Radiotherapy and Oncology 82 (2007) 55–62
www.thegreenjournal.com
Lung cancer IMRT
An analysis of 6-MV versus 18-MV photon energy plans
for intensity-modulated radiation therapy (IMRT) of lung cancer
Elisabeth Weissa,b,*, Jeffrey V. Siebersa, Paul J. Kealla
a
Department of Radiation Oncology, Virginia Commonwealth University, Richmond, VA, USA, bDepartment of Radiotherapy,
University of Göttingen, Göttingen, Germany
Abstract
Purpose: To analyse the supposed benefits of low over high photon energies for the radiotherapy of lung cancer.
Materials and methods: For 13 patients, 6- and 18-MV IMRT planning was performed using identical planning
objectives and dose constraints. Plans were compared according to dose–volume histogram (DVH) analysis including
conformity and homogeneity indices (CI and HI) and overall plan quality (composite score CS), considering also magnitude
and location of planning target volumes (PTVs).
Results: With 6-MV plans, CSs were better in 11/13, HIs in 10/13 and CIs in 6/13 patients compared with 18-MV plans.
Six-MV plans resulted in a better normal tissue sparing except for specified dose levels to the thorax and spinal cord. On
average differences between 6 and 18 MV both for the PTV and normal tissues were not statistically significant
(p > 0.05). Considering size and location of the PTVs as well as their relative position to normal tissue, overall no
significant differences between 6 and 18 MV were observed.
Conclusions: On average no clinically or statistically significant differences between 6- and 18-MV plans were observed.
High photon energies should therefore not be excluded a priori when a dose-calculation algorithm is utilized that
accurately accounts for heterogeneities.
c 2006 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 82 (2007) 55–62.
!
Keywords: Non small-cell lung cancer; Intensity-modulated radiotherapy; Normal tissue toxicity; Dose conformity; Dose homogeneity
Photon energies between 6 and 10 MV are typically used
for the radiation treatment of lung cancer. Presently, low
photon energies 610 MV are required in several lung cancer
treatment protocols, such as RTOG 0412/SWOG S0332 and
other active RTOG protocols. The use of low photon energies is also recommended in national and international
guidelines [1,33].
The reason for preferring low to high energy photons
when irradiating lung tumors lies primarily in an increased
lateral electron transfer in low density tissue, such as lung,
when using photon energies P10 MV. The resulting electron
disequilibrium leads to a wider penumbra for high photon
energies and a consequent decrease in central axis dose
[26,40]. To compensate for the degraded dose to the tumor,
field widths need to be enlarged. This phenomenon is well
known and has repeatedly been described, particularly in
experiments using single or opposing photon fields
[5,10,12,43].
The choice of the optimal energy is a trade-off between
better depth penetration and higher lateral spread as
energy increases. At higher beam energies, inaccuracies in
dose algorithms that utilize only photon path-length
!
corrections are aggravated, further dictating the choice
for low beam energies.
Superposition/convolution (SC) algorithms are increasingly used for treatment planning in clinical routine. Compared to pencil beam algorithms, kernel-based models
such as SC algorithms account for photon transport and,
on a macroscopic scale, the increased electron range in
low density tissue such as lung [2,17,29]. Calculations of
the dose to the lung PTV, the V20 and the mean lung dose
showed an overestimation of more than 10% with correction-based algorithms such as pencil beam compared to SC
algorithms [9,32]. Limitations of SC algorithms have been
demonstrated in extreme situations for small-field, high
energy, single beam phantom geometries, however, the effects are reduced for multibeam clinical cases
[19,20,27,44,46]. In a recent publication, Monte Carlo algorithms have been used to benchmark the accuracy of SC calculations for lung IMRT, using the Pinnacle SC algorithm that
was also used in this study. The mean relative difference
for PTV dose parameters was below 2% for both 6 and
18 MV photons, indicating that the Pinnacle algorithm is
capable of calculating the dose also for high energies with
0167-8140/$ - see front matter c 2006 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.radonc.2006.10.021
56
6-MV versus 18-MV photon energy for lung cancer IMRT
an excellent accuracy. The comparison of mean lung doses
showed a difference below 4% between MC and SC algorithms again both for 6 and 18 MV photons [38]. Our own
comparison between MC and SC was in the same range [22].
