1
Submission Packet
Statements
Purpose Statement: The purpose of this study is to compare plan isocenter locations to
determine if a guideline can be established to prevent collisions of the gantry head with the
immobilization device in all directions while maintaining quality treatment plans.
Problem Statement: The problem is that prone breast set ups occasionally result in collisions of
the gantry head with the immobilization device depending on the isocenter location, which can
negatively impact treatment and patient experience.
Hypotheses:
H1A: The research hypothesis (H1) is that an isocenter location guideline can be developed to
prevent collisions with the prone breast immobilization and gantry head, while still creating a
clinically acceptable treatment plan.
H10: The null hypothesis (H0) is that a guideline isocenter location cannot be developed that will
prevent collisions and create a clinically acceptable plan.
Change Matrix
Title of Capstone: Minimizing clearance issues with prone breast patients on Varian linear
accelerators through isocenter placement.
Group: Lauren Wilson & Rob Rohe
Reviewer’s recommendation How addressed Page numbers
where change
appears
Combining these 2 sentences would Combined sentences page 8, intro 1st
make for a stronger introductory paragraph
statement.
When treating breast cancer, the Changed verbiage to include page 8, intro 1st
volume being treated is often the “target” volume and elaborated by paragraph
whole breast, not the "tumor", which saying this is typically the whole
has often already been excised. breast
"Target volume" might be a more
appropriate descrition with some
discussion about what the target
volume includes.
These 2 sentences seem out of place in After further review the sentences page 8, intro 2nd
this paragraph. The paragraph is about on image guidance were deemed paragraph
immobilization, not image guidance. unnecessary and were removed
Either you need a paragraph about
image guidance or omit image
guidance info all together.
Are you saying that only lt sided
patients have set up errors and
2
therefore are allowed more imaging
dose while rt sided patients always set
up perfectly and therefore don't need
imaging? That makes no sense.
This sentence seems out of place here. Better transition sentence used page 8, intro 2nd
You're talking about reproducibility in paragraph
this paragraph and then all of a sudden
throw a sentence in about increased
complexity of treatment delivery and
the risk of collision. Again, the next
paragraph talks about collision in the
first sentence, so this sentence may be
better served there.
What do you mean by "MLC failure Removed part about MLC failure page 9, intro 3rd
with the use of photon blocks"? This since it seemed irrelevant. Combined paragraph
sentence doesn't make sense to me. sentenced about clearance of MLC
This section starts stating that you Reorganized the paragraph to flow page 9, intro 3rd
already have potential resolution to better. First stated the problem, then paragraph
collision issues, then you state that you the potential solutions and why they
have collision issues. I'd potentially don’t cover all issues
rework this a little. Start with the
problem...The problem is that prone
breast set ups result in collision.
Nguyen suggested supplemental
cameras may help see collisions when
they happen, but don't help avoid them
from happening in the first place.
They also suggest the use of as SSD
100 to give additional clearance, but
this requires extra set up time as the
treatment is no longer isocentric
(something like that).
Move sentence to the last paragraph Moved to end of last paragraph of page 9, intro 3rd
of your intro. intro paragraph
This looks like the declarative Statement more clearly stated and Page 9, intro 4th
statement for what you're writing your intent declared paragraph
paper about. It should be much
stronger and more clearly stated....The
goal of this paper is/was to evaluate
the development of guidelines for
isocenter location that would prevent
collision....something like that.
3
Need a hypothesis abbreviation listed H1 and H0 added Page 9, intro 4th
according to the example. paragraph
Inappropriate word choice here Changed sentence structure and Page 9, intro 4th
removed word paragraph
Reference issues: numbers not in Style corrections to references made, Page 10, references
Times New Roman size 12 font, doi added
spacing, italics for journal
abbreviation, missing doi
To me this sentence reads...Further (as Changed to “proper” Page 10, intro 2nd
in we need additional) immobilization paragraph
is key.....Maybe just say that proper
immobilization of the patient is key to
set up reproducibility....
Font Times New Roman Header font changed Page 1
This is your introduction, not your Changed verbiage to “currently” Page 10, intro 3rd
conclusion. I'd start this out with paragraph
something more like Currently no
guidelines exist....
It was decided to remove the null Removed null hypothesis Page 10, 4th
hypothesis statements during this paragraph
last draft review.
