












































International Journal of Cancer Therapy and Oncology
www.ijcto.org

Copyright © Pokharel et al. ISSN 2330-4049

Shyam Pokharel

Department of Medical Physics, Premier Oncology, Fort Myers, Florida, USA

Received August 02, 2013; Revised August 29, 2013; Accepted August 31, 2013; Published Online August 31, 2013

Original Research

Abstract
Purpose: This study investigated the dosimetric impact of mixing low and high energy treatment plans for high prostate cancer
treated with volumetric modulated arc therapy (VMAT) technique in the form of RapidArc. Methods: A cohort of 12 prostate
cases involving proximal seminal vesicles and lymph nodes was selected for this retrospective study. For each prostate case, the
single-energy plans (SEPs) and mixed-energy plans (MEPs) were generated. First, the SEPs were created using 6 mega-voltage
(MV) energy for both the primary and boost plans. Second, the MEPs were created using 16 MV energy for the primary plan
and 6 MV energy for the boost plan. The primary and boost MEPs used identical beam parameters and same dose optimization
values as in the primary and boost SEPs for the corresponding case. The dosimetric parameters from the composite plans (SE Ps
and MEPs) were evaluated. Results: The dose to the target volume was slightly higher (on average <1%) in the SEPs than in the
MEPs. The conformity index (CI) and homogeneity index (HI) values between the SEPs and MEPs were comparable. The dose
to rectum and bladder was always higher in the SEPs (average difference up to 3.7% for the rectum and up to 8.4% for the
bladder) than in the MEPs. The mean dose to femoral heads was higher by about 0.8% (on average) in the MEPs than in the
SEPs. The number of monitor units and integral dose were higher in the SEPs compared to the MEPs by average differences of
9.1% and 5.5%, respectively. Conclusion: The preliminary results from this study suggest that use of mixed-energy VMAT plan
for high-risk prostate cancer could reduce the integral dose and minimize the dose to rectum and bladder, but for the higher
femoral head dose.

Keywords: Prostate Cancer; Mixed Energy Plan; VMAT; RapidArc

Introduction
In external beam radiation therapy, treatment techniques
such as 3-dimensional conformal therapy (3DCRT), intensity
modulated radiation therapy (IMRT), and volumetric modu-
lated arc therapy (VMAT) are generally used to treat prostate
cancer with an objective of delivering conformal dose dis-
tributions to the target while minimizing the doses to the
normal tissues. Since prostate cancer involves the
deep-seated target, the high-energy photon beams are gener-
ally used for 3DCRT due to their greater penetrating power.1

However, the photon beams with energy 10 mega-voltage
(MV) or higher also create the secondary neutrons due to
interaction between the photons and treatment head of the
machine.1 Despite high-energy photon having an advantage
in penetrating power and skin sparing, use of lower energy
(6–10 MV) photon beams have been found to be an effective
energy choices for the majority of IMRT prostate cases.1,2
Furthermore, several studies demonstrated no clear
dosimetric advantages using high-energy photon beams for
IMRT prostate cases when compared to the low-energy
photon beams.2-8

Recently, Park et al.8 investigated the effect of changing
beam energy according to the penetration depths on the
quality of IMRT plans for prostate cancer and made the
comparisons between mixed-energy plans (MEPs) and sin-
gle-energy plans (SEPs) of either low or high energy. In that
study8, Park et al. showed that mixing energy in an IMRT
plan for deep-seated tumors could improve the overall plan
quality. However, the dosimetric impact of MEPs for pros-

Corresponding author: Shyam Pokharel, PhD; Premier Oncology,
4571 Colonial Blvd, Unit 100, Fort Myers, FL 33966, USA;
Email: pokharel@livemail.uthscsa.edu

Cite this article as:
Pokharel S. Dosimetric impact of mixed-energy volumetric
modulated arc therapy plans for high-risk prostate cancer. Int J
Cancer Ther Oncol 2013;1(1):01011. DOI: 10.14319/ijcto.0101.1

