dosimetric effects of air pockets around high–dose rate brachytherapy vaginal cylinders
TRANSCRIPT
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Int. J. Radiation Oncology Biol. Phys., Vol. 78, No. 1, pp. 276–279, 2010Copyright � 2010 Elsevier Inc.
Printed in the USA. All rights reserved0360-3016/$–see front matter
jrobp.2009.11.004
doi:10.1016/j.iPHYSICS CONTRIBUTION
DOSIMETRIC EFFECTS OF AIR POCKETS AROUND HIGH–DOSE RATEBRACHYTHERAPY VAGINAL CYLINDERS
SUSAN RICHARDSON, PH.D., GEETHPRIYA PALANISWAAMY, PH.D., AND PERRY W. GRIGSBY, M.D.
Department of Radiation Oncology, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, MO
Reprindiation OSchool ofFax: (314
Purpose: Most physicians use a single-channel vaginal cylinder for postoperative endometrial cancer brachy-therapy. Recent published data have identified air pockets between the vaginal cylinders and the vaginalmucosa. The purpose of this research was to evaluate the incidence, size, and dosimetric effects of these airpockets.Methods and Materials: 25 patients receiving postoperative vaginal cuff brachytherapy with a high–dose ratevaginal cylinders were enrolled in this prospective data collection study. Patients were treated with 6 fractionsof 200 to 400 cGy per fraction prescribed at 5 mm depth. Computed tomography simulation for brachytherapytreatment planning was performed for each fraction. The quantity, volume, and dosimetric impact of the airpockets surrounding the cylinder were quantified.Results: In 25 patients, a total of 90 air pockets were present in 150 procedures (60%). Five patients had no airpockets present during any of their treatments. The average number of air pockets per patient was 3.6, with theaverage total air pocket volume being 0.34 cm3 (range, 0.01–1.32 cm3). The average dose reduction to the vaginalmucosa at the air pocket was 27% (range, 9–58%). Ten patients had no air pockets on their first fraction but airpockets occurred in subsequent fractions.Conclusion: Air pockets between high–dose rate vaginal cylinder applicators and the vaginal mucosa are present inthe majority of fractions of therapy, and their presence varies from patient to patient and fraction to fraction. Theexistence of air pockets results in reduced radiation dose to the vaginal mucosa. � 2010 Elsevier Inc.
Endometrial cancer, Gynecologic brachytherapy, High–dose rate, Vaginal cylinder.
INTRODUCTION
The most common cancer of the female reproductive system
in the United States is endometrial cancer. Along with sur-
gery, vaginal cuff brachytherapy plays a major role in the
treatment of this disease. According to the 2005 survey of
the American Brachytherapy Society, 90.6% of participants
who use high–dose rate brachytherapy in their management
of endometrial cancer use a standard segmented cylinder as
the applicator (1). Although it may be considered to be the
standard applicator for vaginal vault irradiation, it is not with-
out significant limitations. It has been established that the
1 mm of tissue surrounding the vaginal cylinder is the pre-
dominant location of the vaginal lymphatic channels (2).
To deliver appropriate dose to these submucosal lymphatic
channels, the treatment device must be in direct contact
with the vaginal surface (3). Delivering a uniform dose to
the vaginal mucosa or surface of the applicator requires com-
plex optimization (4). Cameron et al. determined that 32% of
the patients in their study had an air pocket larger than 2 mm,
with the median number of air pockets per patient being
t requests to: Susan Richardson, Ph.D., Department of Ra-ncology, Campus Box 8224, Washington UniversityMedicine, St. Louis, MO 63110. Tel: (314) 286-2640;
) 362-8521; E-mail: [email protected]
276
1 (range, 0–5) (5). These pockets can result in an underdose
of the target that could potentially lead to relapse.
In 2000, the American Brachytherapy Society recommen-
ded that single-fraction planning was acceptable as long as
‘‘the geometry of the implant remains the same for every
insertion’’(3). However, a paradigm shift is emerging in bra-
chytherapy whereby all treatments are becoming image
guided and all fractions are imaged regardless of the implant
type; consequently, reoptimization should be done. Hoskins
et al. investigated the effect of the cylinder tilt on the sensitive
structure volumes and suggested simulation of the applicator
position before each fraction (6). Symon et al. studied the ef-
fects of individualized fraction optimization vs. first fraction
optimization for a multichannel vaginal cuff applicator and
concluded that individualized fraction optimization was
important to minimize doses to critical structures (7). One
important limitation of that study, expressed by the authors,
was that they did not study the vaginal surface dose. In
response to that publication, an editorial by Dr. Small con-
cluded that ‘‘the need for Individualized Fraction
Conflict of interest: none.Received Aug 28, 2009, and in revised form Nov 6, 2009.
