International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 2013, 2, 161-168
Published Online November 2013 (
Open Access IJMPCERO
Dosimetric Analysis of Prone Breast Treatment in
Tomotherapy and Conventional Linear Accelerator
Ching Chong Jack Yang1, Zhihui Hu2, Yie Chen1, Jie Qiu3
1Monmouth Medical Center, Long Branch, USA
2Chinese Academy Medical Science (CAMS), Beijing, China
3Beijing Union Hospital, Beijing, China
Received August 19, 2013; revised September 21, 2013; accepted October 20, 2013
Copyright © 2013 Ching Chong Jack Yang et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Purpose: To evaluate planning quality and dosimetric differences of clinically deliverable 3D conformal plans generated
from Tomotherapy with TomoDirect™ (TD) and conventional field-in-field approach in prone breast treatment. Mate-
rials and methods: Total of twelve randomly selected early stage left breast patients who went through lumpectomy and
were previously treated on traditional Linear Accelerator (LINAC) have been re-planned and tested on Tomotherapy
TomoDirect module. Baseline prescription dose was chosen at 50.4 Gy (1.8 Gy × 28 fractions) to cover 95% of PTV
for planning criteria with other critical structure dose constraints in the thoracic region. Planning outcomes such as D95
(95% of volume of PTV receiving the prescribed dose), D5 and D1, heart, both lungs as well as the contralateral breast
were simultaneously evaluated. Conformity of the prescription isodose/volume to PTV was evaluated as conformity
index (CI) and dose uniformity was also evaluated with homogeneity index (HI) in the same study series. All outcome
parameters were analyzed and summarized to evaluate dosimetric impact of planning qualities between these two plan-
ning platforms. Results: The planning results indicate that CI, HI, D95, D5 and D1 of PTV, critical structures such as
heart, ipsilateral and contralateral lungs as well as contralateral breast doses were comparable but with better overall
statistical end points from TD plans. The D95, D5 and D1 of PTV for TD plans were superior in dosimetric analysis and
more uniform than those plans generated from Pinnacle™ field-in-field planning technique. Overall, TD plans have
superior planning quality than the conventional method does, with straightforward and automated planning process once
the beam delivery parameters were established. Conclusions: From the clinical treatment planning results, plans from
TD in general achieved better uniform tumor coverage with fewer hot spots while sparing more critical structures were
based upon isodose distribution and Dose Volume Histogram (DVH) analysis. Image guidance of TD delivery auto-
mates the setup within the treatment bore without tedious verification process compared to the process with LINAC.
Though all plans are deliverable, TD planning possesses dosimetric advantages due to its modulated optimization pat-
tern. However, TD did present a challenge during the simulation if a patient is oversized with long pendulant breast
which is hard to fit into the Tomotherapy ring structure. From our analysis, TD plans reserve superior dosimetric out-
come with CI, HI, D95, D5, and D1 of PTV, and better sparing contralateral lung and breast doses.
Keywords: Field-In-Field; Tomo Direct; Prone Breast; Modulation
1. Introduction
Breast cancer has been a commonly discovered disease
among US population. Radiation therapy plays a vital
role in breast cancer management. With the radiation
therapy, the mortality rate of breast cancer introduced by
local recurrence is about 14% with 15 years follow-up
[1]. Conventionally, parallel-opposed tangential beams
are used to treat the whole breast and chest wall tissue
with the supine position. Additional abutting megavoltage
photon and electron fields might be added to treat the
supraclavicular, axillary, and internal mammary lymph
nodes for advanced stages of breast cancer. A typical
complication for breast treatment is radiation induced car-
diovascular side effects, and radiation-induced pneumon-
titis has been reported in approximately 30% of breast
cancer patients treated with definitive radiotherapy [2-4].
Poor cosmesis or fibrosis of those larger breasts was also
observed with different study groups with the stage I and II
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patients treated with lumpectomy and radiotherapy for
curative intent [5,6]. One of the biggest challenges for tra-
ditional breast treatment setup lies in the immobilization
with large or pendulant breast tissues and the daily setup
has been a tedious process for the positional uncertainty.
Those setup uncertainties may be a contributing factor for
the complication that have been observed. Prone breast
treatment was adopted for women with large and pendu-
lous breasts, which achieve better dose uniformity with less
hot spots, and better cosmetic and fewer toxicities [7,8].
Clinical evidence of 3-D conformal radiation therapy
shows positive impact of intensive modification treat-
ment with Multi-leaf Collimators (MLC) combination
which can also produce uniform dose distribution com-
pared to the wedge paired, opposing treatment fields.