Above you stated that seventeen Changed to 12 RT sided patients Page 10, methods
patients were chosen for the study. 1st paragraph
These numbers only add up to 10
patients, 5 Lt sided and 5 rt sided.
Where are the other 7 patients?
Do you need a section for We discussed this and decided it Page 11, methods
contouring critical structures? was not relevant for our topic 3rd paragraph
This reads poorly to me. I'd say Suggestion made Page 11, methods
something more like, "...objectives 4th paragraph
from the institution as described
below."
Do you really have a volume of a This was a typo. Fixed Page 11, methods,
volume? Would it be more accurate 4th paragraph
4
to say, at least 95% of the clinical
target volume at the prescription
dose, a maximum of 118% of the
CTV at 118% of the prescription
dose....
Is it 20Gy or 16Gy to 5% of the 20Gy, 16Gy taken out Page 11, methods,
heart? 4th paragraph
By this point in the paper I'm totally More details about isocenter Page 11, methods
confused. No where are you talking shifts/measurements added. 3rd paragraph
about or addressing the potential for
collision. you're just talking about
plan evaluation and comparison. Do
the plans you are compaing still
have the potential for collision? At
this point I don't know.
Need to explain potential for Measurements for collision Page 12, methods
collision in new plans/if potential for avoidance outlined. 4th paragraph
collision still exists
Format on references last number & Last number corrected and periods Page 13,
take period away after DOI removed references
Page Break
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table description
Minimizing clearance issues with prone breast patients on Varian linear accelerators
through isocenter placement
9
Lauren Wilson; Rob Rohe, BS, R.T.(T)
Medical Dosimetry Program at the University of Wisconsin-La Crosse, WI
ABSTRACT
Keywords:
Introduction
The conventional breast radiation treatment technique is executed with the patient in the
supine position, however, some evidence indicates that radiation treatment delivered in the prone
position is beneficial in order to decrease excess dose delivered to the lung and heart.1 The intent
of prone breast treatment plans is to provide sufficient coverage to the target volume, typically
the whole breast, while avoiding the inclusion of the lungs and heart in the treatment field. The
prone breast position allows for increased target coverage and better sparing of the heart, thyroid,
esophagus and contralateral breast and lung in contrast to supine irradiation treatments.2 Prone
breast setups are known to be particularly helpful in women with large breast size due to the
displacement of the breast from the chest wall and infra-mammary fold, which often develop
skin toxicities from supine breast treatments.3 The prone set up technique enables improvement
of dose conformity and dose-volume parameters associated with toxicities but poses new
challenges for set up reproducibility.4
A study by Huppert et al5 showed treatment position replication was challenging for
prone breast irradiation and that immobilization devices are crucial to ensuring accurate
positioning of the patient. The setup can be managed by use of a prone breast board placed on
the patient positioning system, which aligns the patient in an “arms-up” position and can be
made more comfortable with the addition of a memory foam mattress or Vac-Lok.4 The Civco
Horizon is a prone breast board model that includes scale rulers and setup sheets to assist with
reproducibility (Figure 1 & 2). A study by Lakosi et al6 analyzed a sample of patients who
received whole breast irradiation in the prone position for patient satisfaction and reproducibility.
The results indicated that set-up accuracy was comparable with other prone systems and average
patient satisfaction was reported as good.6 Proper immobilization is key to set up reproducibility
in prone breast irradtion.7
Although immobilization devices are necessary for the setup and reproducibility of prone
breast patients, they can occasionally cause collision issues with the linear accelerator (Figure 3
10
& 4).8 According to Nguyen et al,7 the addition of supplemental live-view cameras can help
reduce the risk of collision but cannot keep collisions from occurring. Varian linear accelerators
have a tertiary collimation system with multi-leaf collimators (MLC) located beneath the X and
Y Jaws.9 The tertiary MLC system decreases the distance from the head of the gantry to the
isocenter which can lead to clearance issues pertaining to the head of the gantry and the patient
or patient positioning system.9 Isocenter location can contribute to additional clearance issues in
Varian linear accelerators.10 The use of source to skin distance (SSD) of 100 cm may help to
provide sufficient clearance for certain prone breast treatments, but the treatment will no longer
be isocentric and set-up time is increased.6
Currently, no guidelines exist to determine appropriate isocenter placement and assure
collisions do not occur while treating prone breast patients in radiation therapy. The problem is
that prone breast set ups occasionally result in collisions of the gantry head with the
immobilization device depending on the isocenter location, which can negatively impact
treatment and patient experience. The goal of this study was to compare plan isocenter locations
to determine if a guideline can be established to prevent collisions of the gantry head with the
immobilization device in all directions while maintaining quality treatment plans. The
researchers hypothesized that (H1A) an isocenter location guideline could be developed to
prevent collisions with the prone breast immobilization and gantry head, while still creating a
clinically acceptable treatment plan.
