Dosimetric Study of Coplanar and Non-Coplanar Intensity-Modulated Radiation Therapy Planning for Esophageal Carcinoma

doi:10.4236/ijmpcero.2013.24018

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International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 2013, 2, 133-138

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

http://dx.doi.org/10.4236/ijmpcero.2013.24018

Open Access IJMPCERO

Dosimetric Study of Coplanar and Non-Coplanar

Intensity-Modulated Radiation Therapy Planning for

Esophageal Carcinoma*

Ying Li, Bing Liu, Fushan Zhai#, Yongfeng Yang, Ming Liu, Chaoen Bao, Qingxiang Zhou

Department of Radiation Oncology, Third Hospital of Hebei Medical University, Shijiazhuang, China

Email: #[email protected]

Received April 25, 2013; revised May 15, 2013; accepted June 5, 2013

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

ABSTRACT

Purpose: To compare the dosimetric impact of coplanar intensity modulated radiation therapy (IMRT) and non-copla-

nar IMRT for the esophageal carcinoma. Methods: There are forty-five esophageal carcinoma patients, fifteen of whom

were cervical and upper thoracic (Group 1) and thirty were middle and lower thoracic (Group 2). Gross tumor volume

(GTV), clinical target volume (CTV), and organs at risk (OAR) were contoured by the chief physician in the CMS-XiO

treatment planning system. For each patient, one coplanar plan and two non-coplanar plans have been created using the

same physical objective function. A detailed dose-volume histogram (DVH) comparison among three plans was then

carried out in a tabulated format. Results: 1) In Group 1 patients with PTV volume less than 100cc, the mean dose and

dose gradient of non-coplanar plan were much better than those in coplanar plan. 2) In Group 2 patients, the conformity

index (CI) for coplanar and two non-coplanar plans were 0.69 ± 0.13, 0.41 ± 0.13, and 0.68 ± 0.15, respectively. The

V5, V10, V20, and the mean dose to the lung were lower in the non-coplanar plans compared to ones in coplanar plan.

However, the non-coplanar plans resulted in an increase in a dose to the heart, but the dose was still within heart toxicity

tolerance. Conclusion: For Group 1 patients, the non-coplanar IMRT plan had less dose gradient and better mean dose

than the coplanar IMRT plan. For Group 2 patients, the non-coplanar IMRT could the decrease dose to the lung tissue,

thus lowering the probability of radiation pneumonia to esophageal cancer patients. The drawback of non-coplanar IMRT

is that, even within toxicity tolerance, it could deliver a higher dose to the heart and spinal cord compared to the coplanar

plan. Therefore, for patients with cardiology and neurology concern, non-coplanar IMRT should be used with caution.

Keywords: Esophageal Carcinoma; Coplanar IMRT; Non-Coplanar IMRT

1. Introduction

Radiotherapy is the primary treatment modality for the

inoperable or unresectable esophageal carcinoma. The

goal of radiotherapy for esophageal cancer is to kill the

cancer cell inside the target volume while sparing the

normal tissues. In the recent years, intensity-modulated

radiation therapy (IMRT) has been broadly used in treat-

ing esophageal carcinoma [1]. IMRT has been proven to

be superior to Three Dimensional Conformal Radiother-

apy (3DCRT) with respect to dose conformity in Plan-

ning Treatment Volume (PTV) and the normal tissue

preservation [2,3]. However, depending on the volume

and location of the esophageal cancer tumor, the normal

lung tissue may be exposed to high doses of radiation.

Several studies have shown that the incidence and sever-

ity of radiation pneumonia were related to the irradiation

to normal lung tissue in esophageal carcinoma radio-

therapy. For instance, Wang et al. [4] found that, among

100 patients, 49% of them have developed various sever-

ity pneumonia, in whom 27% have grade 1, 16% have

grade 2, 6% have grade 3 pneumonia. From a clinical

aspect, decreasing the side effect of radiotherapy is an-

other way to improve the survival rate.

