PURPOSE

� A test protocol was designed to evaluate the delivered dose from the Axxent® System (50 kVp x-ray source) using 30 mm and 35 mm vaginal applicators in comparison with the intended dose planned with Varian BrachyVision software. The delivered dose was measured by radiochromic film in a plane parallel to the applicator axis.

METHODS

� The simulated treatment was planned with Varian BrachyVision™, using the Xoft 50 kVp source TG-43 parameters.

� The prescription dose was 7 Gy per fraction to a point 5mm from the applicator surface.

� Prototype vaginal applicators with 30 mm and 35 mm diameters were used to deliver the simulated treatment in a water phantom.

� The applicator and a 5” square of GAFChromic EBT film (International Specialty Products, Wayne, NJ) were held in a Solid Water™ frame (Gammex rmi® Middleton, WI).

� The film plane was parallel to the long axis of the applicator and offset from the axis by 5mm.

� The Axxent® system controller and miniature x-ray source operated at 50 kVp and 300 µA.

� The treatment consisted of a series of dwell times along the central axis of the applicator. Treatment time was under 7 minutes.

ANALYSIS

� The exposed film was scanned and processed to create a calibrated dose profile. An Epson® Expression 10000XL flatbed scanner (Epson America, Inc., Long Beach, CA) was used as recommended by the film manufacturer. The red portion of the 48 bit RGB image was used for the analysis.

� For this study the interest was in determining the degree to which dose along each predicted isodose contour was indeed constant; therefore an approximate film calibration was deemed satisfactory for initial analysis.

� To compare film results to the BrachyVision plan, an image processing technique was employed which provided quantitative results over the entire isodose contours. The BrachyVision isodose-line plot was transformed into an image matching the 5 inch spatial extent and 150 pixels per inch of the scanned film.

• An image of the isodose contours calculated by BrachyVision was screen captured and processed in the NIH ImageJ program. All marks other than the isodose contours were erased, and the contours were highlighted with values of 1 while the background was set to 0, effectively creating an image “mask” as shown in Figure 3.

• The isodose mask was superimposed on the calibrated film image as shown in Figure 4. This allowed for horizontal alignment of the images. Vertical alignment was set absolutely by the inner edge of the film and an arrow indictor on the isodose mask. (The calibrated film image for each size of applicator is shown separately in the Results, Figures 5 and 8.)

• The isodose mask was multiplied by the calibrated film image resulting in a new image with dose exposure values only along the planned isodose contours and zero’s elsewhere (See Results, Figures 6 and 9).

• Further processing with a computer algorithm written specifically for this purpose identified and recorded all pointsalong each contour, along with their angular position with respect to the center, creating a set of points for each contour. These points were plotted as dose versus angle (See Results, Figures 7 and 10).

� Ionization chamber data were taken during the film exposure. A

PTW model 34013 soft x-ray chamber was placed at the

prescription depth of 5mm from the surface, near the midpoint

along the shaft. Absolute dose values in three runs with each

applicator were recorded and compared with plan values.

Poster (SU-GG-T-36) presented at the American Association of Physicists in Medicine (AAPM) Annual Meeting, Houston, Texas, USA, July 2008.

Abstract Published in Medical Physics, June 2008.

RESULTS

30 mm Applicator

� Visual comparison of isodose contours

from the BrachyVision treatment plan

(Figure 3) and the calibrated film image

from treatment using the 30 mm

applicator (Figure 5) showed qualitatively

good agreement of the delivered

treatment with the plan (Figure 6).

� Further image processing quantified the

agreement by extraction of the dose

values along each isodose curve

(Figure 7).

� The approximate film calibration

employed proved reasonably accurate,

especially below 7 Gy. Average values of

dose on each isodose curve were all close

to the plan values.

� The primary focus of the study was on

variation of dose along each contour.

These were determined to be about 6%

(2 sigma) for dose contours from 2 to 9

Gy. See Table 1.

Film Based Treatment Plan Validation for a New Vaginal Applicator Using the Xoft Axxent® 50 kVp Miniature X-ray Source

S Axelrod, L Kelley, L Mantese, T Rusch, Xoft, Inc., Sunnyvale, CA

BACKGROUND: AXXENT® SYSTEM

� The Axxent® Electronic Brachytherapy (eBx) System utilizes a proprietary miniaturized X-ray source to apply radiation directly to a tumor bed within the body. The Axxent X-ray Source delivers high-dose rate, low energy radiation treatment without the use of radioactive isotopes. The Axxent Source mimics certain characteristics of the most common HDR brachytherapy isotope, 192Ir, but the low energy of the Axxent Source enables the procedure to be done in a minimally shielded setting under the supervision of a radiation oncologist.