Methods and Materials
Patient Selection & Setup
Seventeen patients from a single institution were chosen for this study. The inclusion
criteria were female patients with left or right-sided breast cancer, treated using 3D conformal
treatment technique with tangential fields in the prone position. Patients with regional lymph
node involvement were excluded from this study. The patient data was collected retrospectively
to include 5 patients with left-sided breast cancer and 12 patients with right-sided breast cancer.
Fractionation and prescription doses varied amongst patients but all patients were treated with a
curative intent and with a mix of 6 MV and 10 MV energies, dependent on the size of the patient
and planning restrictions at the institution. Any isocenter shifts from boost plans were not
included in the final comparison.
11
All patients were simulated on a Philips CT scanner using slice thickness of 2 mm for the
scan. Patients were positioned head first with both arms above the head in the prone position
(Figure 5). For simulation, the physician placed wire around the breast tissue to mark landmarks
for contouring. The scan was exported to the RayStation (Version 8A SP1) treatment planning
system (TPS).
Isocenter Location
Prior to planning, a clearance threshold was developed using patient positioning system
locations relative to isocenter to ensure safe treatment of patients with the Civco Horizon
immobilization on Varian Truebeam linear accelerator. The isocenter location threshold was
measured to be within 6 cm mediolaterally of midline and less than or equal to 16 cm from the
top of the patient position system. Superior and inferior shifts were determined to not be a cause
of concern regarding collision issues with prone breast patients, but were used at times due to the
field size limitations in prone breast planning. Isocenters located in above range were found to
allow for clearance on the Varian Truebeam linear accelerator in regards to the patient
immobilization and patient positioning system. After a new isocenter location was determined
for each patient, a new treatment plan was created following the clinical objectives from the
institution, which are described below.
Objectives
Objectives that had to be achieved following isocenter shifts included no more than 95%
of the clinical target volume (CTV) at the prescription dose, no more than 30% volume of CTV
at 118% of prescription dose, and no more than 50% volume of CTV at 112% of prescription
dose. In addition, no more than 400 cGy average dose of the heart and no more than 2000 cGy
dose at 5% volume of the heart. For the lungs, objectives were set for no more than 55% volume
of the right lung at 400 cGy dose, and no more than 15% volume of the left lung at 400 cGy
dose. These organs at risk (OAR) constraints followed the Radiation Therapy Oncology Group
(RTOG) 1005 constraints and institution guidelines. The plan doses for the OAR objectives and
volumes were documented for the initial plans and re-plans after isocenter shifts.
Plan Comparison
The evaluated metrics for this study involved the isocenter, OAR, and target volumes.
Isocenter shifts from the original plan were generated and data from all plans was averaged. The
new plan OAR data was statistically evaluated compared to the original plans to help determine
12
if the new treatment plans were clinically acceptable. The new isocenter was evaluated based on
its location relative to the previous plan isocenter and whether the new location was within the
clearance threshold of plus or minus 6 cm medially/laterally from midline and within 16 cm from
the top of the patient positioning system. Next, the OAR were examined specifically looking at
the mean lung dose, D95 heart dose, D1 heart dose, D95 lung dose, and D1 lung dose. These
parameters were intended to meet RTOG 1005 constraints and the percent difference from the
original plan for D95, D1 and mean dose were recorded for the lungs and heart. The planning
target volume (PTV) doses from each re-plan was evaluated based on percent of PTV receiving
90%, 95% and 100% of the prescribed dose. The PTV dose objectives must have been within 3%
difference of the original plan.