Since non-coplanar IMRT plan usually takes longer

treatment planning time and it requires complex patient

setup and treatment, the majority of the esophageal car-

cinoma cases are treated by coplanar IMRT, and clini-

cians have yet to explore the feasibility of non-coplanar

IMRT in treating esophageal cancer. The main purposes

*Thanks to Dr. Junfang (Jeff) Gao who is medical physicist at Procure

roton therapy center at Oklahoma city in USA for language assistance!

#Corresponding author.

Y. LI ET AL.

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134

of this study were to optimize the treatment approach for

the esophageal cancer and improve the treatment quality

by investigating the dosimetric impact of non-coplanar

IMRT in our hospital. Additionally, this study aims to

establish radiotherapy treatment strategies for future cli-

nical trials, especially for esophageal carcinoma.

2. Methods and Materials

2.1. Patient Selection

Forty-five patients with esophageal carcinoma were en-

rolled in this study. Fifteen of them had cervical and up-

per thoracic tumors (Group 1), and the remaining thirty

patients had middle and lower thoracic section tumors

(Group 2). Median age of patients was 61 years (range,

48 - 72 years), and the median PTV was 114.98cc (range,

30.7cc - 235.12cc). All patients in this study had tumors

located away from the distal esophagus and gastro-eso-

phageal junction.

2.2. Simulation

Both the Groups 1 and 2 patients were simulated in a

supine position under the thermoplastic mask immobili-

zation of the head, neck and shoulders. Patients were

simulated on a computed tomography (CT) scanner (So-

motom-sensation Plus-16) using slice thickness of 5-mm

from mandible to the costophrenic angle. This extended

CT scan volume was used to fully visualize the non-co-

planar beams in treatment planning system (TPS). The

acquired CT images covered the entire thorax and upper

abdomen. The CT images were then transferred to the

TPS via a local area network.

2.3. Contouring

The gross tumor volume (GTV) was contoured by a ra-

diation oncologist. The clinical target volume (CTV) was

expanded with a 0.5 cm in radial direction and a 3 to 5

cm superior-inferior direction, which followed our clini-

cal guidelines. The PTV was defined as an additional 0.5

cm expansion around the CTV. The organ at risk (OAR)

included the spinal cord, lung, and heart.

2.4. Treatment Planning

For each patient, one coplanar IMRT plan (referred as

Plan A) and two non-coplanar IMRT plans (referred as

Plan B and C) were created in the XiO TPS, version 4.40

(ELEKTA, CMS St Louis, USA). Specifically, the co-

planar plan (Plan A) was considered as the reference plan

with three to five beams arrangement. For Group 1 pa-

tients, if the plan A has three beams, we used the beam

setup of one posterior-anterior (PA) and two anterior-

oblique (AO) beams (gantry angle at 180˚ ± 10˚, 50˚ ±

10˚, and 310˚ ± 10˚). If it is a four-beam Plan A, we used

one PA, one anterior-posterior (AP), and two AO (gantry

angle at 0˚, 50˚ ± 10˚, 230˚ ± 10˚, and 310˚ ± 10˚). Simi-

larly, five-beam Plan A consisted of one AP, two AO,

and two PO beams (gantry angle at 0˚, 50˚ ± 10˚, 130˚ ±

10˚, 230˚ ± 10˚, and 310˚ ± 10˚).

For Group 2 patients, if it is a three-beam Plan A, one

AP and two PO (gantry angle at 0˚, 130˚ ± 10˚, and 230˚ ±

10˚) were used; if it is four-beam Plan A, one PA, one AP,

and two AO or one parallel-opposed oblique beams (gan-

try angle at 0˚ and/or 180˚, 50˚ ± 10˚ and/or 310˚ ± 10˚,

130˚ ± 10˚ and/or 230˚ ± 10˚) were used; if it is five-beam

Plan A, one AP, one PA, two AO, and one PO beams (gan-

try angle at 0˚, 50˚ ± 10˚, 130˚ ± 10˚, 180˚ ± 10˚, and 310˚ ±

10˚) were used. The couch angle was always set to 0˚.

For each patient case, the non-coplanar Plan B was

morphed from the reference Plan A by converting one or

two of the coplanar beams in Plan A into the non-copla-

nar beam/s. For Plan B, the total beams of 3 to 5 were

used, and the gantry angle of the non-coplanar beam was

set to 330˚ or 30˚ or 150˚, and couch angle was set to 90˚.