Intensity-modulated radiotherapy has found increasing
use also for the treatment for lung cancer patients. Considering typical characteristics of high energy photon beams,
such as lower entry doses and higher depth dose penetration
compared with lower energies, an analysis of the choice of
photon energy for multiple beam arrangements appears to
be meaningful [5,42].
Many lung tumors treated with radiotherapy have a central rather than a peripheral location. In addition, all lymph
nodes potentially requiring radiation treatment are situated
in the mediastinum or peribronchial regions. Therefore, in
general, treatment techniques in which beams pass through
the mediastinum, giving only low weight to lateral or oblique beams that transverse the lung, are preferred. Since
lateral electron transfer is much lower in the mediastinum,
tumor location is expected to influence the choice of beam
energy during treatment planning.
It is the aim of this study to compare 6- and 18-MV photon
energies for IMRT of lung cancer patients and to investigate
whether recommendations to use photons 610 MV for the
treatment of lung cancer are also applicable to IMRT, when
an accurate algorithm such as superposition/convolution is
used during plan optimization.
Materials and methods
To compare 6- to 18-MV isodose plans, treatment planning was performed retrospectively on 13 selected CT scans
from lung cancer patients. Locoregional tumor stages ranged from T1N0 to T4N3 with both peripheral (4 pat.) and
central tumor localizations (9 pat.). The tumor was situated
in the upper or middle lobes in 10/13 patients. The mean
macroscopic tumor volume was 45.2 cm3 (range 0.9–
299.8 cm3). Affected mediastinal lymph nodes were diagnosed in seven patients.
Expiration is the most stable respiratory position with the
longest duration in the breathing cycle [35]. It was therefore assumed to be the most representative phase, despite
the potentially beneficial effect of larger lung volumes during inspiration with regard to lung toxicity [4]. Moreover,
since planning was performed for gated IMRT delivery, lung
tumor motion, usually claimed to be a limiting parameter
for lung IMRT, was minimized during expiration [11,39].
The computed tomography (CT) scans selected for this study
were therefore acquired in expiration with continuous
2.5 mm slices using a multislice CT scanner (GE Healthcare
Technologies, Waukesha, WI). CT scans were part of respiration-correlated 4D CT data sets provided by MD Anderson
Cancer Center, Houston, TX.
Contouring was performed manually using a commercially
available planning system (Pinnacle, Version 6.2, Philips
Medical Systems, Milpitas, CA). The gross tumor volume
(GTV) was defined as all macroscopically identifiable tumor
including lymph nodes with a diameter of at least 1 cm in
the short axis on CT. The clinical tumor volume (CTV)
enclosed the GTV with an 8 mm margin towards lung tissue
[13] and a 5 mm margin around affected lymph nodes. The
CTV was edited to not cross lobe boundaries and to not include the chest wall or organs situated in the mediastinum
unless infiltrated by the tumor. For the planning target volume (PTV), an 8 mm margin was added isotropically to the
CTV. The mean volume of the CTVs was 110.9 cm3 (range
6–552.2 cm3), while the mean volume of the PTVs was
264.9 cm3 (range 37.3–1009.2 cm3).
IMRT appeared to be the most appropriate planning
method for the purposes of a parametric plan comparison
study, since a set of well-defined constraints and the use
of a single metric, i.e. a composite score (CS) as an indicator for plan quality, allow objective plan comparison. The
CS takes into account the actually achieved dose distribution for PTV and organs at risk after plan optimization, relative to the initially given constraints. The CS is the
weighted sum of the plan subscores for individual anatomic
structures. A plan subscore is a measure for the deviation
from the specified dose, dose–volume, or dose–response
objective for that structure. The greater the deviation,
the greater the plan subscore for that structure.
A further advantage of using IMRT is avoidance of the
complex and subjective determination of an appropriate
PTV-to-beam block margin required for conformal therapy.
