patient number
Fig. 3. (a) The combined DVH for both techniques. (b) Uniformity
Index (UI) comparison for 10 patients.
Fig. 2. The minimal PTV dose for HT plans and Corvus plans. (a) The
comparison of the PTV minimum point dose. (b) The comparison of
the mean dose to the 1 cm3 of the PTV that receives the lowest dose.
dosimetry and subsequently could contribute to the results
of our comparison. The dose comparison also relies on the
operator. We had two experienced dosimetrists optimizing
the plans on each of the planning systems independently.
Different operators may weight organ and tumor constraints
differently, and since there are more than 10 organs involved in each plan, the comprehensive quality index
(CQI) is a more dependable measure of dose to OARs than
comparing the individual organs.
Dose uniformity is critical for the treatment of nasal
cavity and paranasal sinus tumors since the CTVs of tumors
with orbital invasion frequently abut the optic structures
and the PTV expansion may include the optic structures.
Hot spots in the PTV adjacent to the optic structures could
178
Dosimetric comparison of IMRT and Tomotherapy
Table 2
The quality index of eight OARs and the comprehensive QI
Organ
1. Eye (I)
2. Eye(C)
3. Lens (I)
4. Lens (C)
5. ON (I)
6. ON (C)
7. OC
8. BS
CQI
QI
0.73 ± 0.12
1.08 ± 0.35
0.53 ± 0.51
0.80 ± 0.81
0.97 ± 0.07
0.98 ± 0.08
0.97 ± 0.07
1.05 ± 0.17
0.98 ± 0.23
put the patient at increased risk of visual loss given the
interfraction setup and intrafraction motion uncertainties
in treatment delivery. The improved dose homogeneity of
HT over the non-coplanar LINAC-based plans could have
clinical significance in preventing visual complications.
Also, given the rapid dose falloff of plans to treat these tumors while minimizing the dose to adjacent critical structures, any patient setup errors could also result in PTV
underdosing. Clearly this would be even more of an issue
if definitive doses of 65–70 Gy were prescribed as opposed
to preoperative doses of 50 Gy studied here as the dose
falloffs would even be steeper for definitive radiotherapy
cases. Daily image guidance reduces the risk of patient setup errors with PTV hot spots being delivered into adjacent
critical structures or underdosing of the PTV and we plan
to use the shift measurements made under daily image
guidance after laser setup of marks on aquaplast masks
to calculate the dose that would have been delivered with
and without image guidance to determine the potential
benefit that this adds to the treatment delivery process.
The combined time to acquire a megavoltage CT (MVCT)
and perform the MVCT/kVCT co-registration and deliver
the daily dose on HT is 12–15 min and this compares
favorably to the delivery time for non-coplanar LINAC
treatments which required an average of 25 min mostly
due to the fact that automatic field sequencing was not
possible with the non-coplanar fields.
Conclusions
We compared the dosimetry of 10 nasal cavity and
paranasal sinus tumor patients planned for non-coplanar
LINAC-based IMRT and Helical Tomotherapy (HT) with the
PTV prescribed 50 Gy/25 fractions and both planning
systems satisfied the PTV prescription requirement but HT
delivered a significantly more uniform dose to the PTV and
slightly better PTV coverage. The comparison of organs at
risk by CQI did not yield a statistically significant difference
except the ipsilateral eyes and lenses, for which HT
achieved significant lower dose. We conclude that the coplanar geometry of HT resulted in equivalent or slightly
superior plans comparing the step-and-shoot standard noncoplanar approach.
* Corresponding author. Ke Sheng, Department of Radiation
Oncology, University of Virginia, Box 800375, Charlottesville, VA
22908, USA. E-mail address: [email protected]
Received 9 June 2006; received in revised form 1 January 2007;
accepted 3 January 2007; Available online 31 January 2007
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