International Journal of Medical Physics, Clinical Engineering and Radiation Oncology, 2013, 2, 125-132
Published Online November 2013 (
Open Access IJMPCERO
Patient-Specific and Generic Immobilization Devices for
Prostate Radiotherapy
Adam D. Melancon1,2, Rajat J. Kudchadker1, Richard Amos1, Jennifer L. Johnson1, Yongbin Zhang1,
Zhiqian H. Yu1,2, Lifei Zhang1, Lei Dong1, Andrew K. Lee3
1Departments of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston, USA
2Program in Medical Physics, The University of Texas Graduate School of Biomedical Sciences at Houston,
Houston, USA
3Departments of Radiation Oncology, The University of Texas M. D. Anderson Cancer Center, Houston, USA
Received July 10, 2013; revised August 5, 2013; accepted September 3, 2013
Copyright © 2013 Adam D. Melancon et al. This is an open access article distributed under the Creative Commons Attribution Li-
cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The purpose of our study was to compare interfractional bony setup variations in pelvic anatomy with two immobiliza-
tion devices, the patient-specific Vac-Lok and the generic Dual Leg Positioner system (both Civco Medical Solutions,
Kalona, IA), for bilateral proton radiotherapy of the prostate. Two groups of 10 patients were studied. Computed tomo-
graphy (CT) was performed three times a week, yielding 233 CT image sets for the vacuum system group and 252 for
the other group. The translational shifts of the pelvic bone and prostate and rotation of the upper femurs of the femoral
heads with respect to the simulation CT images were analyzed. Along the anterior-posterior and lateral axes, mean and
systematic translational variations of the pelvic bone and prostate, relative to skin fiducials, were significantly lower in
the Vac-Lok group (all p < 0.01) than in the Dual Leg Positioner group. Abduction of the upper femur, the dominant
rotation, had random rotational variations of 1.9˚ and 2.0˚ and systematic rotations of 3.1˚ and 2.9˚ for the vacuum and
generic system groups, respectively. Femoral abduction was highly correlated with anterior prostate displacement for
both femurs in both groups (p < 0.01). We conclude that image guidance may be needed to correct systematic transla-
tion introduced during simulation CT, particularly with the generic immobilization system. High degrees of femoral
rotation may introduce prostate translation and distal misalignment of lateral proton beams with the prostate.
Keywords: Femoral Head; Interfractional Variation; Pelvic Immobilization; Proton Therapy; Prostate Cancer
1. Introduction
Pelvic immobilization during daily patient setup is a cru-
cial step in managing treatment uncertainty during pros-
tate radiotherapy. Modern conformal radiotherapy re-
quires that all treatment uncertainties are minimized to
enable the use of narrow target margins and an escalated
target dose. Immobilization and daily target localization
have become common at many cancer centers, creating a
demand for precise analysis of residual treatment uncer-
tainty and determination of the appropriate margins to
accommodate it.
Previous studies have used port film or electronic por-
tal imaging devices to compare patient setup uncertain-
ties between pelvic immobilization devices [1-5] with
patient setup uncertainties with and without rigid immo-
bilization [6-10]. Analysis of pelvic anatomic variation
with computed tomography (CT) interfractional patient
data and CT registration software provides a precise and
objective measure of pelvis translation relative to exter-
nal setup reference points (e.g., BBs). In addition, this
type of analysis provides the opportunity to analyze the
three-dimensional rotation of the upper femur during
daily setup, which could be particularly important for
proton therapy with a lateral beam arrangement, as is
typically used to treat the prostate. In such treatments, the
proton beams pass directly through the dense bones of
the upper femur and femoral heads (FHs) before reaching
the prostate. It is still unknown how much range uncer-
tainty is caused by these bony variations and whether the
use of a particular immobilization device can reduce it.
It has been well documented that prostate position can
change during daily patient setup for radiotherapy.
