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Dosimetric analysis of volumetric arc therapy on localized prostate cancer
with unilateral hip prosthesis -the "Hip-Skip" method
Seth Duffy1, Jonathan Gonzales, R.T.(T)2, Raheel Khan, R.T.(T)3
1
Cancer Care of Western New York, Cheektowaga, NY
2
University of Michigan Health Center, Ann Arbor, MI
3
McLaren Cancer Center, Flint, MI.
ABSTRACT
Many treatment planning techniques for irradiation of the prostate have been discussed as well as
institutions utilizing class solutions, templated planning schemes, or institution specific doselimiting structures. While many of these approaches are highly efficient in producing highquality treatment plans for traditional prostate cases, to the best of the authors' knowledge, little
discussion on the most efficient way of adapting to prostate treatment planning around a metal
hip prosthesis exists. More specifically, with volumetric arc therapy (VMAT) becoming a gold
standard in treating prostate cancer, the benefits of avoidance sectors cannot be understated when
approaching a case where a prosthesis is within the treatment field. Evaluation of optimal
avoidance sectors in VMAT, while maintaining prescription PTV coverage and minimizing
prosthesis dose, is the goal of this paper.
Key Words: Prostate cancer, hip prosthesis, VMAT, avoidance sectors
Introduction
Prostate cancer is the most common cancer in American males after skin cancer. The
evolution of external beam radiation therapy (EBRT) has changed the way we treat patients
diagnosed with this disease. From 3D-Conformal to now Intensity-Modulated radiation therapy,
the disease management of prostate cancer has become increasingly effective. With more precise
methods of delivering radiation to the prostate, there is now a steady trend in prescription dose
escalations that still can maintain minimal toxicities to surrounding critical
structures.1Volumetric Modulated Arc Therapy (VMAT) is one form of this innovative
technique. The goals of delivering high doses to the prostate and minimizing dose to adjacent
critical structures has now become more achievable with this arc therapy modality. VMAT has
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separated itself from its predecessor because of its capability of administering dose while in
motion and due to the utilization of all 360 degrees of gantry rotation. Dose can now be spread
throughout the entire pelvic area while still tightly focusing 100% of the prescription around the
target. There are, however, situations where it has been recommended to not utilize all 360
degrees of the arc. When considering EBRT for prostate cancer, the presence of a prosthetic hip
can introduce significant treatment planning obstacles and uncertainties. Hip prostheses are
usually manufactured using high Z atomic number materials and create significant
inhomogeneity within the treatment fields. Approximately 1% to 2% of patients with prostate
cancer have a hip prostheses.2With about 220,800 new cases of prostate cancer diagnosed each
year, this can pose a significant problem to a large population of people. The presence of the
prosthesis usually complicates the planning process because of dose perturbation around the
prosthesis, radiation attenuation through the prosthesis, and the introduction of computed
tomography artifacts in the planning volume.2In an ideal plan, the entire volume would have the
same electron density but in the real world that is never the case. The pelvis is one of the thickest
areas of any patient and is home to large pelvic bones which already attenuate beams. With the
addition of a metal prostheses, this problem increases. It is common practice to not treat through
the prostheses because it is well known that presence of artifact and treating through a high
atomic z material can pose many dosimetric uncertainties.The first thought that comes to mind is
to treat with beams that do not pass through the prosthesis. Doing so will definitely avoid
dosimetric complications but may significantly increase dose to the critical structures
surrounding the prostate.3With studies indicating that dose increases 15% in tissue at the
interface between the metal implant and tissue and that there are dose reductions of 5–25% or
10–45% in the shadow of the hip prosthesis4, avoiding the prosthetic is justified and accounting
for the inhomogeneities of the artifact is crucially important to treatment planning. Avoiding the
prosthetic hip limits the possible treatment angles of an arc and in doing so creates a challenge in
wrapping dose around the PTV while sparing dose to organs specifically the rectum and bladder.
With VMAT, there is an option to include an avoidance sector where the beam will produce 0
monitor units (MU) through those specific beam angles while maintaining a full rotating arc.
This study aims to assess various avoidance angles using a single full arc VMAT plan on treating
patients with intact prostate cancer and a unilateral prosthetic hip. Using Varian Eclipse
treatment planning systems, an assessment of increasing avoidance angles will be tested to
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evaluate the most ideal avoidance section that offers 100% of PTV coverage with at least 98% of
the prescription dose while also adhering to dose constraints for the bladder and rectum under
RTOG 0126.