Statistical Analysis
Each patient plan was evaluated individually for data collection. The data were
statistically evaluated for normality using the Shapiro-Wilk test. In addition, the Wilcoxon
Signed Rank test (WSR) was used for all OAR and target metrics, including the mean heart dose,
mean lung dose, D95 heart dose, D95 lung dose, D1 heart dose, and D1 lung dose. In addition
the WSR was performed on the PTV for percent volume receiving 90%, 95% and 100% of the
prescribed dose. For each of the OARs evaluated, Wilcoxon signed rank tests were performed to
compare the distributions for the new isocenter compared to the original isocenter. Data was
collected for Lungs D95 (cGy) but no hypothesis testing was needed because all of the measured
values for both treatment plans were 0, thus showing no difference between treatment isocenters.
The Benjamini-Hochberg adjustment, or false discovery rate, was applied to control the
type 1 error rate for multiple testing with a family-wise error rate of 5% for the 17 tests overall.11
Each sample of differences was examined for normality both graphically and with Shapiro-Wilk
normality tests. It was determined that Wilcoxon signed rank tests were preferable to paired t-
tests due to non-normality observed in several of the samples.
Results
Objectives
The WSR test was performed to investigate the relationship between OAR metrics of
original and new plans. A positive difference indicated that the dose using the new isocenter was
higher than the dose for the original isocenter and a negative difference indicated that the dose
for the new isocenter was less than the dose for the original isocenter (Table 1). Only one OAR
13
objective registered a statistically significant difference in the median dose for the population of
all patients under the new and original isocenter. The population median dose for CTV Breast
D95 was higher in using the new isocenter (Padj = 0.034) and no statistically significant
differences were found for the median doses for CTV Breast Mean or D1. No significant
differences in population medians were seen in any of the remaining 16 OARs tested (all Padj >
0.05). The non-parametric related samples evaluated by the WSR test revealed no statistically
significant differences for Heart Mean D95 and D1. In addition, no statistically significant
differences were calculated in the median dose for the Lung Mean and D1.
The WSR test evaluated the relationship between target volumes of original and new
plans. The population median doses for PTV D95, PTV Mean, CTV D95, CTV Mean, CTV D1
and GTV D95 revealed a positive difference indicating that the dose using the new isocenter was
higher than the dose for the original isocenter. The population median doses for PTV D1, GTV
Mean and GTV D1 revealed a negative difference indicating that the dose for the new isocenter
was less than the dose for the original isocenter. No metrics registered a statistically significant
difference in the median dose for the population of all patients under the new and original
isocenter (all Padj > 0.05).
Isocenter Location
No original plan isocenter fell into clearance threshold and all isocenters from new plans
were located within clearance threshold metrics defined as within 6 cm mediolaterally of midline
and less than or equal to 16 cm from the top of the patient position system (Table 2). Superior
and inferior shifts were not considered to contribute to collisions. The average isocenter shift
from the original plan to acceptable parameters measured 3.28 cm medially and 3.1 cm anterior.
Discussion
Conclusion
14
I would like to thank the Statistical Consulting Center at UW-La Crosse for its assistance with
statistical analysis; however, any errors of fact or interpretation remain the sole responsibility of
the author.
15
References
1. Yao S, Zhang Y, Nie K, et al. Setup uncertainties and the optimal imaging schedule in the
prone position whole breast radiotherapy. Radiat Oncol. 2019;14(1):76.
https://doi.org/10.1186/s13014-019-1282-4
2. Deseyne P, Speleers B, De Neve W, et al. Whole breast and regional nodal irradiation in
prone versus supine position in left sided breast cancer. Radiat Oncol. 2017;12(1):89.
https://doi.org/10.1186/s13014-017-0828-6
3. Boyages J, Baker L. Evolution of radiotherapy techniques in breast conservation
treatment. Gland Surg. 2018;7(6):576-595. https://doi.org/10.21037/gs.2018.11.10
4. Fahimian B, Yu V, Horst K, Xing L, Hristov D. Trajectory modulated prone breast
irradiation: A LINAC-based technique combining intensity modulated delivery and
motion of the couch. Radiother Oncol. 2013;109(3):475-481.
https://doi:10.1016/j.radonc.2013.10.031
5. Huppert N, Jozsef G, Dewyngaert K, Formenti SC. The role of a prone setup in breast
radiation therapy. Front Oncol. 2011;1:1-8. https://doi.org/10.3389/fonc.2011.00031
6. Lakosi F, Gulyban A, Ben-Mustapha Simoni S, et al. Feasibility evaluation of prone
breast irradiation with the Sagittilt system including residual-intrafractional error
assessment. Cancer Radiother. 2016;20(8):776
782.https://doi.org/10.1016/j.canrad.2016.05.014
7. Nguyen SM, Chlebik AA, Olch AJ, Wong KK. Collision risk mitigation of varian
TrueBeam linear accelerator with supplemental live-view cameras. Prac Radiat Oncol.