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[11-13]. It is arguable that the role of immobilization
devices may be diminished with daily image-guided
setup. However, it is important to note that a simple
couch shift (the common type of correction under image
guidance) cannot correct complicated anatomical changes
resulting from improper immobilization. Few studies
have addressed the magnitude of FH and upper femur
variations resulting from patient setup and the potential
effect and these variations have on daily target localiza-
tion of the prostate, particularly for conventional bilateral
proton treatments. To fill this gap in knowledge, we retros-
pectively compared two immobilization devices used for
radiotherapy of the prostate. The first immobilization
device, the Vac-Lok (Civco Medical Solutions, Kalona,
IA), is a patient-specific device that conforms tightly to
the patient’s body contours. The second immobilization
device, the Dual Leg Positioner (also from Civco Medi-
cal Solutions), is a non-patient-specific positioning sys-
tem that reduces storage needs. We specifically analyzed
and compared these two systems’ ability to control rota-
tion of the upper femur around the FH and, to a lesser
extent, translation of the pelvic girdle and prostate with
respect to the BBs to determine if the convenience of
generic patient immobilization is obtained at the cost of
patient setup precision.
2. Materials and Methods
2.1. Simulation, Treatment Planning, and
Twenty patients from two institutional review board-
approved protocols at our institution participated in a re-
trospective study to determine the efficacy of the im-
mobilization devices for future proton radiotherapy pa-
tients. Patients enrolled on both protocols underwent
three CT scans per week (“daily CT”) for tumor localiza-
tion using a CT-on-rails linear accelerator system (EX-
aCT; Varian Oncology Systems, Palo Alto, CA) immedi-
ately before their radiation treatment [14]. The CT-on-
rails CT-linear accelerator combination radiotherapy sys-
tem allowed for daily CT imaging and treatment without
removing the patient from the treatment couch. Patients
on both protocols received 75.6 Gy with 8-field IMRT
delivered in 42 fractions, yielding an average of 24 im-
ages sets per patient.
The first 10 patients were set up with the Vac-Lok sys-
tem, a patient-specific vacuum positioning system in
which a radiotranslucent cushion fixes the patient’s ana-
tomy from the upper thighs to the feet (Figure 1(a)). The
patients were placed in the treatment position on a par-
tially inflated cushion. The air in the cushion was then
partially evacuated until the cushion was semi-rigid, and
the therapist molded it around the area to be immobilized.
Once the desired shape was achieved, more air was re-
moved until the cushion became rigid. The second 10
patients were immobilized with the Dual Leg Positioner
system, a vinyl-covered foam immobilization device that
is indexable to improve reproducibility and that can be
easily cleaned and reused (Figure 1(b)).
CT scans for treatment simulation were acquired while
the patients had full bladders and empty rectums. For
these scans, the patients were marked with one anterior
and two lateral skin marks that reflected the position of
the isocenter inside the prostate and coincided with the
treatment room laser lines marked on the patient. After
external fiducial markers (BBs) were placed on the pa-
tients’ permanent skin marks, the patients were scanned.
Each CT image had a 3-mm slice thickness, a 512 × 512
pixel matrix size, and a 1-mm axial resolution. The CT
dataset was transferred to the Pinnacle3 treatment plan-
ning system (Philips Medical Systems, Andover, MA),
and one radiation oncologist contoured the prostate, se-
minal vesicles, bladder, rectum, and FHs, as described
2.2. Anatomical Contouring and Registration
In order to compare the changes in FH and upper femur,
the left FH, right FH, left femur, and right femur were
contoured on each treatment planning CT as reference
structures. In addition, a region of interest from the pel-
vic bone surrounding the prostate (including the ischium,
pubic arch, and pubic symphysis) was selected to repre-
sent the pelvis bony anatomy in the bony registration (Fig-
ure 2). The reference images were exported along with
the daily CT image sets into a non-commercial CT regis-
tration software [15]. The CT-assisted targeting software
is capable of both rigid and deformable registration of
soft tissue and bony structures. In this study, the software
automatically registered the bony structures using rigid
transformations. Each patient’s pelvic anatomy was reg-
istered to the corresponding pelvic anatomy on the ref-
erence CT, and the treatment couch correction (transla-
tion relative to the position indicated by registra-
Figure 1. Two commercially available pelvic immobilization
devices. The Vac-Lok system (a), a patient-specific radio-
translucent cushion that fixes the patient’s anatomy from
the upper thighs to the feet. The Dual Leg Positioner system
(b), a non-patient-specific vinyl-covered foam immobiliza-
tion device.