Accepted for publication Nov 9, 2009.
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Dosimetric effects of air pockets d S. RICHARDSON et al. 277
Optimization for single channel vaginal cylinders is yet to be
determined ’’(8). We hope to investigate both authors’ ques-
tions by determining the impact of air pockets on the vaginal
surface dose conformity on a per-fraction basis for a single
channel applicator.
Significant air pockets around vaginal cylinders in patients
have been observed in our clinic. For example, the patient
shown in Fig. 1 had good applicator conformance at the
introitus of the vagina, but at the vaginal apex there was bal-
looning of the anatomy, which resulted in air surrounding the
cylinder. The purpose of this research was to evaluate the
incidence, size, and dosimetric effects of these air pockets.
Fig. 1. Axial (above) and coronal (below) CT images of the vaginawith a cylinder inserted. It demonstrates a small air pocket and pointdose calculation of the dose to the lymphatic channel. The distancewas measured from the surface of the cylinder to the point of max-imum displacement. The dose was then determined at that point andcompared with the surface dose of the cylinder.
Number of gaps over 6 fraction treatment
4
5
6
atien
ts
METHODS AND MATERIALS
The study population consisted of 25 consecutive patients with
pathologically proven carcinoma of the endometrium referred to
the Department of Radiation Oncology, Mallinckrodt Institute of
Radiology. The Washington University Human Research Protection
Office approved this prospective dosimetric analysis, and all pa-
tients signed informed consent.
Initially, each patient had a pelvic examination under anesthesia,
and three gold markers were placed at the vaginal cuff. The vaginal
vault size was determined upon examination, and a snugly fit vagi-
nal cylinder was inserted. Each patient received pelvic computed to-
mography simulation for each treatment fraction on a Philips
Brilliance computed tomography (CT) scanner (Philips Medical,
Chesterfield, MO) with 3-mm slice thickness. Patients were treated
using a standard segmented cylinder applicator with a Varian Varis-
ource 200 Iridium-192 high dose afterloader (Varian Medical Sys-
tems, Inc. Palo Alto, CA). Twenty-four patients were treated with
a 2.5-cm diameter cylinder, and one was treated with a 3.0-cm diam-
eter cylinder. The dose was prescribed to 5-mm depth for the cranial
3 to 5 cm of the vagina (9). The prescriptions ranged from 200 to 400
cGy per fraction (depending on the clinical indications) for six
fractions total. Seventeen patients also received external pelvic irra-
diation.
Each patient was imaged for each treatment fraction (6 per pa-
tient). Using BrachyVision 8.1, each treatment plan was retrospec-
tively evaluated for the presence of air pockets in the cranial 5 cm
of the vagina. The air pockets were contoured in the axial plane,
and the volumes were calculated by the treatment planning system.
The average number of air pockets, average air pocket volume, and
maximum displacement of the vaginal mucosa were recorded per
patient fraction. The dose ratio between the vaginal mucosa dis-
placed by the air pocket and the surface dose of the cylinder was cal-
culated by the treatment planning system as demonstrated in Fig. 1.
The doses were calculated using the TG 43 formalism.
0
1
2
3
0 1 2 3 4 5 6 7 >8
Number of Gaps
Nu
mb
er o
f P
Fig. 2. Number of air pockets experienced by the number of pa-tients over the course of treatment (six fractions). Five patientshad 0 pockets, and 20 patients had at least one pocket. The maxi-mum number of air pockets for a single patient was 14.