This computation is included in the optimization process
itself.
IMRT planning was performed using the Pinnacle treatment planning system (version 6.2) with the collapsed cone
convolution implementation of the superposition algorithm
[2,26]. The SC algorithm was chosen for this work as it accounts for the electron disequilibrium in lung and, compared to Monte Carlo algorithms, is increasingly available
on commercially available planning systems. During optimization, shortcomings of the pencil-beam scatter estimation
are accounted for using the delta-pixel-method, where pencil beam is used during optimization and then corrected for
with a SC algorithm after five iterations. Full scatter is
therefore considered during optimization. Furthermore, following convergence, individual segment weights for the MLC
leaf segments are re-computed using the SC algorithm, and
segment-weight re-optimization is used to further refine the
plan. Convergence errors should therefore be minimal. During our TPS commissioning, differences between doses calculated with the SC algorithm and film measurements at
5 cm depth in a water-equivalent phantom were <2% or
<2 mm for IMRT fields for both 6 and 18 MV photon energies.
The prescribed dose was 74 Gy (5 · 2 Gy/week) to follow
the maximum tolerated dose of the currently active RTOG
protocol 0117. The objective of inverse planning was to
deliver the prescribed dose to at least 95% of the PTV with
a dose range not exceeding "10% and +20% of the prescribed
dose [25,30]. In general, six coplanar non-opposed predominantly anterior–posterior beams were chosen with angles
depending on the tumor location. Beam arrangements did
not vary between the 6- and 18-MV plans.
For plan optimization a Quasi-Newton gradient-based
method was employed, using a sequential quadratic programming method to minimize a quadratic objective
function which is constructed from a set of dose- or dose–
volume based objectives for individual regions of interest
E. Weiss et al. / Radiotherapy and Oncology 82 (2007) 55–62
(Pinnacle3 P3IMRT User Guide). Plan optimization for 6 and
18 MV was performed using identical constraints. Constraints were defined as maximum doses or dose–volume
histogram (DVH) limitations with weight factors depending
on the respective normal tissue. The total lung volume
was defined as the combined volume of the right and left
lungs after subtraction of the GTV. Treatment planning
was designed to limit the total lung volume receiving
20 Gy (V20) or higher to 30% of the lung volume [16]. Esophagus and spinal cord were expanded isotropically by 5 mm to
get the respective planning organ-at-risk volumes (PRVs).
Plan optimization was performed with the aim to keep the
esophagus PRV dose of 55 Gy (V55) to 30% of the organ volume and to limit the maximum spinal cord PRV dose to
45 Gy [3,36,37]. The heart was not to receive more than
40 Gy (V40) in 50% of the heart volume [14]. To reduce high
dose volumes outside the PTV, a constraint was defined that
limited the dose to the thorax (except the PTV) to 80 Gy as a
maximum [22].
After conversion to segmental multileaf collimator (MLC)
(step-and-shoot) leaf sequences, all plans were deliverable
on the linear accelerators of the department (Varian
2100EX, Varian, Palo Alto, CA). Although it became evident
during the inverse planning process that in individual patients, modifications of objectives, constraints and the
respective weights may have resulted in clinically more
acceptable plans, we adhered to the initially chosen parameters in order to allow optimum plan comparability. Due to
the retrospective nature of this study, none of the IMRT
plans were actually delivered.
Based on the developed 6- and 18-MV IMRT plans, DVHs
were calculated for the PTV and for all above-mentioned
normal tissue structures. The following questions were
analysed:
1. Concerning the PTV, do 6-MV plans result in improved
dose distributions compared with 18-MV plans? For general plan comparison, composite scores were calculated.