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Figure 2. Pelvic bony anatomy used for CT registration (top
left) and imported pelvic bone contours on daily registered
pelvic bony anatomy (top right). Femoral head (FH) and
upper femur contoured on the simulation CT image and
contours generated from 2˚ femoral rotations (middle left)
and daily femur images matched to the rotated femurs on
the planning CT image (middle right) in the axial plane. FH
and upper femur contoured on the simulation CT image
and contours generated from 2˚ femoral rotations (bottom
left) and daily femur image s matched to the rotated femurs
on the planning CT image (bottom right) in the coronal
tion to the external skin fiducials) was recorded. The
amount of translation of the pelvic bone relative to ex-
ternal skin fiducials is a good estimator of patient setup
uncertainty for protocols with little or no image guidance
for daily setup. Image guidance may correct translational
misalignment of the target and the treatment beams.
To evaluate femoral rotation, each patient’s daily FH
was registered to the corresponding FH on the reference
CT and the femur’s contour was rotated in the axial plane
around the center of the contoured FH volume in 2˚ in-
crements from the 16˚position to the –16˚ position. The
contour that best matched the “daily” femur with the fe-
mur on the reference CT was manually chosen and re-
corded (Figure 2). This process was repeated with each
daily FH and femur to generate the daily rotation of the
left and right FHs.
2.3. Analytical Methods
The “daily” translations of the pelvic girdle and prostate
relative to the external skin fiducials were measured first.
The deviation of each patient’s daily CT image set rela-
tive to the simulation CT image set was recorded in the
superior-inferior (SI), anteroposterior (AP), and medio-
lateral (or right-left [RL]) directions. The absolute mean
and standard deviations were chosen to evaluate overall
alignment quality between the two immobilization de-
The daily pelvic and prostate translations were further
subdivided into two components, systematic and random.
The systematic component (S) and random component(s)
of setup uncertainty were calculated as previously de-
scribed [16,17]. S represents the patient-to-patient mean
anatomical deviation from the position at simulation. s
represents daily variation of the patient’s anatomy with
respect to that patient’s mean position during the course
of radiotherapy. Similarly, the total daily rotation of each
patient’s FH was recorded. The left and right FH data
were combined for each group, and the mean, total un-
certainty (stotal), random component of variation (s) and
systematic component of variation (S) were compared.
The Student’s t-test was used to analyze the statistical
significance of differences in the mean and random s
values between patient groups, for both translational and
rotational variations. The F-test for equivalence of vari-
ance was used to analyze the statistical significance of
differences in S values between the two patient groups
(SPSS Inc., Chicago, IL). The Pearson’s parametric cor-
relation was use to determine correlation between rota-
tional and translational uncertainties.
3. Results
There were 233 CT image sets in the vacuum system
group and 252 CT image sets in the foam immobilization
system group. Table 1 shows the mean translation, s, and
S values for daily translations of the pelvis relative to
BBs in each cardinal direction. The absolute mean trans-
lation of the vacuum system group (1.7 mm) was statis-
tically less than that in the other group (2.9 mm) along
the AP axis (p < 0.01). The random component of uncer-
tainty in the AP direction was greater for the vacuum
system group, but the systematic component was greater
in the foam system group.
The translations along the SI axis were nearly identical
between the two patient groups. The generic foam im-
mobilization system is indexed along the axis of the table
and SI axis of the patient. Rigorous alignment along the
SI axis when using this system appeared to provide im-
mobilization equivalent to that afforded by the rigid vac-
uum immobilization system.
The absolute mean translation of the vacuum system
group (0.9 mm) was statistically less than that in the
foam immobilization system group (2.5 mm) along the
RL axis (p < 0.01). The systematic components of un-
certainty in the RL direction were 1.0 mm for the
Vac-Lok group and 3.3 mm for the Dual Leg Positioner
group (p < 0.01). In general, alignment to the BBs was
equivalent in the SI direction and mean components of
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variations were greater along the lateral and AP axis for
the generic immobilization device. This indicates BB
alignment with the generic device may be less reproduci-
ble from the position on the reference CT than the vac-
uum device.
The translation of the prostate relative to the BBs dur-
ing the same treatment sessions is summarized in Table
2. The misalignments along the SI and RL axis were
nearly identical to the misalignments of the pelvic bony
anatomy. However, the misalignments along the AP axis
were substantially greater for the prostate than for the
pelvic bony anatomy. Additional factors may have con-
tributed to the larger uncertainty along the AP axis of the
prostate, including prostate motion caused by rectal and
bladder filling and deformation of the pelvic region (such
as by femoral rotation).