Planning principles of field-in-field concept have also
been utilized with prone breast treatment in order to gen-
erate much more uniform dose distribution while avoid-
ing the setup positional uncertainty on a daily basis with
the supine, unsettled location [9-12]. With the TD mod-
ule provided by Tomotherapy with the 3D treatment,
fixed angle modality with constant couch movement, the
Tomotherapy unit could also deliver static beam with the
modulated fields of various MLC combinations. For pa-
tients who are not suitable for the supine position, prone
breast treatment has been widely accepted as a protocol
to reduce many unacceptable hot and cold spots and un-
wanted side effects such as dermatitis and possible car-
diac perfusion defects [13-18]. LINAC based prone
breast treatment encounters certain challenges such as
collision possibility, setup uncertainty and most likely,
the hot spots close to the border of the pendulant breast
PTV volume which are dosimetrically challenging. Though
Tomotherapy IMRT technique has been adopted to pro-
duce acceptable breast treatment, the low dose distribu-
tion to many other critical structures such as heart, con-
tralateral lung and breast has been questionable for the
gain of better dose uniformity coverage [19-21]. This
study is to prove the benefits of utilizing TD in managing
the prone breast treatment similar with the current field-
in-filed intensity modulated approach, with possibly more
homogeneous PTV dose distributions.
TD is a discrete angle, and a non-rotational treatment
delivery mode designed to complement the Tomother-
apy IMRT
delivery technique. It applies the principles
by continuously translating the couch while using fewer
fixed beams for treatment. TD delivers all beams for
each target sequentially, with a single run of the couch
movement to cover the full length of the breast with
margins [22]. While still utilizing the beamlet-based
delivery, it also allows users to plan and treat routine
cases with fixed beam direction, in the same format of
CT pilot scanning process with constant treatment
couch movement.
2. Materials and Methods
Total of twelve patients diagnosed with early stage left
breast cancer with lumpectomy treatment were randomly
selected (n = 12) with prone breast delivery of 50.4 Gy
prescription dose. Target volume and critical structure
definition with PTV is limited to 5mm from the skin sur-
face to avoid hot spot spill to the skin tissue. Two skin
structures were contoured, a 5 mm and a 2.5 mm strip ex-
tending from patient surface towards the breast PTV in
TD planning. The volume of complete PTV structure
ranged from 143.45 cm3 to 1541.29 cm3 with the mean
volume of 647.80 cm3 (standard deviation [SD] ±391.07
cm3). All other critical structures such as lungs, heart,
contra-lateral breast tissue, and unspecified tissue (for
planning avoidance) were also delineated by the same
physician. The unspecified tissue was defined as the bo-
dy contours but excluding all the above delineated targets
and critical structures in test cases.
Beam geometry (Opposing beam ports with pseudo
half beam characteristics to avoid divergence at planning
isocenter) was defined in each system (Tomo plan ver-
sion 4.03 and Philips Pinnacle version 9.2). Since the TD
planning is a non-uniform fluence delivery pattern with
constant couch movement, binary MLC pattern has pro-
duced optimized MLC pattern with couch movement
(pitch index) to generate conformal dose distribution,
similar to the field-in-field technique. Optimization and
calculation were performed in TD without further manual
fine tuning; therefore, plan can be created within 15 min-
utes. LINAC based prone breast treatment is with field-
in-field segmental optimization method, with a purpose
of producing homogeneous PTV coverage.
Planning criteria of the twelve selected left breast pa-
tients are based on the following instructions:
1) Both plans consists the same planning isocenter,
while treatment delivery of TD is off centered;
2) Goals of planning are to at least 95% of the PTV
with the prescription dose;
3) Minimize V107% (Volume at 107% of the prescrip-
tion dose) of the PTV coverage close to zero;
4) Philips Pinnacle Version 9.2: conventional 3D-CRT
with field-in-field segmental optimization method. For
each tangential field, 2 or 3 segments were used to ho-
mogenize the dose at the planning target volume (PTV);
5) Tomo plan version 4.2: TomoDirect 3D-CRT with
two tangential beams. A field width of 2.5 cm and the
default pitch of 0.25 were used to generate suitable pro-
jected DRR comparable to the field-in-field technique.
One major difference of the TD plan versus conven-
tional 3D planning is that planning process is similar to
the IMRT technique, dose constraints need to be entered
to generate the optimized dosimetry results. Though the
beam angles have to be specified for static beam entries,
flash of the beam to cover the breast volume (usually
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around 2 cm) is automatically added to provide enough
field width for the beam ports. Couch movement is con-
tinuous; with the pitch and projected DRR patterns gen-
erated from MLC shapes does represent the modulated
characteristics of prone beam delivery in Tomotherapy.
