1. Study Comparing Five Rectal Immobilization Devices on Intra-
Fraction Motion for Prostate Proton Therapy
Rachel Rendall, B.S. R.T.(T), Robert Foster, M.S. CMD,
Megan Dunn, PhD, MSHS
CDH Proton Center – Warrenville, IL, USA
Purpose
Intra-fraction motion during prostate radiation therapy
treatments can affect the relative position of the treatment
volume and dose given to normal tissue structures. Maintaining a
reproducible setup and internal stabilization can be achieved
with variations in organ filling, especially in the rectum. To
better evaluate the effect of rectum stabilization on prostate
mobility, a study was completed comparing our current rectal
immobilization with commercial and in-house made rectal
immobilization.
Materials and Methods
Data was collected from 95 prostate cancer patients treated with
proton therapy. All patients had three fiducial markers placed in
the prostate to aid in target alignment using daily flat portal
imaging. Following fiducial placement, each patient was
simulated in the supine position, with either a commercial water
filled rectal balloon (90cc, 110cc, or 120cc), a 120cc in-house
design water filled rectal balloon, or our current immobilization,
free water (100cc), inserted in the rectum approximately 10cm,
using a catheter.
Treatment plans were developed to deliver either 79.2CGE in 44
fractions or 38CGE in 5 fractions for low risk prostate cancer,
70CGE in 28 fractions for intermediate risk prostate cancer, and
50.4CGE in 28 fractions for high risk prostate cancer. Low risk
planning target volume (PTV) was generated by expanding the
clinical target volume (CTV) with a margin of 3mm radially with
the exception of a 2mm posterior margin. Intermediate risk PTV
margin were generated by expanding the CTV 4mm radially with
the exception of a 3mm posterior margin. High risk PTV was
generated combining the prostate (4mm radially, 3mm posterior)
and the nodes (7mm radially).
Endorectal balloons used in the study: A – Radiadyne® Immobiloc, B – Qfix Pro-
Tekt™, C – In-house design balloon.
Two orthogonal portal images
were taken daily to match the
fiducial markers within a 1mm
margin for treatment. After
proton beam treatment
delivery, a second set of
orthogonal portal images were
taken. These post-treatment
images were taken during the
first five or ten fractions
(dependent on diagnosis) and
then weekly.
Portal images showing fidicial motion
from treatment position (left) to post
treatment position (right).
If the fiducial markers were displaced from the original
position, a positional magnitude was calculated to determine
an intra-fractional fiducial motion, using offsets from three
axes: lateral, anterior-posterior, and craniocaudal.
After treatment completion, an average vector magnitude was
calculated for each patient and each rectal immobilization
device was compared to assess intra-fractional movements of
the target volume. Physician assessments were recorded at
treatment completion and at three and six month follow-ups to
track any adverse side effects for both the bladder and
rectum.
Results
The mean magnitude ( in mm) for the 90cc rectal balloon (n=34)
was 1.10 (SD=1.33). The mean magnitude for the 110cc rectal
balloon (n=11) was 0.78 (SD=1.08). The mean magnitude for the
120cc rectal balloon (n=29) was 0.96 (SD=1.23). The mean
magnitude for the 120cc in-house rectal balloon (n=5) was 2.29
(SD=1.46). The mean magnitude for the 100cc free rectal water
(n=16) was 1.43 (SD=1.43). An
Graph 1: Mean Magnitude for each patient. The corresponding color line represents
the Magnitude average for the rectal immobilization device (C – Commercial
balloon, HM – In-House balloon, WF – free rectal water).
ANOVA was performed to test
the differences in the mean
magnitude among the five
groups. This yielded statistically
significant results (p < .05). A
second ANOVA was performed to
compare the differences in
mean magnitude among the
three commercially available
rectal balloons and free water,
which also yielded statistically
Graph 2: Mean Magnitude per rectal
immobilization device, plus standard
deviation (C – Commercial balloon, HM
– In-House balloon, WF – free rectal
water).
significant results (p < .05). The in-house designed balloon was
removed from the study after the first five patients showed
poor results. There was no significant difference in acute
bladder complications, the most frequently reported being
grade 1 urinary frequency (n=37) and grade 1 urinary urgency
(n=37), both of which improved at the 6-month follow up (n=18
and n=15, respectively). Acute rectum complications were
rarely seen in all patient groups.
Conclusion
Intra-fraction motion is an important consideration for prostate
treatments. Based on our study, commercial rectal balloon
devices are more effective internal immobilization devices
compared to free water inserted into the rectum. All three
volumes studied reduced intra-fraction motion, improved target
localization, and did not result in any significant acute toxicity
to the patient. Analysis of the immobilization devices along
with the knowledge of our treatment margins gives us
confidence that our treatments are robust against typical intra-
fraction movements. With the findings of this study, we want to
stress the importance of rectum immobilization in prostate
proton therapy, as target volume stabilization and appropriate
set-up margins reduce dose given to normal tissue structures
while achieving reproducible target coverage.
Table 1: Acute toxity