Dosimetric impact of mixed-energy volumetric modulated
arc therapy plans for high-risk prostate cancer

http://dx.doi.org/10.14319/ijcto.0101.1


Pokharel: Dosimetric impact of mixed energy in VMAT plans International Journal of Cancer Therapy and Oncology
www.ijcto.org

Copyright © Pokharel et al. ISSN 2330-4049

2

tate cancer using VMAT technique remains to be addressed.
Thus, we investigated the effect of mixing the low energy (6
MV) and high energy (16 MV) treatment plans for prostate
cancer treated with VMAT technique in the form of
RapidArc (Varian Medical Systems, Palo Alto, CA, USA).
The dosimetric comparisons between SEPs and MEPs were
done for 12 prostate cases.

Methods and Materials

A cohort of 12 prostate cases involving proximal seminal
vesicles and lymph nodes was selected for this retrospective
study. All 12 cases were treated with RapidArc technique at
Premier Oncology, Fort Myers. Florida, USA. The computed
tomography (CT) simulation of patients was performed in a
supine position on the Phillips Brilliance CT Scanner (Philips
Healthcare, Andover, MA, USA), and the CT images were
acquired with a 3 mm spacing. The contouring of prostate,
proximal seminal vesicles, lymph nodes, and organs at risk
(OARs) (rectum, bladder, and femoral heads) was done on
the axial slices of the CT in the Eclipse treatment planning
system (TPS), version 11.1 (Varian Medical Systems, Palo
Alto, CA, USA). The primary clinical target volume (CTVp)
was defined as the prostate, seminal vesicles, and lymph
nodes, whereas the boost clinical target volume (CTVb) was
defined as the prostate only. The primary and boost planning
target volume (PTVp and PTVb, respectively) was generated
with a margin of 7 mm around the CTVp and CTVb, respec-
tively, in all directions except in the posterior direction,
where a margin of 0.5 cm was used.

The RapidArc treatment plans of all 12 cases were generated
in the Eclipse TPS using 6 and 16 MV X-ray beams  Varian
Clinac iX (Varian Medical Systems, Palo Alto, CA, USA).
Each treatment plan consisted of primary and boost plan,
and the total prescription dose was 81 Gy with a daily dose
of 1.8 Gy over 45 fractions. Furthermore, the prescription
dose to the primary plan was 45 Gy to the PTVp, and the
prescription dose to the boost plan was 36 Gy to the PTVb.
For each prostate case, the SEPs and MEPs were generated.

FIG. 1: A transversal view of VMAT (RapidArc) plan set up for boost
PTV (case #7) using one arc in Eclipse treatment planning system.
Abbreviations: VMAT = volumetric modulated arc therapy, PTV =
planning target volume.

First, the SEPs were created using a 6 MV photon beam for
both the primary plan and separate boost plan. The treat-
ment plan was set up using one, two or three arcs depending
on the size of the target volume. [Figure 1] The length of
gantry rotations, collimator angle, and field sizes of the co-
planar arcs for the primary as well as boost plans were cho-
sen based on the location of the PTV and OARs using the
beam-eye-view (BEV) graphics. [Figure 2]

FIG. 2: Beam's-eye-view of case #7 showing (a) primary planning
target volume (PTV), and (b) boost PTV in the Eclipse treatment
planning system.

The isocenter of the plan was placed at the center of the
target volume (i.e., PTVp or PTVb). The primary and boost
plans were optimized using Progressive Resolution Optimiz-
er (PRO) (version 11.1). The dose-volume constraints and
their weightings were adjusted during the optimization pro-
cess of SEPs such that at least 95% of the target volume was
covered by the prescription dose while keeping the dose to
the OARs as minimum as possible. The plan optimization
process was carried out with an objective of meeting the
planning criteria listed in Table 1.

TABLE 1: Dose specifications for rectum, bladder, and femoral heads
in the composite plan

Organ Limit* D15% D25% D35% D50%
Rectum < 75 Gy < 70 Gy < 65 Gy < 60 Gy
Bladder < 80 Gy < 75 Gy < 70 Gy <65 Gy
Femoral Mean Dose < 45 Gy

*Normal organ limit refers to the volume of that organ that should
not exceed the dose limit. Abbreviation: Dx% = Dose received by x%
of total OAR volume, where x % = 15, 25, 35 and 50; OAR = Organ
at risk.