Further, plans were analysed comparing mean, minimum
and maximum doses to the PTV as well as dose homogeneity and dose conformity. The homogeneity index (HI)
was defined as D5%/D95% (minimum dose in 5% of the
PTV/ minimum dose in 95% of the PTV). The lower (closer
to 1) the HI is, the better the dose homogeneity. Since
not all parts of the PTV were covered by the prescribed
dose, the conformity index (CI) was calculated as follows: CI = CF (cover factor) · SF (spill factor), where
the CF was defined as the percentage of the PTV volume
receiving at least 74 Gy and the SF as the volume of the
PTV receiving at least 74 Gy relative to the total 74-Gy
volume (see also RTOG protocol 98-03). The closer the
CI value is to 1, the better the dose conformity.
2. Concerning normal tissue, do 6-MV plans result in better
sparing of organs at risk? Plan comparison for normal tissue was performed for clinically relevant dose and volume levels. For lung and thorax, volumes receiving low
doses such as 5 and 10 Gy were calculated to evaluate
the potentially higher volumes irradiated with IMRT compared with conventional conformal radiotherapy and to
analyse the effect of photon energy on those low-dose
volumes. The irradiated partial volumes of the total lung
57
volume were compared for different dose levels (V5/10/
20/30/50) as well as the mean lung dose [16,45]. Similarly,
the partial volumes of the thorax were calculated for different dose levels from V5 to V50 and the mean thorax
dose. The effects of different photon energies on dose
to heart and esophagus were compared for the respective mean doses and specified dose levels (V35/55 esophagus, V30/40 heart) [3,6,14,23,31,36,37]. For the spinal
cord, D0.1% doses were compared.
3. Are there tumor-related characteristics that predict
when higher photon energies are beneficial? Although
the main aim was to investigate whether the dogma to
use only low energy photons for lung cancer is true,
despite the limited patient numbers, we also tried to
determine if subgroups of patients could be identified
who would potentially benefit from higher photon energies before planning in order to avoid unnecessary planning efforts. For this purpose, CSs, HIs and CIs for the
two photon energies were related to size and location
of the PTVs. In addition, the position of organs at risk,
relative to the tumor, were analysed – taking into
account the dose deposition in the respective organs
depending on photon energy.
A two-tailed t-test was applied to analyse statistical significance of the two photon energies (Microsoft Excel analysis
functions). A significant difference was assumed for
p < 0.05.
Results
Ad 1. Comparison of 6- versus 18-MV plans for the
PTV
For all patients together, the mean CS was lower (better)
with 6 MV (0.1437 versus 0.1595, p = 0.86), implying that on
average 6-MV plans met the given planning objectives and
constraints better than the 18-MV plans. The difference
was, however, not statistically significant. For 2/13 patients, the composite score was lower with 18 MV than 6 MV.
On average, 6-MV plans, compared with 18-MV plans,
resulted in lower D5% (78.91 versus 79.16 Gy) and higher
D95% doses (70.88 versus 70.61 Gy). Consequently, mean
HIs improved with lower photon energies. With the mean
V74 being larger for 6 MV both inside and outside the PTV
(V74: 347.49 cm3 for 6 MV versus 336.34 cm3 for 18 MV), 6MV plans had better CIs compared with 18 MV. Fig. 1 compares a 6-MV plan and an 18-MV plan for a left-sided central
tumor and shows only marginal differences between the two
photon energies both for the PTV and normal tissue. For all
evaluated patients, differences were in general small and
not significant between the two photon energies (Table 1).
Eighteen-MV showed superior results in individual patients
with CI being better in seven and HI in three patients.
Ad 2. Comparison of 6- versus 18-MV plans for
normal tissue
Lungs: The mean dose to the total lungs was reduced
with 6-MV compared to 18-MV plans (Table 2). Lungs
58
6-MV versus 18-MV photon energy for lung cancer IMRT
Fig. 1. Energy comparison for a large medial lung lesion. (a) Six- and (b) 18-MV isodose distributions. The 74-Gy (dark blue), 40-Gy (purple)
and 20-Gy (light blue) isodose curves are shown. The PTV is shaded red. (c) DVH comparison between 6 MV (solid lines) and 18 MV (dashed
lines).
volumes receiving 5, 10, 20, 30 or 50 Gy were smaller with 6
than 18 MV.