The non-coplanar Plan C was using the identical beam

parameters as in the Plan B, with addition of two more

non-coplanar beams with gantry angles of 330˚ and 30˚,

thus, making the total of 5 to 7 beams in Plan C.

All the IMRT plans were generated using 6 mega-

voltage (MV) X-ray beam. The intermediate dose pre-

scribed to the PTV was 60 Gy in 30 fractions. Dose to

the OARs, such as the lung, spinal cord, and heart, were

minimized to the acceptable tolerances. The treatment

planning was done with an objective of meeting the

planning criteria: Dose to the 95% (D95%) of the PTV

volume receives the prescribed dose (60 Gy); D100% of

the PTV is greater than 57 Gy; D5% of PTV is less than

63 Gy. The spinal cord dose was limited to 45 Gy for

0.1cc. For the total lung, the V5, V20, and mean dose were

expected to be lower than the 50%, 25%, and 13 Gy,

respectively (in absolute percentage of the lung volume

at 5 and 20 Gy). For the heart, the planning goals were to

keep V40 less than 40% and mean dose less than 30 Gy

(in absolute percentage of the heart volume). All the

plans were calculated using superposition algorithm with

a dose calculation grid size of 0.2 cm.

2.5. Plan Evaluation

Each plan was evaluated with respect to the dose distri-

bution, dose-volume histograms (DVHs), and additional

dosimetric parameters described below. Comparisons of

treatment plans were based on doses delivered to the

PTV and OARs. The dose distribution of PTV was as-

sessed by evaluating the maximum dose, mean dose, and

minimum dose.

To evaluate the plan quality with respect to the dose

delivered to the tumor, the conformity index (CI) and

heterogeneity index (HI) were computed.

Y. LI ET AL.

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135

ref ref

ref

VT VT

CI= VT V

× (1)

where, VT is the volume of PTV; VTref is the volume of

PTV enclosed by the 95% prescription isodose cloud;

Vref is enclosed by the 95% prescription isodose cloud.

The CI is usually 0 - 1, with a larger value indicating

better conformity.

95%

HI= D (2)

where, D5% and D95% correspond to the dose delivered to

5% and 95% volume of the PTV, respectively. Greater

HI values indicate doses exceeding the prescription dose

and, thus, a greater degree of dose heterogeneity in the

PTV.

To evaluate the effect of IMRT on normal lung tissue,

heart, and spinal cord irradiation, we computed several

different dosimetric indices, including V5, V10, V20, and

V30 for the normal lung and mean dose delivered to the

normal lung (MLD). The rationale behind using V5-V30

for the normal lung evaluation in comparing the different

plans was based on observations that lung tissue tends to

have a low dose tolerance. We also calculated V30, V40,

V45, V50, and V55 for the heart, as well as mean dose and

D1cc volume of spinal cord dose.

2.6. Statistical Analysis

The different plans were compared using mean statistics.

Quantile-quantile plots showed the data to be approxi-

mately normally distributed, so the differences between

means were tested for significance using a two-tailed

paired Student’s t-test. The null hypothesis was that there

was no difference between the coplanar IMRT technique

and the non-coplanar IMRT treatment techniques. Statis-

tical significance was set at P < 0.05.

3. Results

3.1. Group 1 Patients

Table 1 shows the doses to the PTV, CI, and HI for three

different IMRT plans (A, B and C). The results showed

that all three different plans are very similar. We further

classify this group patient into two sections based on the

PTV volume: First section includes the patients with

PTV volume less than 100cc, and the second section con-

sisted of patients with PTV volume greater than 100cc.

In the section with PTV volume less than 100cc, the PTV

mean doses (cGy) were 6528.33 ± 286.93, 6354.00 ±

270.87, 6354.00 ± 270.87 for Plans A, B and C, respec-

tively. The PTV mean dose of Plan B was closest to the

prescription dose and showed statistical difference com-

pared with Plan A (P = 0.037). The HI of Plan B was

closest to 1, and showed statistical difference compared

with Plan A (P = 0.020). More detailed comparisons of

the Plan A, B, and C are presented in Table 2.