Methods and Materials
Patients
All individuals analyzed in this study were patients diagnosed with non-metastatic
prostate cancer, all of which had an intact prostate with a unilateral prosthetic hip implant. There
are a total of X cases in this study that have all been gathered from three different clinical
facilities. All patients were simulated head first, supine, and scanned using a Phillips Big Bore
and or a General Electric scanner at 2.5 mmor 3 mmthick CT images. Patients were immobilized
during the CT simulation using a Vac-Lok bag systemor a more traditional set up with headrest
and angle sponge under the knees. All patients were required to have an empty rectum and full
bladder during the scan.All cases were prescribed 81 Gy to the PTV with a planning directive of
100% of the PTV to be covered with 98% of the prescription dose.
Contouring
Varian Eclipse treatment planning software was used during the contouring portion of
this study. All datasets used in this study were performed on patients with intact prostate with
unilateral prosthetic hip. The organs contoured were the prostate, bladder, and rectum. The
prosthetic hip was contoured by window/leveling the CT image until you were left with the
highest density visible. In this view we were able to localize the high density prosthetic hip from
the surrounding tissues and bone. The artifact produced by the prosthetic hip was contoured to
include the most obvious level of density changes. Again proper image window/leveling was
used to best identify where this most discrepancies were occurring. For some of the patients of
this study, three gold seed markers were implanted within in the prostate to assist in effective
localization of the target during daily treatment. These gold seeds also produces image artifact
and they too were contoured. After all contours were complete, we assigned each a specific label
and color, refer to Table 1 for details to the guidelines. The PTV was designed as a 0.5 cm
marginal expansion from the Prostate contour;refer to Table 2 for details on target structures.
Finally, the contoured artifacts were both assigned a density of 35 HU; refer to Table 3 for
details on density assignments.
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Treatment Planning
All plans were performed on Varian Eclipse TPS and each was calculated using
anisotropic analytical algorithm (AAA). Each case was planned using VMAT and was
geometrically set to utilize 1 or 2 (should the field carriage be split due to size limiting factors)
full rotational arcs with a 6 MV energy setting. A Varian 2100iX and Varian TrueBeam were the
intended treatment units for each of these plans. All plans were designed in a clockwise (CW)
rotation starting at 180° and ending at 179° and a counter clockwise rotation starting at 179°
ending at 180° for double arc plans. 6 plans were generated from each case, all of which had
different avoidance sectors. For each case, the initial plan began with avoiding angles where the
central axis of the beam would not directly pass through the prosthesis, but there is a minimum
of 15° of avoidance for Varian Eclipse TPS requirements so this angle was used if the starting
angle was under 15°.This was done by switching to beam’s eye view (BEV) in Eclipse TPS and
following the arc at each degree and recording where the prosthesis contour was directly against
the central axis. 5 more plans were created, but each subsequent plan increased the avoidance
sector by 4° in each direction, a total increase of 8°.
Each plan received a prescription of 81 Gy to the PTV in 45 fractions at 1.8 Gy fractions.
The target goals of all plans were to normalize 100% of the prescribed dose to cover at least 98%
of the PTV with a maximum dose limit of 109% of the prescription. The dose constraints
adhered to RTOG protocol 0126 which focused on bladder and rectal dose limits. Dose
constraints to the bladder are no more than 15%, 25%, 35%, and 50% of the volume should
receive more than 80 Gy, 75 Gy, 70Gy, and 65 Gy respectively. For the rectum, no more than
15%, 25%, 35%, & 50% of the volume should receive more than 75 Gy, 70 Gy, 65 Gy, and 60
Gy respectively. Normal Tissue Objective tool was used during plan optimization with consistent
parameters being implemented across all institutions participating in the study. No artificial
structures were used during the optimization process.
Results
For this study we chose to analyze our data using mean dose values across the given
PTV, bladder, and rectal constraints. Additionally, graphical representation and plotting of linear
regression lines allow us to evaluate the relative variation in dose distribution across the
avoidance sectors chosen. This type of data evaluation was selected due in part to the goals of the
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research, determine what angles of avoidance best meet the constraints and prescribed doses
while also considering how the angle impacts the overall isodose distribution throughout the
pelvis. It is possible to determine whether or not a linear relationship between dose to a critical
structure and a specific avoidance sector exists, and even PTV coverage and avoidance sector,
but a larger case population and additional investigation is necessary.Refer to Table 4 for graphs.
Statistical evidence indicated that the initial avoidance sector showed the least amount of
variation in dose deposited across the bladder. This is offset by the mean dose deposited to the
bladder, with the zero avoidance trials yielding the highest average dose to 50% bladder volume.
Figure 1 is an example taken from one of our cases that shows the how the 50% isodose line is
encompassing more of the bladder from an initial avoidance plan when compared to plans of
increasing avoidance sector. It should also be noted that the 16° avoid and the 40° avoid both
saw slight improvements in mean dose to the bladder, over the preceding trial (8 avoid) as well
as (32 avoid) This is not to say the dose was not escalating very slowly in these regions, as it
was, but the mean dose seemed to plateau across the majority of monitored constraints when
using these avoidance angles.