Thorax: Although the mean dose to the thorax was lower
for 6 MV, the volumes covered by the 20-, 30- and 50-Gy isodose were on average smaller for 18 MV compared with 6 MV
(Table 2). As a measure for dose concentration to the PTV
and for sparing of normal tissue, the ratio between the
mean PTV and the mean thorax dose was calculated. This
ratio was on average 7.43 for 6-MV and 7.33 for 18-MV plans
(p = 0.92), showing a slightly better sparing of overall normal tissue with 6 MV.
Esophagus: 6 MV plans resulted in smaller volumes
receiving at least 35 and 55 Gy as well as lower mean organ
doses (Table 3).
Heart: The mean heart dose, V30/40 was lower with 6 MV
than with 18 MV (Table 3).
Spinal cord: Mean D0.1% doses of the spinal cord were
higher with the 6-MV than the 18-MV plans (Table 3).
For the studied normal tissue none of the observed differences were found significant (Tables 2 and 3).
Ad 3. Comparison of 6- and 18-MV plans with
respect to tumor-related characteristics
Influence of PTV magnitude and location on CS, CI and
HI
Patients were divided into groups with large PTVs (median 589.75 cm3), small PTVs (median 86.30 cm3), and patients with central and peripheral tumor location.
Differences in CS between large and small PTVs were
marginally significant both for 6 and 18 MV (p = 0.055 both
E. Weiss et al. / Radiotherapy and Oncology 82 (2007) 55–62
Table 1
Plan comparison for the PTV
6 MV
Mean
Dmean (Gy)
D95% (Gy)
D5% (Gy)
CI
HI
74.73
70.88
78.91
0.566
1.116
18 MV
SD
Mean
0.48
2.94
2.37
0.096
0.079
74.76
70.61
79.16
0.563
1.125
59
Table 3
Comparison of relevant mean dose and volume levels for
esophagus, heart and spinal cord
p
SD
6 MV
0.53
3.15
2.48
0.099
0.087
Mean
SD
Mean
SD
23.41
27.37
14.47
9.32
15.66
14.38
24.81
28.33
14.69
9.69
14.76
14.71
0.71
0.87
0.97
16.62
22.74
15.65
14.32
22.04
15.94
18.01
24.85
16.98
15.43
24.41
17.92
0.81
0.82
0.84
39.97
8.67
39.80
8.36
0.96
0.90
0.82
0.80
0.94
0.80
for 6 and 18 MV), implying that objectives and constraints
could be met more easily for small PTVs as opposed to large
PTVs. Differences in HI were marginally significant between
large and small tumors for both photon energies (p = 0.06
for 6 and 18 MV). No significant differences were found between the 6- and 18-MV plans for CSs, CIs and HIs comparing
large PTVs, small PTVs, and peripheral and central PTVs
(Table 4).
The composite score was found to be better with 18 MV
for two patients compared with 6 MV. These patients either
had small or intermediate sized central tumors. Although
several patients had better CI (7) and HI (3) values with
18-MV plans, no correlation to tumor position and tumor
size was observed in these patients. Fig. 2 shows the better
sparing of esophagus and heart with the 6-MV plan in a patient with a small peripheral tumor, but a better conformity
of the 74-Gy isodose to the PTV for the 18-MV plan.
Influence of the relative position of organs at risk and
PTV on the ability to meet overall planning
constraints for different energies
Lungs: Irradiated lungs volumes were smaller with 18 MV
in three patients for V20 and four patients for V5, V10, V30
and V50. All of those patients had central tumor locations.
In patients with peripheral tumors, no significant difference
between 6 and 18 MV was observed.
Thorax: The thorax volumes covered by a specified dose
level were smaller with 18 MV for V5 in seven, V10 in five, V20
in eight, V30 in ten and V50 in seven patients. In four of 13
patients, the ratio between mean PTV dose and mean thorax dose was higher for 18 MV, indicating a relatively better
sparing of the thorax compared to 6-MV plans. All four patients had central tumor positions.