3.2. Group 2 Patients

Table 3 shows that the CI for Plans A, B, and C were

0.69 ± 0.13, 0.41 ± 0.13 and 0.68 ± 0.15, respectively.

The CI of plan B was the lowest and had statistical dif-

ference compared to the reference Plan A (P = 0.000).

Plan C and Plan A has no statistical difference (P =

0.807). More detailed comparisons of the three plans are

presented in Table 3.

The OAR analysis is presented in Table 4, Figure 1,

and Figure 2. The results showed that the lung V5(%),

V10(%), V20(%), V30(%), and mean lung dose (MLD)

(cGy) of reference Plan A were 42.33 ± 8.24, 29.65 ±

8.57, 15.71 ± 4.42, 7.66 ± 4.74, and 882.70 ± 191.83,

respectively. The results were consistent with the finding

from the isodose distribution as lung V5(%), V10(%),

V20(%), V30(%), and MLD (cGy) were reduced to 16.27,

10.59, 5.81, 2.35 and 248 using the Plan B (P < 0.000).

Table 5 shows the dosimetric results of the heart. The

V30(%),V40(%), V45(%), V50(%), V55(%), and MLD (cGy)

of Plan A were 19.00 ± 10.23, 13.00 ± 11.98, 9.09 ± 9.35,

5.88 ± 6.46, 3.30 ± 5.02, and 1729.71 ± 1025.13, respec-

tively. Compared to these values in Plan A, the Plans B

had an increase of 11.05 for V30(%), 9.79 for V40(%),

7.84 for V45(%), 5.86 for V50(%), 4 for V55(%), and

626.94 for MLD with statistical significance (P < 0.05).

Additionally, when compared to the Plan A, the Plan C

also showed an increment in V30(%), V40(%), V45(%), but

not in V50(%), V55(%), and MLD when a non coplanar

field was used. The spinal cord D1cc（cGy）of reference

Plan A was 3127.36 ± 740.8. Compared to Plan A, both

the Plan B and C had slightly higher dose; however, all

sets of plans had D1cc ≦ 4500 cGy for the spinal cord.

4. Discussion

In external beam radiation therapy, the choice of beam

parameters plays an important role when we optimize the

IMRT treatment plan. The selection of beam angle is par-

ticularly important as the beam angle directly affects the

patient setup and treatment plan quality, especially in the

cases where the tumor target is wrapped around multiple

OARs [5].

Beam angle selection in IMRT plan optimization has

been investigated by several institutions [5-8]. Allen [9] and

Tucker [10] reported that non-coplanar IMRT significantly

can improve the dose distribution when tumor is close to

the spinal cord. Bedford et al. [11] reported that the use of

inverse planning algorithm to generate 3 to 6 beam

non-coplanar plans without intensity-modulation could

provide better rectal sparing in conformal prostate plan

compared to a three-field coplanar plan. Olivier et al. [12]

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Table 1. Averaged dosimetric results of target volume in Group 1 patients.

Dmax (cGy） D

min (cGy） D

mean (cGy） CI HI

Plan A 6810.51 ± 777.00 5096.70 ± 930.80 6239.52 ± 623.31 0.76 ± 0.17 1.11 ± 0.08

Plan B 6616.70 ± 470.80 5144.51 ± 1021.33 6191.50 ± 569.60 0.75 ± 0.19 1.09 ± 0.06

Plan C 7012.22 ± 821.00 5222.73 ± 917.21 6339.74 ± 645.13 0.80 ± 0.10 1.14 ± 0.08

T value

P value

1.13* - 0.76#

0.340* 0.498#

−0.67* - 4.11#

0.546* 0.054#

1.46* - 0.87#

0.240* 0.448#

0.65* - 0.79#

0.561* 0.484#

1.14* - 0.80#

0.334* 0.479#

Note:* = P l an B compared with Plan A; # = Plan C compared with Plan A.

Table 2. Average dosimetric results of PTV less than 100cc in Group 1 patients.