Figure 1 has shown the typical beam arrangement and
isocenter selection during the prone breast setup. For 3D
CRT, patient markings were carefully selected at the
rigid points which can be easily reproduced with SSD
recordings (Figure 1(a)). In Tomotherapy setting, though
there were no SSD readings, the relative shift in MVCT
image verification helped to position the patient inside
the gentry bore with lasers in image guidance (Figure
Figure 1. (a) Beam arrangement in conventional 3D-CRT
with field-in-field, optimization methodology; (b) Beam ar-
rangement in TomoDirect, the isocenter location is located
at the center of the bore (green lines), and beam delivery
center is off the machine isocenter (red lines).
3. Results
Planning dosimetry outcome has indicated superior dose
conformity and better sparing of critical structures with
the TD plans. Tomotherapy static beam delivery with the
modulated MLC and couch movement combination can
reduce the hot spots during the planning stage while
making the setup easier compared to the conventional
LINAC delivery methodology. One of the benefits for
prone breast in Tomotherapy is that clearance has been
confined inside the gantry ring structure with automatic
couch movement function. Therefore, image guided pre-
treatment verification can be achieved with efficiency
and collision possibility can be eliminated. However,
with the ring gantry delivery, other dilemma is mechani-
cal limitation for treating large sized breast with over-
weighed patients. During the pre-screening process in the
patient simulation stage with our large bore CT simulator
(large bore is 85 cm radius, the same size as Tomother-
apy bore) certainly reduce the possibility of not being
able to perform TD delivery if the patients can be setup
and screened during this stage.
3.1. Dose Distribution Analysis
We observed that with the pre-defined prescription cov-
erage (95% to the breast PTV), the hot spot of TD is less
compared to the field-in-field conventional planning ap-
proach. Figure 2 indicates the typical isodose compari-
Figure 2. Dose displays in three orthogonal views (Left: Pin-
nacle; Right: TomoDirect).
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son between Pinnacle field-infield plans versus TD 3D
plan. It is obvious that relationship in between these two
delivery methodologies, a noticeable hot spot reduction
is observed with TD. Exposures of ipsilateral lung and
heart are also minimized, due to the hard constraint post-
ed in the TD plan.
3.2. DVH Analysis
DVHs of PTV coverage and critical structure are also
displayed from Figures 3(a) -(f), which indicate the plan-
ning advantages for TD, because of the automated proc-
ess. The planning time for TD can be greatly reduced to
within 15 minutes for all calculated dose criteria.
3.3. Dosimetric Data Analysis
Mean dose coverage of PTV for both TD and conven-
tional field-infield plans are without statistically signifi-
cance of a p-value of 0.893. However, TD plan has better
D95 and D99 coverage, while the hot spot of D1 is less
than field-in-field 3D plan. The V100% (volume for re-
ceiving the full prescription dose) is also higher in TD
plan, with much homogeneous dose distributions. How-
Figure 3. (a), (b) DVH comparisons of PTV and heart (solid-Pinnacle, dash-T D); (c), (d) DVH comparisons of ipsilateral and
contra-lateral lungs (solid-Pinnacle , dash-TD); (e), (f) DVH comparisons of contralateral breast and external volumes (solid-
Pinnacle, dash-TD).
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ever, conformal index (CI) of TD is less than optimum,
which indicates TD plan has missed some PTV at the
isocenter junction planes. Table 1 summarizes dose and
volume planning results, also indicates the homogeneous
distribution of TD inside the PTV, with superior V100%
coverage (Table 1).
In Table 2, all dosimetric data related to the quality of
plans have been reported. The 1 cc heart and ipsilateral
doses are significant lower in TD plan compared to the
3D field-infield approach. Dose constraints addressed in
TD has been optimized while at the same time, PTVcov-
erage is maintained with comparable coverage. One of
the key factors for the heart and lung sparing difference
is that the 3D field-in-field plan in Pinnacle still relies on
planner’s personal skill in selecting the treatment areas or
critical structures to achieve dose uniformity, therefore,
the maximum heart and ipsilateral doses in planning
process may be neglected in these cases.
Table 1. Target volume coverage.