Esophagus: V35, V55 and mean esophagus dose were lower
with 18 MV in six, four and three patients, respectively. In
Esophagus
Mean dose (Gy)
V35 (%)
V55 (%)
Heart
Mean dose (Gy)
V30 (%)
V40 (%)
Spinal cord
D0.1% (Gy)
18 MV
p
two of these patients, the PTV was located close to the
esophagus.
Heart: Four (mean heart dose) and three patients (V30/40)
had improved heart sparing with 18-MV plans. These were
patients with small central tumors away from the heart.
For the two patients with PTVs directly adjacent to or covering parts of the heart, 6 MV resulted in better heart
sparing.
Spinal cord: D0.1% doses were higher with 6 MV than 18 MV
in 6/13 patients. None of the patients had a PTV close to the
spinal cord.
Overall, there was no statistically significant relation between any of the evaluated dose parameters, photon energy
and the relative position of the respective normal tissue
structure to the tumor (p = 0.77–0.99).
Discussion
The present planning study showed no significant differences between 6- and 18-MV photon energies for any of
the evaluated parameters. This is in accordance with the
findings by Liu et al., who in their comparison between conventional 3D conformal radiotherapy and IMRT planning also
included 18-MV IMRT plans and found no noticeable difference between 6- and 18-MV IMRT plans. Results of a detailed
analysis were, however, not shown [25].
Six-MV plans resulted in a slightly better average plan
quality expressed by the composite score, and improved
dose conformity and homogeneity. Similar to the PTV, the
Table 2
Volumes of total lung (minus GTV) and thorax (minus PTV) covered by selected dose levels
Lungs
Thorax
Energy (MV)
V5 (%)
6
18
6
18
49.44
50.00
36.81
36.82
(18.31)
(18.30)
(14.88)
(14.71)
V10 (%)
V20 (%)
V30 (%)
V50 (%)
Mean dose (Gy)
38.99
39.70
27.98
28.28
24.10
24.91
17.40
17.12
17.05
17.51
11.40
10.76
10.26
10.41
4.77
4.50
15.88
16.68
11.37
11.65
(16.77)
(17.67)
(12.71)
(12.88)
(9.37)
(9.77)
(7.52)
(7.82)
The table gives mean values (SDs) for all patients.
Differences between 6 and 18 MV were not significant at any dose level (p = 0.72–0.93).
(7.48)
(7.19)
(5.39)
(5.33)
(6.40)
(6.29)
(3.10)
(2.75)
(5.54)
(5.73)
(3.88)
(4.13)
60
6-MV versus 18-MV photon energy for lung cancer IMRT
Table 4
Comparison of the mean composite score, the mean conformity and homogeneity index for size and location of the PTV
CS
CI
HI
Energy (MV)
Large PTVs
Mean (SD)
Small PTVs
Mean (SD)
Peripheral PTVs
Mean (SD)
Central PTVs
Mean (SD)
6
18
6
18
6
18
0.2854 (0.2524)
0.3085 (0.2739)
0.507 (0.092)
0.492 (0.084)
1.178 (0.066)
1.193 (0.075)
0.0252 (0.0423)
0.0359 (0.0649)
0.605 (0.072)
0.625 (0.074)
1.068 (0.045)
1.070 (0.045)
0.2947 (0.3305)
0.3272 (0.3329)
0.569 (0.04)
0.578 (0.106)
1.167 (0.092)
1.178 (0.104)
0.0765 (0.1044)
0.0849 (0.1082)
0.549 (0.114)
0.549 (0.107)
1.101 (0.067)
1.107 (0.072)
All differences between 6 and 18 MV were not significant for any of the analysed parameters (p = 0.64–0.99).
Fig. 2. Energy comparison for a small peripheral lung lesion. (a) Six- and (b) 18-MV isodose distributions. The 74-Gy (dark blue), 40-Gy (purple)
and 20-Gy (light blue) isodose curves are shown. The PTV is shaded red. (c) DVH comparison between six MV (solid lines) and eighteen MV
(dashed lines).
application of 6- and 18-MV plans had no significant impact
on normal tissue sparing, although on average 6 MV appeared to spare most of the analysed organs minimally
better.