The daily rotations of the upper femur around the FH
were random about the origin (Figure 3), so the standard
deviations (total uncertainty) of each group were more
indicative of overall immobilization quality than the di-
rection-specific uncertainty values. The standard devia-
tions of the daily femoral rotations in both groups were
nearly identical, 3.67˚ for the vacuum system group and
3.57˚ for the foam system group.
The random components of uncertainty (Table 3 ) were
1.9˚ and 2.0˚ for the vacuum and generic foam system
groups, respectively. The systematic component of un-
certainty was 3.1˚ for the former group and 2.9˚ for the
latter group. There were no statistically significant dif-
ferences between the two groups when daily femoral
rotations were compared.
Uncertainty along the AP axis was found to be greater
for the prostate than the pelvic bony anatomy. We corre-
lated prostate misalignment with daily FH rotation to
investigate whether rotation might have negatively im-
pacted daily target localization of the prostate (Figure 4).
Table 1. Daily translation of pelvic bony anatomy relative to external skin marks.
AP shift, mm (A = +) SI shift, mm (I = +) RL shift, mm (L = +)
Immobilization Device Mean σ Σ Mean σ Σ Mean σ Σ
Vac-Lok 1.7 2.6 2.8 1.3 1.2 1.5 0.9 2.7 1.0
Dual Leg Positioner 2.9 2.1 3.4 1.3 1.1 1.5 2.5 2.9 3.3
AP = anteroposterior, SI = superior-inferior, RL = mediolateral, σ = random variation, Σ = systematic variation.
Table 2. Daily translation of the prostate relative to skin marks.
AP shift, mm (A = +) SI shift, mm (I = +) RL shift, mm (L = +)
Immobilization Device Mean σ Σ Mean σ Σ Mean σ Σ
Vac-Lok 2.6 3.1 3.6 1.4 1.9 1.5 0.9 2.8 1.0
Dual Leg Positioner 3.2 2.8 4.1 1.6 1.9 1.5 2.3 2.9 3.2
AP = anteroposterior, SI = superior-inferior, RL = mediolateral, σ = random variation, Σ = systematic variation.
Figure 3. Total daily femoral head (FH) rotations for both groups. Histograms represent the rotations of both left and right
FH combined. The total uncertainty indicated by the standard deviation suggests that the immobilization devices have nearly
identical immobilization quality during treatment.
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Table 3. Total uncertainties for daily FH rotation.
FH Rotation
Immobilization Device Mean  σtotal σ Σ
Vac-Lok 2.4 3.7 1.9 3.1
Dual Leg Positioner 2.5 3.6 2.0 2.9
FH = femoral head, σtotal = total uncertainty, σ = random variation, Σ = systematic variation.
Figure 4. Correlation of femoral head rotation and prostate translation in the anteroposterior direction for the Vac-Lok
group (top) and Dual Leg Positioner group (bottom). A (+) indicates anterior translation or clockwise rotation. All correla-
tions were statistically significant (p < 0.01). The correlation coefficients were generated with Pearson’s parametric correla-
tion. Each color corresponds to a particular patient’s variations.
Right and left FH rotations for both groups were highly
correlated with prostate AP interfractional motion (p <
0.01, all groups), which suggests that special care to mi-
nimize FH rotation during treatment setup has the poten-
tial to improve prostate coverage during radiotherapy
4. Discussion
On the basis of our findings, we conclude that the Vac-
Lok pelvic immobilization device outperforms the Dual
Leg Positioner particularly when considering systematic
translation along the lateral axis. Treatment beam and
target translational misalignments are correctable with
proper image guidance and the appropriate couch shifts
for correction. Additionally, for the case of lateral proton
beam treatment, lateral translation of the patient does not
alter the water-equivalent depth from the patient’s skin
surface to the distal end of the target. Thus, translational
uncertainty will have minimal effects on our current pro-
ton treatment protocols.
Rotation, however, is difficult to correct. The largest
component of FH rotation was rotation in the axial plane.