Conventional 3D-CRT (field-in-field) TomoDirect 3D-CRT
PTV Range Mean ± SD Range Mean ± SD p Valuea
Mean Dose (Gy) 51.07 - 52.04 51.52 ± 0.29 51.13 - 51.73 51.53 ± 0.17 0.893
D1 (Gy) 53.00 - 53.88 53.53 ± 0.29 52.11 - 53.16 52.71 ± 0.29 0.000
D95 (Gy) 47.98 - 48.61 48.28 ± 0.18 50.06 - 50.58 50.39 ± 0.16 0.000
D99 (Gy) 45.88 - 47.24 46.40 ± 0.38 49.01 - 50.08 49.76 ± 0.38 0.000
V90% (%) 99.3% - 99.9% 99.58% ± 0.20% 99.9% - 100.0% 99.98% ± 0.03% 0.000
V95% (%) 95.3% - 97.7% 96.34% ± 0.60% 99.7% - 100.0% 99.93% ± 0.08% 0.000
V100% (%) 74.9% - 84.9% 80.70% ± 3.10% 91.0% - 97.0% 94.98% ± 1.84% 0.000
V105% (%) 1.5% - 41.7% 16.38% ± 12.43% 0.0% - 2.9% 0.74% ± 0.95% 0.001
V107% (%) 0.0% - 0.7% 0.18% ± 0.27% 0.0% - 0.1% 0.02% ± 0.04% 0.081
HIb 1.09 - 1.12 1.10 ± 0.01 1.02 - 1.05 1.04 ± 0.01 0.000
CIc 0.66 - 0.79 0.75 ± 0.04 0.74 - 0.87 0.81 ± 0.05 0.009
aThe p values were calculated with paired Student’s t-test. p < 0.05 indicates that the difference between the compared parameter sets is statistically significant.
This test has small compared data sets; bHomogeneity Index (HI) = D5/D95; cConformality Index (CI) = (TVRI/TV) × (TVRI/VRI), where TVRI is the target vol-
ume covered by the prescription dose, TV is the target volume; VRI is the volume of the prescription dose. Ideal CI should be close to 1.0 [23].
Table 2. Normal tissue sparing.
Conventional 3D-CRT TomoDirect 3D-CRT
PTV Range Mean ± SD Range Mean ± SD p Value
Mean Dose (Gy) 0.83 - 1.74 1.29 ± 0.27 0.55 - 1.24 0.77 ± 0.20 0.000
D1cc (Gy) 3.56 - 33.16 15.65 ± 7.97 1.93 - 27.52 7.15 ± 7.61 0.000
D5 (Gy) 1.90 - 4.88 3.08 ± 0.81 1.18 - 2.78 1.75 ± 0.45 0.000
Ipsilateral Lung
Mean Dose (Gy) 0.37 - 0.93 0.67 ± 0.16 0.27 - 0.73 0.42 ± 0.14 0.000
D1cc (Gy) 2.28 - 31.74 15.16 ± 11.29 1.44 - 36.62 10.38 ± 12.16 0.039
D5 (Gy) 1.04 - 2.55 1.87 ± 0.42 0.67 - 1.98 1.09 ± 0.38 0.000
Contralateral Lung
Mean Dose (Gy) 0.12 - 0.30 0.21 ± 0.06 0.13 - 0.28 0.19 ± 0.05 0.015
D1cc (Gy) 0.66 - 1.45 1.04 ± 0.21 0.52 - 1.07 0.73 ± 0.14 0.000
D5 (Gy) 0.37 - 0.69 0.56 ± 0.08 0.27 - 0.58 0.39 ± 0.08 0.000
Contralateral Breast
Mean Dose (Gy) 0.42 - 0.85 0.59 ± 0.12 0.30 - 0.46 0.39 ± 0.05 0.000
D1cc (Gy) 1.29 - 2.38 1.91 ± 0.32 0.77 - 1.72 1.29 ± 0.32 0.000
D5 (Gy) 0.90 - 1.58 1.24 ± 0.18 0.55 - 0.98 0.81 ± 0.14 0.000
Unspecified Tissue
Mean Dose (Gy) 1.87 - 4.01 3.00 ± 0.69 1.68 - 3.68 2.28 ± 0.53 0.004
D1cc (Gy) 51.48 - 53.61 52.54 ± 0.66 52.32 - 55.16 53.74 ± 0.83 0.001
D5 (Gy) 7.46 - 40.14 25.09 ± 10.46 2.90 - 40.04 11.56 ± 10.85 0.004
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4. Discussion
Prone breast radiation treatment is the preferred tech-
nique at our facility since 2005 because it offers several
potential advantages compared to supine patient posi-
tioning. With enhanced immobilization technique, treat-
ment time and planning optimization can be improved
with expected less toxicity with prone breast setup. Do-
simetric studies have consistently demonstrated im-
proved normal tissue toxicity with prone positioning,
with clear dose reductions to the lung and potential bene-
fits in dose to cardiac structures compared with supine
treatment. Prone breast treatment for those patients with
pendulant anatomy is critical in delivering uniform do-
simetry to the whole breast volume. Though field-in-field
prone breast planning has been implemented, however,
with similar modality in Tomotherapy, it has achieved
superior plans with better dose coverage in D95 and D99.