Eighteen MV were found superior in subsets of patients,
but patient numbers were not sufficient to determine statis-
tically significant indicators of when it is superior. Neither
PTV location nor PTV size played a significant role for the
photon energy leading to the superior plan quality. In clinical routine, usually the frequency of patients with peripheral and lower lobe tumors is higher than in the analysed
cohort of patients. This will, however, not interfere with
E. Weiss et al. / Radiotherapy and Oncology 82 (2007) 55–62
the general finding that 6-MV photons are not always superior to higher energies.
We also performed planning with mixed energy beams for
a small peripheral tumor, a large peripheral and a central
tumor. Eighteen-MV beams were used primarily for those
fields where the tumor was situated distant to the beam entry to use the advantages of higher depth dose penetration
with higher photon energies. For all three patients, no systematic improvement in plan quality was discernible when
combining 6- and 18-MV beams in one plan.
All observed differences between 6- and 18-MV plans
were small. A comparison of monitor units (MUs) per
fraction showed no significant difference between 6- and
18-MV plans (707 versus 663 MU, p = 0.67), indicating only
clinically irrelevant elongations of treatment time with
6 MV compared to the total IMRT treatment time. From
the clinical point of view, it therefore needs to be added
that the observed variations in dose delivered to the PTV
and normal tissue are certainly of a much lower scale compared with other more fundamental issues in the treatment
of lung cancer patients, e.g. total dose, definition of the
clinical target volume, tumor motion [15,34,35,39,41].
For IMRT it is most important to use an accurate algorithm, such as a SC algorithm, to, e.g. avoid convergence
errors during optimization, so that dose deficiencies in the
delivery of one beam can be compensated for by adjusting
the intensity from the other beams [18]. Of further importance when comparing low to high energy photons is the
generation of nuclear particles, in particular neutrons, induced by high energy photons [8]. However, in clinical routine the contribution of nuclear particles to total dose is
presently not considered. From phantom measurements of
penumbra width and PTV dose, low energy beams were recommended for tumors surrounded by lung parenchyma
[24,28]. We are aware that phantom measurements might
show deviations from even advanced dose-computation
algorithms, such as the superposition/convolution algorithm, to a significant amount [7]. The awareness of the limitations of the used planning algorithm is, however, only one
factor that influences the radiooncologist’s decision on an
isodose plan. Also important is the quantitative assessment
of dose delivered to normal tissue, which is usually not considered in phantom studies.
Selection of the optimum plan is particularly important in
all patients with a high risk for treatment-related sequelae,
such as combined modality treatment, compromised lung
function and dose escalation protocols. High photon energies should particularly be considered for central tumor
locations. This observation also supports the recommendations of the National Comprehensive Cancer Network NCCN,
where higher photon energies are suggested for large tumors surrounded by consolidated or atelectatic lung and
bulky lymphadenopathy (http://www.nccn.org).
Conclusion
For the treatment of patients with lung carcinomas, on
average no significant differences between the 6- and 18MV plans were observed when a superposition/convolution
61
algorithm was used during plan optimization. In several
patients, 18-MV plans were superior to 6-MV plans for the
analysed indices. Sufficiently accurate planning algorithms
are a prerequisite for decisions on high energy beams in
the thorax region involving lung tissue [1,21]. Further
studies on larger patient data sets are needed to identify
subgroups where low/high photon energies are more
applicable.
Acknowledgments
The authors acknowledge Drs L. Dong and R. Mohan from the UT
MD Anderson Cancer Center for supplying the CT data used for this
study. The authors wish to thank Dr. C.F. Hess, University of Göttingen, and Dr. N. Dogan for their critical comments. Thank you to
Devon Murphy for improving the clarity of the manuscript. This work
was partially supported by NCI R01 CA 93626 and R01 CA 98524.
* Corresponding author. Elisabeth Weiss, Department of Radiation Oncology, Virginia Commonwealth University, PO Box 980058,
Richmond, VA 23298, USA. E-mail address: [email protected]
Received 19 October 2006; accepted 25 October 2006; Available
online 5 December 2006
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