Second, the MEPs were created using a 16 MV photon beam
for the primary plan and a 6 MV photon beam for the boost
plan. Specifically, the primary MEP used the identical beam
parameters and same optimization dose-constraints and their
weightings as in the final primary SEP plan for the corre-
sponding case. Similarly, the boost MEP and boost SEP had
the same beam parameters and plan optimization values for
the corresponding case. No modifications of dose-volume
constraints and weightings were made during the optimiza-
tion processes of MEPs.

The optimized SEPs and MEPs plans were calculated with
the anisotropic analytical algorithm (AAA), version 11.1,
using dose calculation grid size of 2.5 mm. All calculated



Volume 1 • Number 1 • 2013 International Journal of Cancer Therapy and Oncology
www.ijcto.org

Copyright © Pokharel et al. ISSN 2330-4049

3

plans were then normalized such that at least 95% of the
PTV volume was covered by the prescription dose. The pri-
mary and boost plans were combined to generate a compo-
site (COMP) plan. This allowed us to perform the dosimetric
comparison between the SEPs and MEPs using the
dose-volume histograms (DVHs) of the COMP plans that
were generated in the Eclipse TPS. The DVH parameters
evaluated for the target volume (PTVb) were: mean dose,
maximum dose, conformity index (CI) defined as the ratio of
volume of the isodose cloud receiving 100% of the prescrip-
tion dose (V100%) to volume of the PTVb, and homogeneity
index (HI) defined as the ratio of dose at 5% of the PTVb
(D5%) to dose at 95% of the PTVb (D95%). For rectum and
bladder, the volumes that received 70 Gy, 40 Gy, and 20 Gy,
(V70Gy, V40Gy, and V20Gy, respectively) as well as mean dose
were compared. The mean dose to the femoral heads was
evaluated. In addition, the number of monitor units (MUs)
and normal tissue integral dose were compared too.

For the purpose of comparison, the average percentage dif-
ference (Davg.) between the SEPs and MEPs at the corre-
sponding dosimetric parameter of the same case was calcu-
lated using Equation 1.

where x is a corresponding dosimetric parameter in the
COMP SEPs and MEPs for the nth case. In Equation 1, the
Davg. is expressed in percentage and averaged over all twelve
cases in this study. At a given dosimetric parameter, a posi-
tive Davg. means higher dosimetric value in the SEPs com-
pared with the MEPs, and a negative Davg. means higher
dosimetric value in the MEPs compared with the SEPs. The
statistical analysis was done using paired two-sided student’s
t-test in a Microsoft Excel spreadsheet, and a p- value of less
than 0.05 was considered to be statistically significant.

Results

Table 2 and Figures 3, 4, 5, and 6 summarize the dosimetric
results in the COMP plans, and the values are averaged over
the twelve analyzed cases. The dosimetric results obtained in
this study were clinically acceptable.

The maximum and mean doses to the target volume were
slightly higher in the SEPs than in the MEPs by an average
difference of less than 1%, and the results showed the statis-
tical significance with p-values of 0.001 and 0.044 for the
maximum and mean dose, respectively. The CI and HI values
between SEPs and MEPs were comparable with average dif-
ferences of 1% for the CI (p = 0.009) and 0.4% for the HI (p =
<0.000) showing statistical significance.

The dose to the rectum was always higher in the SEPs and

FIG. 3: The Davg. (%) between SEPs and MEPs for the PTV doses, CI,
and HI. The values are averaged over the twelve analyzed cases.
Note: The error bars represent the standard deviations. The Davg. (%)
is defined in equation 1 (Materials and Methods). Abbreviations:
Davg. = average difference,  SEPs = single energy plans, MEPs =
mixed energy plans, PTV = planning target volume, CI = conformity
index, HI = homogeneity index.