Tomotherapy with TomoDirect also made the optimiza-
tion process easier to accomplish the planning goals in
eliminating hot spots. Prone positioning appears to be ex-
tremely well tolerated and reproducible. Finally, a recent
report from our institution of the long-term clinical out-
comes of prone whole breast treatment confirmed excel-
lent tumor control parameters similar to that published
with the conventional supine radiation therapy [13].
Table 1 has shown that with Tomotherapy planning, a
better homogeneity and conformality indexes can be
achieved the same time. This systematic comparison and
the clinical finding can serve as the guidelines for further
planning parameters selection with template implementa-
tion. As recommended in the Phase 3 NSABP B-39/
RTOG 0413 study, a comparison PTV, PTV_EVAL, was
created to remove target volume outside the breast [19].
To allow comparison of normal structure radiation ex-
posure in this series, treatment planning was performed
to cover excellent PTV coverage (V95%) with two tech-
niques. For field-in-field and TomoDirect the coverage
were calculated of 96.34% and 99.93%, respectively. The
critical structures such as heart, ipsilateral and contralat-
eral lungs, contralateral breast, etc. all presented less
mean doses in TD plans. Importantly, to achieve optimal
PTV coverage, a much larger portion of normal tissue
(higher integral dose) is required to receive radiation
exposure for traditional helical Tomotherapy plans. This
integral dose has been dramatically reduced to the level
comparable to the LINAC prone breast plans with the TD
software module. Whether this would lead to increased
clinical complications remains to be seen with further
This study allows a true dosimetric comparison of both
modalities; field-in-field versus TD optimized plans, as
the planning CT scans were performed in the same pa-
tient at same beam geometry. In the past, a significantly
higher portion of ipsilateral breast tissue (to cover chest
wall lymphatic nodes) needs to be irradiated to achieve
higher target coverage with the supine external beam
techniques, which is a risk of poor immobilization and
repositioning problems on a daily basis. It is important to
note that the clinical validity of using a prone Tomother-
apy remains to be demonstrated and has to be studied in
detail using setup reproducibility data. Tighter dose-
volume constraints to PTV and normal tissues have been
proposed and will be evaluated in ongoing trials to assess
the feasibility and safety of this image guided therapy TD
plan; thereby making MVCT a great pre-treatment veri-
fication in the clinical setting. The dosimetric benefits of
TD and image verification for the positioning and plan-
ning approach must be tailored to the patient’s anatomy
and location of the lumpectomy cavity, which could be
translated to homogeneous dose distribution and better
cosmetic results.
5. Conclusion
A field-in-field planning approach with MLC combina-
tion is feasible for prone breast treatment and improves
the dose homogeneity of those women with larger and
pendulous breasts. With a different delivery platform, TD
also provides another alternative by changing the deliv-
ery pattern with the ring gantry system and moving
couch positions. We have evaluated and compared the
dosimetry variation in two different planning modalities
(3D field-in-field technique versus TomoDirect). Due to
the planning and the delivery nature of TD, the hot spot
of the opposing beams can be minimized with better dose
uniformity and conformity in the breast volume (PTV).
The lung and heart with TD sparing can achieve better
dosimetric results comparatively and the hot spots are
also lower in dosimetric results. Contralateral breast dose
is also minimized compared to the conventional 3D tech-
nique. Prone breast treatment in Tomotherapy is a choice
for patients not suitable for the supine position, while at
the same time like to achieve image guided verification
with smooth and easier setup process. TD prone breast
treatment means to minimize the patient positional un-
certainties, and reduce the separation and cardiopulmon-
ary irradiation. In TD, more reduction in critical organ
doses is also feasible by the modulated binary MLC de-
livery nature, with easier setup and image guided pre-
treatment verification. TD planning is relatively straight-
forward with carefully selected patient breast size and all
those parameters can be properly screened during the
virtual simulation stage. TD offers an innovative alterna-
tive to the current LINAC based prone breast treatment
with optimum dosimetric outcomes.
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