RESULTS
All treatment plans showed the cylinder abutting the vag-
inal cuff and the gold markers. All treatment plans were also
observed for cylinder conformance at the introitus, effec-
tively verifying that the appropriate cylinder diameter was se-
lected for patient treatment. Of 25 patients, 20 (80%) had one
or more air pockets present in the upper vagina in at least one
of their six treatment fractions, 5 of 25 (20%) had no air
pockets present in any of their treatment fractions. The total
number of air pockets found throughout treatment for all
25 patients was 90. The frequency of air pockets per patient
is shown in Fig. 2. There was an average of 3.6 pockets over
the course of treatment, with the average total pocket volume
being 0.34 cm3 (range, 0.01–1.32 cm3). Twenty-one of the
pockets were located in the apex, and 69 were located lateral
to the cylinder surface but within the upper 5 cm of the va-
gina. The average distance the mucosa was displaced was
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Fig. 3. Persistent air pocket for 1 patient. Each image displayed is from a new fraction and new insertion of the cylinder.The first fraction CT image is shown in the upper left, and the sixth fraction in the bottom right. Arrows indicate air in thevaginal canal.
278 I. J. Radiation Oncology d Biology d Physics Volume 78, Number 1, 2010
3.7 mm (range, 1.3–8 mm). Out of 90 total pockets, 80
(88.8%) displaced the vaginal mucosa away from the cylin-
der surface by a distance of 2 mm or greater. The average
dose reduction to the vaginal mucosa was 27% (range,
9–58%). Ten patients (40%) did not have any air pockets
present when scanned for their first fraction but air pockets
subsequently occurred during the course of treatment. Ten
patients had air pockets on the first fraction.
Some patients had air pockets in the same anatomic loca-
tion from fraction to fraction. Twelve pockets were in the
same anatomic location for two fractions, four were the
same for three fractions, and three were present for four frac-
tions. An example of a patient who had a single air pocket in
the same area for four fractions of her treatment is shown
in Figure 3. For this patient, the dose received by the mucosa
in that region for each fraction was as follows: 100%, 78%,
77%, 58%, 59%, and 100%, for an average of 79% of the pre-
scribed dose. If the patient had been imaged for only the first
fraction, the air pockets would have been undetected.
DISCUSSION
Each patient’s vaginal anatomy is unique. Patient age, par-
ity, hormonal status, and surgical technique of closure of the
vagina after hysterectomy can result in variations in the size
and shape of the vaginal apex. The size of the vaginal cuff can
vary by several centimeters, and the shape can vary from con-
ical to balloon-shaped. Frequently the vaginal introitus is
smaller in diameter than the vaginal apex. One solution
when air pockets in a vaginal cylinder patient treatment are
observed is to simply increase the size of the vaginal cylinder
being used. However, this may not result in conformance of
the vaginal surface to the applicator because the patient’s va-
gina may not be perfectly cylindrical in nature after surgery
and may even have a ‘‘dog-ear’’ configuration. Additionally,
increasing the cylinder diameter increases the total integral
dose to the patient, which can increase toxicity. Finally, the
larger cylinder may not be comfortable or tolerable to the pa-
tient. An alternative approach would be to have a more flex-
ible and variable-size applicator that is individualized to the
patient’s anatomy. One existing solution is the use of ovoids
for vaginas with a ‘‘dog-eared’’ configuration. Another op-
tion is a custom mold as described by Potter et al. (9). For
this reason, we are developing a flexible patient-specific
treatment device that is more comfortable for the patient
and can adapt to the patient’s anatomy.
Vaginal relapse rates in postoperative endometrial carci-
noma patients receiving vaginal vault brachytherapy have
been found to be as high as 18% (10), although other pub-
lished rates of treatment failure in the vaginal cuff are 5%
or less (11–13). One possible cause of the relapse is that
the target volume does not receive the prescribed dose be-
cause of the presence of the air pockets. Additionally,
many institutions do not image their patients on a frac-
tion-by-fraction basis, therefore the severity and magnitude
of these air pockets are widely undetermined. Though the
total dose required to eradicate the disease is unknown
(14), the American Brachytherapy Society recommends
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Dosimetric effects of air pockets d S. RICHARDSON et al. 279
maintaining contact between the vaginal mucosa and the
applicator.
Ten patients (40%) did not have an air pocket present on
their first brachytherapy fraction but did have an air pocket
present on at least one subsequent fraction. This indicates
that scanning only the first fraction of a patient’s treatment
is insufficient for determining vaginal mucosa conformality
to the cylinder throughout the course of treatment. Yapar-
palvi et al. (15) found that interfraction changes in the cylin-
der insertion (and also fluctuations in bladder and rectal
volumes) resulted in varying bladder and rectal doses; they
recommended assessing the dose to organs at risk on an indi-
vidual fraction basis. Therefore, to obtain an accurate and
complete geometric and dosimetric assessment of the pa-
tient’s treatment, the patient should be imaged for every treat-
ment fraction.