2019;9(1):e103-e109. https://doi.org/10.1016/j.prro.2018.07.001
8. Gupta A, Ohri N, Haffty B. Hypofractionated radiation treatment in the management of
breast cancer. Expert Rev Anticancer Ther. 2018;18(8):793-803.
https://doi.org/10.1080/14737140.2018.1489245
9. Mohan R, Jayesh K, Joshi R, Al-idrisi M, Narayanamurthy P, Majumdar SK. Dosimetric
evaluation of 120-leaf mulileaf collimator in a Varian linear accelerator with 6-MV and
18-MV photon beams. J Med Phys. 2008;33(3):114-118. https://doi.org/10.4103/0971-
6203.42757
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10. Boyer A, Biggs P, Gavin J, et al. AAPM report 72: basic applications of multileaf
collimators. Madison, WI: Medical Physics Publishing, American Association of
Physicists in Medicine; 2001.
11. Benjamini Y, Hochberg Y. Controlling the False Discovery Rate: A Practical and
Powerful Approach to Multiple Testing. Journal of the Royal Statistical Society: Series B
(Methodological). 1995;57(1):289-300. https://doi.org/10.1111/j.2517-
6161.1995.tb02031.x
17
Figures
Figure 1. Civco Horizon prone breast board for prone breast treatment immobilization.
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Figure 2. Civco Horizon prone breast board for prone breast treatment immobilization.
19
Figure 3. Civco Horizon prone breast board collision with Varian gantry head.
20
Figure 4. Civco Horizon prone breast board collision with Varian gantry head.
21
Figure 5. Philips CT (computed tomography) machine with Civco Horizon prone breast board
for prone breast simulation.
Table 1. 95% confidence intervals for medians and FDR-adjusted Wilcoxon signed rank P-
values for the mean difference are given for each OAR.
95% CI
Response Padj Lower Upper
CTV Breast D95 (cGy) 0.034* 21.5 73.5
CTV Breast Mean (cGy) 0.561 -1.5 18.5
CTV Breast D1 (cGy) 1.000 -15.5 21.0
PTV TumorBed D95 (cGy) 1.000 -16.5 18.0
PTV TumorBed Mean (cGy) 1.000 -10.0 13.5
PTV TumorBed D1 (cGy) 0.561 -5.5 24.5
CTV TumorBed D95 (cGy) 0.924 -16.5 11.5
CTV TumorBed Mean (cGy) 1.000 -10.0 10.0
CTV TumorBed D1 (cGy) 1.000 -11.0 20.0
22
GTV TumorBed D95 (cGy) 1.000 -16.0 25.0
GTV TumorBed Mean (cGy) 1.000 -10.5 16.0
GTV TumorBed D1 (cGy) 1.000 -18.5 24.0
Heart D95 (cGy) 0.561 -0.5 5.0
Heart Mean (cGy) 0.561 -1.0 7.5
Heart D1 (cGy) 0.561 -11.0 21.0
Lungs Mean (cGy) 1.000 -3.5 3.0
Lungs D1 (cGy) 0.799 -33.5 53.0
*Significantly different at the 5% level
Table 2. Isocenter locations and shifts from original plan isocenter to isocenter location within
clearance threshold.
Isocenter Location From top of PPS Isocenter Shifts Isocenter Shifts
Plan ID from Midline (cm) (cm) Medially (cm) Anteriorly (cm)
1 5.98 14.92 6 3
2 5.51 14.98 1 4.5
3 5.04 14.93 2 3
4 5 16 2 4
5 5.74 14.89 3.76 3.11
6 5.98 14.89 5.52 3.11
7 5.98 14.95 3.02 2.05
8 5.98 15.11 5.02 5.39
9 6 15 5.5 3.5
10 6 15 3 2.5
11 6 15 7 4
12 6 15 2 3
13 6 15 3 6
14 6 15 5 2.5
15 5 15 0 1.1
16 4.5 15 0 2
17 6 15 2 0
Average: 3.28 3.1
23