Dmax (cGy） D

min (cGy） D

mean (cGy） CI HI

Plan A 7119.66 ± 576.36 5391.00 ± 883.36 6528.33 ± 286.93 0.73 ± 0.19 1.11 ± 0.06

Plan B 6797.33 ± 370.04 5424.00 ± 1046.88 6354.00 ± 270.87 0.72 ± 0.21 1.08 ± 0.04

Plan C 7385.66 ± 418.06 5535.00 ± 822.72 6661.66 ± 49.16 0.79 ± 0.12 1.13 ± 0.07

T value

P value

2.01* - 0.73#

0.182* 0.537#

0.33* - 4.11#

0.767* 0.054#

5.05* - 0.85#

0.037* 0.483#

0.38* - 0.98#

0.736* 0.429#

3.34* - 1.44#

0.020* 0.209#

Note: * = Pl an B compared with Plan A; # = P l an C compared with Plan A.

Table 3. Averaged dosimetric results of target volume in Group 2 patients.

Dmax (cGy） D

min (cGy） D

mean (cGy） CI HI

Plan A 6198.00 ± 1155.47 4413.53 ± 965.83 6230.32 ± 1171.13 0.69 ± 0.13 1.08 ± 0.04

Plan B 6156.21 ± 1021.97 4484.68 ± 1020.97 5382.15 ± 980.93 0.41 ± 0.13 1.09 ± 0.03

Plan C 6230.32 ± 1171.13 4482.15 ± 980.93 5828.52 ± 1053.96 0.68 ± 0.15 1.09 ± 0.05

T value

P value

0.80* - 1.40#

0.432* 0.178#

−0.79* - 1.50#

0.438* 0.149#

2.00* 1.01#

0.064* 0.326#

7.98* 0.24#

0.000* 0.807#

−0.80* - 0.80#

0.434* 0.434#

Note: * = Plan B compared with Plan A; # = Plan C compared with Plan A.

Table 4. Averaged dosimetric results of normal lung tissue in Group 2 patients.

5 (%) V10 (%) V20 (%) V30 (%) MLD (cGy)

Plan A 42.33 ± 8.24 29.65 ± 8.57 15.71 ± 4.42 7.66 ± 4.74 882.71 ± 191.83

Plan B 26.06 ± 10.14 19.06 ± 8.50 9.90 ± 5.03 5.31 ± 3.32 634.00 ± 244.85

Plan C 40.04 ± 7.83 25.51 ± 7.85 12.88 ± 5.18 5.05 ± 4.02 775.94 ± 186.12

T value

P value

7.53* 3.66#

0.000* 0.002#

5.82* 5.07#

0.000* 0.000#

6.97* 4.09#

0.000* 0.001#

2.37* 4.39#

0.020* 0.000#

6.86* 6.64#

0.000* 0.000#

Note: * = Pl an B compared with Plan A; # = Pl an C compared with Plan A.

Table 5. Averaged dosimetric results hear t in Group 2 patients.

30 (%) V40 (%) V45 (%) V50 (%) V55 (%)

Plan A 19.00 ± 10.23 13.00 ± 11.98 9.09 ± 9.35 5.88 ± 6.46 3.30 ± 5.02

Plan B 30.05 ± 15.35 22.79 ± 17.34 16.93 ± 13.15 11.74 ± 10.12 7.30 ± 8.95

Plan C 21.14 ± 13.83 15.85 ± 14.52 10.80 ± 10.27 6.39 ± 7.14 3.73 ± 5.56

T value

P value

−5.32* - 2.25#

0.000* 0.020#

−5.36* - 2.23#

0.000* 0.043#

−5.99* - 2.87#

0.000* 0.010#

4.88* - 2.05#

0.000* 0.061#

−3.43* - 1.25#

0.004* 0.225#

Note: * = Pl an B compared with Plan A; # = Pl an C compared with Plan A.

found that using non-coplanar fields in three-dimensional

conformal radiotherapy (3DCRT) and IMRT can dra-

matically reduces the dose to the heart in irradiation of

middle and lower lung tumors. Liu et al. [13] compared

and analyzed the 3DCRT plans with coplanar beam ante-

rior field and non-coplanar beam anterior field, as well as

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Figure 1. Discrepancy be tween Plan B and Plan A in Group

2 patients (Note: Dashed line = Plan B; Solid line = Plan A.