FIG. 4: The Davg. (%) between SEPs and MEPs for the V70Gy, V40Gy,
V20Gy, and mean does to the rectum. The values are averaged over
the twelve analyzed cases. Note: The error bars represent the stand-
ard deviations. The Davg. (%) is defined in Equation 1 (Materials and
Methods). Abbreviations: Davg. = average difference,  SEPs = single
energy plans, MEPs = mixed energy plans, VnGy = percentage volume
irradiated by n Gy or more of a certain structure

lower in the MEPs with an average difference ranging from
0.6% (at V40Gy) to 3.7% (at V20Gy). The statistical significance
was obtained for the mean dose (p = 0.009) and V20Gy (p =
0.003), whereas the statistical significance was not seen for
the V70Gy (p = 0.427) and V40Gy (p = 0.277).  Similar to the
dosimetric results for the rectum, the dose to the bladder was
higher in the SEPs and lower in the MEPs. However, the
range of average difference values between the SEPs and
MEPs were larger for bladder compared to the one for rec-
tum. Specifically, the average difference values in bladder
ranged from 0.1% (at V20Gy) to 8.4% (at V40Gy). Furthermore,
the statistical significance was obtained for the mean dose (p
<0.000), V70Gy (p = 0.007), and V40Gy (p = 0.002), whereas the
results for V20Gy (p = 0.384) were not statistically significant.

   
 

12
n n

avg.
n=1 n

SEP  MEP1
D  (x) = ×100          Eq.1

12 EP

–
S

 
 
  




Pokharel: Dosimetric impact of mixed energy in VMAT plans International Journal of Cancer Therapy and Oncology
www.ijcto.org

Copyright © Pokharel et al. ISSN 2330-4049

4

FIG. 5: The Davg. (%) between SEPs and MEPs for the V70Gy, V40Gy,
V20Gy, and mean does to the bladder. The values are averaged over
the twelve analyzed cases. Note: The error bars represent the stand-
ard deviations. The Davg. (%) is defined in equation 1 (Materials and
Methods). Abbreviations: Davg. = average difference, SEPs = single
energy plans, MEPs = mixed energy plans, VnGy = percentage volume
irradiated by n Gy or more of a certain structure

In contrast to the results seen for the rectum and bladder in
this study, the mean dose to the femoral heads was higher in
the MEPs by an average difference of 0.8% with no statistical
significance (p = 0.684). In comparison to the MEPs, the
number of MUs and integral dose were higher in the SEPs by
average differences of 9.1% (p < 0.000) and 5.5% (p < 0.000),
respectively, showing the statistical significances.

FIG. 6: The Davg. (%) between SEPs and MEPs for the femoral head
mean dose, normal tissue integral dose, and MUs. The values are
averaged over the twelve analyzed cases. Note: The error bars rep-
resent the standard deviations. The Davg. (%) is defined in equation 1
(Materials and Methods).  Abbreviations: Davg. = average difference,
SEPs = single energy plans, MEPs = mixed energy plans, MUs =
Monitor Units.

Discussion

In this study, we investigated the dosimetric impact of mix-
ing low energy (6 MV) and high energy (16 MV) treatment
plans for prostate cancer treated with RapidArc technique.
The results from this study showed no clear dosimetric dif-
ferences between the SEPs and MEPs for the target volume.
However, the results suggested that the use of mixed energy
treatment plans for prostate cancer could potentially reduce
the dose to the OARs, especially for bladder and rectum.

TABLE 2: Comparison of dosimetric parameters for the single and mixed energy composite (primary + boost) plans.

