Dosimetric Analysis of Prone Breast Treatment in Tomotherapy and Conventional Linear Accelerator

doi:10.4236/ijmpcero.2013.24021

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International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 2013, 2, 161-168

Published Online November 2013 (http://www.scirp.org/journal/ijmpcero)

http://dx.doi.org/10.4236/ijmpcero.2013.24021

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

Email: [email protected]

Received August 19, 2013; revised September 21, 2013; accepted October 20, 2013

License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ABSTRACT

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

1(b)).

(a)

(b)

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

Heart

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

follow-up.

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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