Brown = PTV, Green = Heart, Red = Spinal cord, Blue =

Lungs).

Figure 2. Discrepancy be tween Plan C and Plan A in Group

2 patients (Note: Dashed line = Plan B. Solid line = Plan A,

Brown = PTV, Green = Heart, Red = Spinal cord, Blue =

Lungs).

assess the dose distribution in the planning target volume

PTV and OARs in treating thoracic esophagectomy un-

der the conditions of the PTV length 19 cm. That study

[13] found that the use of non-coplanar beam can get

better dose distribution on target area and reduce the dose

to spinal cord.

To date, this is the first study, which investigates the

dose distribution around esophageal target and OARs in

the coplanar IMRT and non-coplanar IMRT in detail for

a large group of patients (n = 45). In our study, for Group

1 patients, we found that Plan B produced the mean PTV

dose close to the prescribed dose and HI was also closer

to 1. It showed some potential benefit of using non-co-

planar IMRT plan as non-coplanar beam can reduce the

effect of the dose distribution to the body surface to some

extent, especially when the target volume is relatively

small (<100cc).

Radiation pneumonitis is the main factor to limit cli-

nical therapeutic effect and our goal was to reduce the

irradiation to the lung. Graham [14] found that the inci-

dence and degree of radiation pneumonitis are closely

correlated with radiation volume and dose. There was a

significant association between the degree of the radia-

tion pneumonitis and the V20, V30, and mean dose of lung.

Meanwhile, Allen et al. [15] and Tucker et al. [16] found

that, with the increase of V5, the mortalition of radiation

pneumonitis will increase. In our study, non-coplanar

IMRT plans could be an effective tool to reduce irradia-

tion volume of lung. However, as shown for Plan B, re-

duction in the irradiation of lung may also reduce the

conformality of plan. The reason we found that, compare

to Plan A, the volume of PTV enclosed by the 95% pre-

scription isodose cloud was similar, but the 95% pre-

scription isodose cloud significantly increase. Using Plan

C, it was possible to limit the irradiation of lung without

reducing the conformality of plan.

We also found that non-coplanar IMRT plans in-

creased the dose to the heart while reducing dose to the

lung. In comparing our data with those from other studies

on prediction of radiation injury of the heart, we found

that two types of non-coplanar IMRT plans investigated

in this study can meet the heart clinical constraints,

which traditionally have been considered acceptable [17,

18]. Based on these results, it clearly appears that non-

coplanar IMRT Plan C provided better benefit. It will

decrease the severe acute toxicity of lung without creat-

ing the radioactivity injury of heart.

In our treatment planning study, the delivery technique

was limited to the IMRT. Currently, the volumetric in-

tensity modulated arc therapy (VMAT) technique is

available for the treatment of cancer. The VMAT is gain-

ing more popularity since VMAT requires less number of

monitor units and treatment time when compared to

IMRT. Recently, Rana et al. utilized the VMAT planning

technique on esophageal cancer treatment plans to inves-

tigate the impact of dose calculation algorithms in terms

of dosimetric [19] and radiobiological study [20]. The

findings from those studies [19,20] of Rana et al. showed

that the VMAT has a great potential of reducing dose to

the lung tissue and heart while providing adequate target

coverage for the esophageal cancer patients. Since Rana

et al. [19,20] used the coplanar arcs in their studies, it

would be interesting to further investigate the impact of

non-coplanar arcs on dosimetric results of esophageal

cancer treatment plans generated by the VMAT tech-

nique.

5. Conclusion

For cervical and upper thoracic (Group 1) patients, the

non-coplanar IMRT plan had less dose gradient and bet-

ter mean dose than the coplanar IMRT plan. For mid- dle

and lower thoracic (Group 2) patients, the non-coplanar

IMRT can decrease the irradiation to the lung, thus low-

ering the probability of radiation pneumonia to the eso-

phageal cancer patients. The drawback of non-coplanar

IMRT is that, even within toxicity tolerance, it will de-

liver the higher dose to the heart and spinal cord com-

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138

pared to the coplanar IMRT plan. Therefore, for patients

with cardiology and neurology concern, non-coplanar

IMRT should be used with caution.

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