It has been suggested that the locations of air pockets are
probably different at each treatment, diluting the overall dosi-
metric effect (5). We have found this not to be the case with all
patients. In our study, there were 12 instances of air pockets
that occurred in the same location during multiple fractions
over the course of treatment. In 1 patient, the average dose
to this region was less than 80% of the prescribed dose.
CONCLUSIONS
Out of 25 patients in this study, 90 air pockets were present
in 150 procedures (60%). The average dose reduction to the
vaginal mucosa was 27% (range, 9–58%). Considering that
standard segmented cylinders typically only come in discrete
sizes, this may limit the ability of the mucosa to maintain
direct contact with the applicator. The use of custom applica-
tors could possibly reduce the number of air pockets and im-
prove the dosimetric coverage of the mucosa.
REFERENCES
1. Small JW, Erickson B, Kwakwa F. American BrachytherapySociety survey regarding practice patterns of postoperative irra-diation for endometrial cancer: Current status of vaginal brachy-therapy. Int J Radiat Oncol Biol Physics 2005;63:1502–1507.
2. Choo JJ, Scudiere J, Bitterman P, et al. Vaginal lymphatic chan-nel location and its implication for intracavitary brachytherapyradiation treatment. Brachytherapy 2005;4:236–240.
3. Nag S, Erickson B, Parikh S, et al. The American Brachyther-apy Society recommendations for high-dose-rate brachytherapyfor carcinoma of the endometrium. Int J Radiat Oncol Biol Phys2000;48:779–790.
4. Li S, Aref I, Walker E, et al. Effects of prescription depth, cyl-inder size, treatment length, tip space, and curved end on dosesin high-dose-rate vaginal brachytherapy. Int J Radiat Oncol BiolPhys 2007;67:1268–1277.
5. Cameron AL, Cornes P, Al-Booz H. Brachytherapy in endome-trial cancer: Quantification of air pockets around a vaginal cyl-inder. Brachytherapy 2008;7:355–358.
6. Hoskins P, Bownes P, Summers A. The Influence of applicatorangle on dosimetry in vaginal vault brachytherapy. Br J Radiol2002;75:234–237.
7. Symon Z, Menhel J, Alezra D, et al. Individual fraction optimi-zation vs. first fraction optimization for multichannel applicatorvaginal cuff high-dose-rate brachytherapy. Brachytherapy2006;5:211–215.
8. Small W Jr.. To plan or not to plan? That is the question. Bra-chytherapy 2006;5:216–217.
9. Potter R, Gerbaulet A, Haie-Meder C. The GEC ESTROhandbook of brachytherapy. Endometrial Cancer 2002;15:365–401.
10. Knocke TH, Kucera H, Weidinger B, et al. Primary treatment ofendometrial carcinoma with high-dose-rate brachytherapy: Re-sults of 12 years of experience with 280 patients. Int J RadiatOncol Biol Phys 1997;37:359–365.
11. Horowitz NS, Peters WA 3rd, Smith MR, et al. Adjuvant highdose rate vaginal brachytherapy as treatment of stage I and II en-dometrial carcinoma. Obstet Gynecol 2002;99:235–240.
12. MacLeod C, Fowler A, Duval P, et al. High-dose-rate brachy-therapy alone post-hysterectomy for endometrial cancer. Int JRadiat Oncol Biol Phys 1998;42:1033–1039.
13. Alektiar KM, Venkatraman E, Barakat RR. Intravaginal brachy-therapy alone for intermediate-risk endometrial cancer. Int J Ra-diat Oncol Biol Phys 2005;62:111–117.
14. Sorbe B, Straumits A, Karlsson L. Intravaginal high-dose-ratebrachytherapy for stage I endometrial cancer: A randomizedstudy of two dose-per-fraction levels. Int J Radiat Oncol BiolPhys 2005;62:1385–1389.
15. Yaparpalvi R, Mutyala S, Thawani N, et al. Variance in bladderand rectal doses in the course of fractionated cylinder high-dose-rate brachytherapy. Brachytherapy 2009;8:145.