SEP MEP
p-value

(Avg. ± SD) (Avg. ± SD)
PTVb

(127.2 ± 35.2 cc)
Mean Dose (Gy) 83.2 ± 0.4 82.9 ± 0.3 0.044
Max. Dose (Gy) 86.3 ± 0.6 85.6 ± 0.6 0.001

CI 1.09 ± 0.05 1.08 ± 0.05 0.009
HI 1.03 ± 0.00 1.03 ± 0.00 <0.000

Rectum
(77.6 ± 47.1 cc)

Mean Dose (Gy) 34.6 ± 3.9 34.3 ± 3.9 0.009
V70Gy (%) 6.5 ± 2.8 6.5 ± 2.8 0.427
V40Gy (%) 26.1 ± 7.0 25.9 ± 6.9 0.277
V20Gy (%) 89.3 ± 3.9 86.1 ± 13.2 0.003

Bladder
(325.9 ± 218.2 cc)

Mean Dose (Gy) 43.2 ± 5.2 42.2 ± 5.0 <0.000
V70Gy (%) 9.3 ± 4.3 9.0 ± 4.1 0.007
V40Gy (%) 45.4 ± 17.2 41.4 ± 15.3 0.002
V20Gy (%) 99.8 ± 0.5 99.7 ± 0.8 0.384

Femoral Heads
(135.7 ± 16.5 cc)

Mean Dose (Gy) 28.0 ± 3.8 28.2 ± 3.4 0.684

Monitor Units (MUs) 590 ± 35 538 ± 34 <0.000
Integral Dose  (105 Gy-cc) 3.2 ± 0.5 3.0 ± 0.5 <0.000

Abbreviations:  SEP = Single Energy Plan, MEP = Mixed Energy Plan, Avg. = Average, SD = Standard Deviation, PTV b = Boost Planning
Target Volume, Max. Dose = Maximum Dose, VnGy = Percentage volume irradiated by n Gy or more of a certain structure, CI = Conformity
Index, HI = Homogeneity Index. (The values are averaged over the 12 analyzed cases. The p-values were obtained from paired two-sided
student’s t-test. The p-values less than 0.05 were considered to be statistically significant).



Volume 1 • Number 1 • 2013 International Journal of Cancer Therapy and Oncology
www.ijcto.org

Copyright © Pokharel et al. ISSN 2330-4049

5

The use of lower energy photon beams generally minimizes
the head leakage, internal scatter, and secondary neutrons.2-7
However, the low-energy photon beams also requires greater
number of MUs to deposit high doses in the area peripheral
to the target, resulting increase in the integral dose and radi-
ation exposure to the OARs.4 The results in our study also
showed that the number of MUs in the lower energy (6 MV)
plans (i.e., SEPs) were about 9% higher (on average) in com-
parison to the MEPs that contained higher energy (16 MV)
photon beam. Furthermore, the integral dose to the normal
tissues was lower in the MEPs by about 5.5% (on average),
and this would also reduce the radiation-induced secondary
cancer. 9, 10

The dosimetric differences in the treatment plans from the
use of low and high energy photon beams depend on the
beam modeling employed within the dose calculation algo-
rithm.11 In this study, we used AAA to calculate the dose in
all treatment plans. Several studies12-17 have documented the
limitation of AAA in estimating the dose more accurately
when heterogeneous media are involved along the photon
beam path. Recently, a number of studies have shown that
Acuros XB, new dose calculation algorithm employed within
Eclipse TPS, is more accurate than AAA for photon dose
calculation, especially in the heterogeneous media.14-17 The
dosimetric and radiobiological impact of Acuros XB on the
prostate cancer treatment plans due to change in photon
beam energy will be an interesting topic for future studies.

Conclusion

The preliminary results from this study suggest that use of
mixed-energy VMAT plan for high-risk prostate cancer
could reduce the integral dose and minimize the dose to
rectum and bladder, but for the higher femoral head dose.

Competing interests
The authors declare that they have no competing interests.

References

1. NCRP. Report No, 79: Neutron contamination from
medical electron accelerators. Bethesda, Maryland;
NCRP: 1987.

2. Soderstrom S, Eklof A, Brahme A: Aspects on the
optimal photon beam energy for radiation therapy.
Acta Oncol 1999; 38: 179–187.

3. Pirzkall A, Carol MP, Pickett B, Xia P, Roach M
3rd, Verhey LJ. The effect of beam energy and
number of fields on photon-based IMRT for
deep-seated targets. Int J Radiat Oncol Biol Phys
2002; 53: 434–442.