Both random (s) and systematic (S) components of FH
rotation were nearly identical between the two patient
groups, at approximately 2˚ and 3˚, respectively. Minor
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rotations, as are seen in the majority of patients’ daily
treatment fractions, will have little effect on the treated
volume within the pelvis. Prostate misalignment relative
to the BBs was nearly equivalent to the misalignment of
the pelvic bones, except along the AP axis. This addi-
tional uncertainty is due to motion of prostate within the
pelvic anatomy, which is generally attributed to rectal
and bladder filling [13]. However, the weak but statisti-
cally significant correlation of FH rotation to interfrac-
tional AP movement of the prostate suggests that femoral
rotation contributes to prostate treatment uncertainty and
should be managed for all radiotherapy treatments.
Femoral and FH rotations have typically been ne-
glected in positioning studies for prostate cancer patients
because their impact on patient dosimetry is minimal for
photon therapy unless they result in geometric translation
of the pelvis or internal pelvic organs. However, femoral
and FH rotations may be critical to treatment uncertain-
ties in prostate proton radiotherapy [18]. The range de-
fined by the Bragg peak in proton therapy is roughly
proportional to the radiologic path length in the beam
path. Range uncertainties in the bilateral proton therapy
of the prostate can potentially compromise the distal
coverage of the target. In addition, dense bony structures,
such as the femur and FH, in the beam path can also
cause uncertainties in dose distributions because the
Bragg peak becomes degraded [19]. This degradation, in
turn, affects the final dose distribution [20]. Hence, de-
pending on the type of immobilization device used,
changes in daily patient setup during the course of proton
radiotherapy could introduce varying amounts of bone in
the treatment field due to FH rotation.
It should be noted that in proton radiotherapy “smear-
ing” [21,22] is typically applied to the range compensator
design in an attempt to correct for range uncertainty.
Range compensators are used in proton radiotherapy to
conform the distal edge of each beam to the target while
accounting for the heterogeneities along each beam path.
With the appropriate application of smearing in the de-
sign of each compensator, the changes in radiologic path
length resulting from tissue motion should not lead to
insufficient proton penetration, thus maintaining distal
coverage of the target. However, compensator smearing
is usually two-dimensional and orthogonal to the beam
path, so significant anatomical rotation may not be fully
compensated. To illustrate this insufficiency, we de-
signed a single lateral proton beam plan with the greatest
rotational misalignment found in our study (Figure 5).
We found that the typical 6.6-mm smearing in our rou-
tine proton plan for prostate treatment did not preserve
the dose coverage in this case: The dose coverage for the
prostate dropped from 75.6 Gy (prescribed dose) to only
60 Gy. While a single fraction of treatment with the mis-
aligned FH may not have clinical impact, a large system-
Figure 5. Changes in dose distribution to the distal edge of
the prostate after a 20˚ femoral head rotation. The refer-
ence CT image set and plan in colorwash (top) and linear
(bottom) isodose lines (a). The reference plan recalculated
on a daily CT image set in the Eclipse treatment planning
system (Varian Medical Systems, Palo Alto, CA) (b).
atic error in FH position would compromise administra-
tion of the planned dose. We plan further work to evalu-
ate the dosimetric effect resulting from patient setup un-
certainties in bilateral proton therapy for prostate cancer.
It is worth mentioning that the dosimetric impact of
FH/femur position in proton therapy cannot be resolved
by simply using image-guided setup techniques. Range
uncertainties due to daily radiologic path length varia-
tions require the design of better immobilization devices
to ensure reproducibility of the patient anatomy.
Numerous studies have been conducted on the use of
immobilization for patients undergoing prostate radio-
therapy. Most of these studies compared patient treat-
ment uncertainty with and without rigid daily patient
immobilization. Bentel et al. [6] compared pelvic immo-
bilization with and without a hemibody foam cradle in 74
prostate cancer patients. They evaluated immobilization
quality by comparing the number of times that the radia-
tion oncologist requested an isocenter shift on the table.
Requests were made when setup error exceeded 5 mm.
Bentel et al. found that 17.4% of patients with hemibody
foam setup and 23.1% of patients without pelvic immo-
bilization required setup correction. Soffen et al. [9]
evaluated the use of rigid immobilization for the pelvic
regions of early stage prostate cancer patients. The me-
dian decrease in median daily variation of patients with
the immobilization devices declined to 1/3 the compari-
son value (1 mm vs. 3 mm) because of the elimination of
the largest 10% of misalignments. Taken together, the
results of these studies suggest that some form of daily
pelvic immobilization is essential to significantly im-
prove target reproducibility during prostate radiotherapy.