Discussion
This study examined the effectiveness of varying avoidance sectors applied on VMAT
plans for patients with a unilateral prosthetics hip implant. Each plan achieved target goals for
PTV coverage and each met RTOG protocol 0126 requirements. We then proceeded to further
investigate the effects each of these avoidance sectors had on the bladder and rectum. Our results
indicated that there was a spike in mean dose for 50% of the bladder volume when the tightest
avoidance section was used. This evidence is something to consider if selection if selection of
similar angles of avoidance sector are chosen.
In addition to monitoring OR doses, the statistics indicated that for 24° avoid and 32°
avoid sectors, that there was a slight increase in PTV coverage above 98%. 24° avoid and 32°
avoid sectors had angles of avoidance that ranged from 40° - 50°. This was merely an
observation that indicated to us that their might be some room for further investigation of
designing plans with these degrees of avoidance with aims of pushing the PTV percent coverage
past 98% without compromising organ dose constraints.
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It should be noted that because of the use of multiple CT simulators with varying
energies and slice thickness, artifact from the prosthesis and general quality of the scan also vary.
In addition, due to the metal prostheses, each patient scan had different degree of artifact
streaking which creates a problem when contouring the prostate volume as well as areas of the
bladder and rectum.5By adjusting the window level of the reconstructed image however, we were
still able to identify the target volume to the best of our ability. However, the amount of artifact
contoured is entirely up to the discretion of the planner, and because of this, there is a percent
chance of variability from plan to plan.
Conclusion
The presence of metal hip prostheses present a major problem when creating a treatment
plan for patients with prostate cancer. The high electron density of the prosthesis causes too
much dose attenuation and scatter so lateral beams should not be used. This in turn makes it
difficult to achieve optimal dose covering the entire PTV while maintaining minimal dose to the
rectum and bladder.With the use of VMAT, all 360° of the gantry can be utilized to achieve the
most efficient PTV coverage. With the addition of avoidance sectors, we can furthermore
minimize dose to the prosthesis without sacrificing PTV coverage that you would by creating a
VMAT plan completely avoiding the prosthesis from the beam’s eye view (BEV).
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References
1. Zelefsky MJ, Fuks Z, Hunt M, et al. High dose radiation delivered by intensity modulated
conformal radiotherapy improves the outcome of localized prostate cancer. J Urol.
2001;166(3):876-81 http://dx.doi.org/10.1016/S0022-5347(05)65855-7.
2. Brooks C, Cheung RM, Kudchadker RJ. Intensity-modulated radiation therapy with
noncoplanar beams for treatment of prostate cancer in patients with bilateral hip prosthesis—
a case study. Med Dosim. 2009;35(2):87-91. http://dx.doi.org/10.1016/j.meddos.2009.03.005
3. Raft C, Alecu R, Das IJ, et al. TG-63: Dosimetric considerations for patients with HIP
prostheses undergoing pelvic irradiation. Med Phys. 2003;30:1162-1182.
http://dx.doi.org/10.1118/1.1565113
4. Ding GX, and Yu CW. A study on beams passing through hip prosthesis for pelvic radiation
treatment. Int J RadiatOncolBiol Phys. 2001;51(4):1167-1175.
http://dx.doi.org/10.1016/S0360-3016(01)02592-5.
5. Keall PJ, Chock LB, Jeraj R, Siebers JV, Mohan R. Image reconstruction and the effect on
dose calculation for hip prostheses. Med Dosim. 2003;28(2):113-7.
http://dx.doi.org/10.1016/S0958-3947(02)00246-7
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Tables
Table 1. Organ/Structures/Target - colors
Organ/Structure
Color
Prostate
Red
Bladder
Yellow
Rectum
Brown
Prosthetic Hip
Red
Prosthetic Seed Artifact
Green
Gold Seed Artifact
Green
Table 2. Targets
Target
Color
PTV = Prostate + 0.5 cm margin
Magenta
Table 3. Inhomogeneity density assignments
Artifact
Density assignment
Gold Seed Artifact
35 HU
Prosthetic Hip Artifact
35 HU
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Figures
Figure 1. Image of the 50% isodose line within the bladder of the initial avoidance compared to
the following 5 plans of increasing avoidance sectors
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Mean Bladder Dose By Avoid Zone
30
25
20
0 Avoid
8 Avoid
15
16 Avoid
Dose (Gy)
24 Avoid
10
32 Avoid
40 Avoid
5
0
15% Vol
25% Vol
35% Vol
50% Vol
Volume
Standard Deviation By Avoidance Zone
16
14
12
0 Avoid
10
Dose (Gy)
8 Avoid
8
16 Avoid
6
24 Avoid
4
32 Avoid
40 Avoid
2
0
15% Vol
25% Vol
35% Vol
Volume
50% Vol