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.i

PHYSICS 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: srichardson@radonc.wustl.edu

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.

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

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

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.

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