Other studies have investigated the performance of
several immobilization devices and techniques. For ex-
ample, in a study of 52 prostate cancer patients, Fiorino
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et al. [1] compared pelvic immobilization with an alpha
cradle device fixed at the pelvic level and that with the
device fixed at the leg level. The group immobilized at
the leg level demonstrated smaller variations than the
group immobilized at the pelvic level. In light of these
findings, given the correlation between femoral rotation
and prostate translation that we found in our investiga-
tion, immobilization at the leg level could have reduced
femoral rotation and thus reduced prostate translation.
However, in a study of 77 patients, Malone et al. [5]
compared pelvic immobilization using a rubber leg cu-
shion, a thermoplastic Hipfix (Civco Medical Solutions),
and an alpha cradle device and found that the hip immo-
bilization device was superior to the other two types.
Because the results from these two studies and our inves-
tigation arrive at no consensus, we can draw no conclu-
sions as to whether hip immobilization reduces femur
Few authors have formally addressed rotation in pelvic
immobilization studies. Hanley et al. [7] measured pelvic
translation and rotation in 50 prostate patients immobi-
lized with a thermoplast body cast. They found that the
pelvic rotations in the AP and SI directions were 0.6˚ and
0.9˚, respectively. The random and systematic compo-
nents of uncertainty were respectively 1.9 mm and 2.0
mm in the RL direction, 1.4 mm and 1.7 mm in the SI
direction, and 1.3 mm and 1.9 mm in the AP direction,
which are in good agreement with our findings. Van
Herk et al. [23] addressed femoral and prostate rotation
in a study of interfractional pelvic anatomy variation
with serial CT. In that study, the only significant correla-
tions of the two presented were leg scissor with SI rota-
tion of the prostate (R = 0.47) and leg roll with SI rota-
tion of the prostate (R = 0.46). Although the magnitudes
of these correlations are similar to those we reported, the
magnitude, or slope, of their regression is much shal-
lower than the regression line of prostate motion with
femoral rotation in our study. The prostate would need to
rotate by a large amount before geometrically shifting
outside a planning target volume. Visual inspection of
our correlations suggests a nearly one-to-one correlation
of prostate AP shift (in mm) with femoral rotation (in˚).
Figure 4 predicts more prostate motion for a given
femoral rotation than these previous results. Thus, femo-
ral rotation contributes more to prostate interfractional
uncertainty than suggested in previous studies.
To deliver a realistic and consistent dose to patients
undergoing radiotherapy, therapists must ensure that pa-
tients are adequately immobilized during simulation and
treatment. Good immobilization devices should achieve
true reproducibility of patient’s anatomy while minimiz-
ing the additional workload introduced by other interven-
tional, image-guided setup procedures. These devices
should be relatively easy for therapists to use so that
setup time is minimized and patient comfort maximized.
In addition, they should be rigid and durable enough to
prevent motion and last the entire course of treatment.
Compared with the Vac-Lok immobilization device, the
Dual Leg Positioner system displays these pivotal char-
acteristics, making it a suitable choice for prostate radio-
therapy. In addition, it can be used on multiple patients,
minimizing the need for storage. Thus, we currently use
this foam immobilization system in our institution for all
our prostate patients undergoing IMRT and proton ther-
5. Conclusion
We found that the Vac-Lok system introduced less sys-
tematic uncertainty, particularly laterally, than the Dual
Leg Positioner system; however, our data suggest that
these differences are minimal for institutions utilizing
daily image-guided setup or treating with lateral proton
beams. Rotational variations due to the two setup tech-
niques were nearly identical. A 2˚ - 3˚ random rotation (1
standard deviation) of the FH will probably not have a
detrimental effect on a prostate cancer patient’s proton
fraction; however, a larger-than-expected systematic ro-
tation of the FH/femur raises concern about the do-
simetric impact of bilateral proton delivery to the pros-
tate. Regardless of the immobilization device used or
treatment modality, additional measures are needed to
improve treatment precision by managing systematic FH
rotations introduced during initial patient setup.
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