4. Subramanian TS. Linear accelerators used for IMRT
should be designed as small field, high intensity,

intermediate energy units [For the proposition].
Med Phys 2002; 29: 2526–28.

5. Söderstrom S, Eklöf A, Brahme A. Aspects on the
optimal photon beam energy for radiation therapy.
Acta Oncol 1999; 38: 179–187.

6. Welsh JS, Mackie TR, Limmer JP. High-energy
photons in IMRT: uncertainties and risks for ques-
tionable gain. Technol Cancer Res Treat 2007; 6:
147–149.

7. Sun M and Ma L. Treatments of exceptionally large
prostate cancer patients with low-energy intensi-
ty-modulated photons. J Appl Clin Med Phys
2006;7: 43–49.

8. Park JM, Choi CH, Ha SW, Ye SJ. The dosimetric
effect of mixed-energy IMRT plans for prostate
cancer. J Appl Clin Med Phys 2011;12 :3563.

9. Brenner DJ, Curtis RE, Hall EJ, Ron E. Second ma-
lignancies in prostate carcinoma patients after ra-
diotherapy compared with surgery. Cancer 2000;
88: 398-406.

10. Hall EJ, Wuu CS. Radiation-induced second can-
cers: the impact of 3D-CRT and IMRT. Int J Radiat
Oncol Biol Phys 2003; 56: 83-88.

11. Madani I, Vanderstraeten B, Bral S, et al. Compari-
son of 6 MV and 18 MV photons for IMRT treat-
ment of lung cancer. Radiother Oncol 2007; 82:
63–69.

12. Rana SB. Dose prediction accuracy of anisotropic
analytical algorithm and pencil beam convolution
algorithm beyond high density heterogeneity in-
terface. South Asian J Cancer 2013; 2: 26-30.

13. Robinson D. Inhomogeneity correction and the an-
alytic anisotropic algorithm. J Appl Clin Med Phys
2008; 9: 112-122.

14. Rana S, Rogers K. Dosimetric evaluation of Acuros
XB dose calculation algorithm with measurements
in predicting doses beyond different air gap thick-
ness for smaller and larger field sizes. J Med Phys
2013; 38: 9-14.

15. Bush K, Gagne IM, Zavgorodni S, Ansbacher W,
Beckham W. Dosimetric validation of Acuros XB
with Monte Carlo methods for photon dose calcu-
lations. Med Phys 2011; 38: 2208-2221.

16. Han T, Mourtada F, Kisling K, Mikell J, Followill
D, Howell R. Experimental validation of determin-
istic Acuros XB algorithm for IMRT and VMAT
dose calculations with the Radiological Physics
Center's head and neck phantom. Med Phys 2012;
39: 2193-2202.

17. Rana S, Rogers K, Lee T, Reed D, Biggs C. Verifica-
tion and dosimetric impact of Acuros XB algorithm
for stereotactic body radiation therapy (SBRT) and
RapidArc planning for non-small-cell lung cancer
(NSCLC) patients. Int J Med Phys Clin Eng Radiat
Oncol 2013; 2: 6-14.

http://dx.doi.org/10.1080/028418699431591
http://dx.doi.org/10.1016/S0360-3016(02)02750-5
http://dx.doi.org/10.1118/1.1513164
http://dx.doi.org/10.1080/028418699431591
http://dx.doi.org/10.1002/(SICI)1097-0142(20000115)88:2%3C398::AID-CNCR22%3E3.0.CO;2-V
http://dx.doi.org/10.1016/S0360-3016(03)00073-7
http://dx.doi.org/10.1016/j.radonc.2006.11.016
http://dx.doi.org/10.1016/j.radonc.2006.11.016
http://dx.doi.org/10.4103/2278-330X.105888
http://dx.doi.org/10.4103/0971-6203.106600
http://dx.doi.org/10.1118/1.3567146
http://dx.doi.org/10.1118/1.3692180
http://dx.doi.org/10.4236/ijmpcero.2013.21002