� The Axxent® System has been used to deliver accelerated partial breast irradiation (APBI) using an inflated balloon placed into the patient’s resection cavity one to two weeks post-lumpectomy.

� Recently FDA-clearance was granted for the Axxent® eBx System in the general treatment of “lesions, tumors and conditions in or on the body where radiation is indicated”.

METHODS

CONCLUSION

Funding provided by

Figure 2. Controller for the Axxent®

Electronic Brachytherapy (eBx) System

INTRODUCTION

� Vaginal cylinders are commonly used for irradiation of the vaginal wall following hysterectomy in the treatment of early stage endometrial cancer.

� Typical vaginal cylinders for use with Ir-192 sources are fabricated from materials that have poor transmission characteristics for low energy x-rays such as those emitted by the Axxent® Model S700 X-ray Source (Xoft, Inc., Sunnyvale, CA).

� New vaginal cylinders were designed for use with the Axxent® System and have been cleared by the FDA (Figure 1). Four sizes of applicators have been fabricated with diameters of 20 mm, 25 mm, 30 mm, and 35 mm.

� The rigid cylinders are made of common medical-grade polymers with near water-equivalent characteristics and were designed to have a 94±5%

X-ray transmission with respect to water.

Figure 1. Axxent® Vaginal Applicator

� The delivered doses from the Axxent® Brachytherapy System (50 kVp x-ray source) using prototype vaginal

applicators show excellent agreement with the intended doses planned with Varian BrachyVision software.

� The delivered doses as measured by radiochromic film exposure in a plane parallel to the applicator axis were found

to be constant along plan isodose contours with standard deviations of about 5 to 6% (2 sigma). The error budget

for the system allows up to 20% 2 sigma error.

� Ionization chamber data showed that absolute dose values in three runs with each applicator were within 3% (2

sigma) of the plan values.

� During 100 cycles of simulated patient treatment, X-ray transmission has been shown to be constant to within ± 2% of this value. See Poster #SU-GG-T-43, “X-ray Transmission of Vaginal Cylinders for Use With

the Axxent® Electronic

Brachytherapy System” by H

Hausen, L Kelley, T Lovoi, L Mantese, T Rusch at the AAPM 2008 Meeting.

� One important aspect of the testing program is to run a typical treatment in a phantom and compare actual results to thetreatment plan. Film measurements provide a way to measure dose at high spatial resolution, but quantitative comparisons of measured dose to planned dose typically are restricted to choosing particular points and thus do not take advantage of the preponderance of data. The technique described here allows for comparison of measured to planned dose all along multiple predicted isodose contours.

� Unique attributes of the Axxent® System:

• No radioactive materials handling and safety issues

• Creates greater access to treatment facilities by requiring a minimally shielded room for use.

• Medical personnel can be present in room during treatment, providing patient comfort and well being.

• Self-contained unit can be wheeled from room to room

• Radiation is attenuated beyond the area under the Axxent® FlexiShield®

protecting other parts of the body.

• Radiation is emitted only during treatment when the X-ray source is turned on.

• Radiation is emitted at a low energy and high-dose rate, providing coverage to the target area and sharper dose fall-off than conventional HDR isotopes.

• Source voltage and current can be modulated, permitting Intensity Modulated Brachytherapy (Xoft IMBT™) in future applications.

Table 1. Values of Unwrapped Isodose Lines with the 30 mm Applicator

Dose Average % Sigmas

9 Gy 8.35 Gy 2.65

7 Gy 6.73 Gy 2.84

5 Gy 4.88 Gy 3.30

4 Gy 3.93 Gy 2.98

3 Gy 3.00 Gy 3.12

2 Gy 2.04 Gy 2.75

30 mm Applicator

35 mm Applicator

Figure 7. Contour values (dose) versus angle for all points on the extracted isodose contours

from treatment using the 30 mm applicator.

Figure 5. Calibrated film image from treatment using the 30 mm

applicator. False color scale is at upper right. Figure 6. Calibrated film image combined with isodose mask to extract dose values on isodose contours.

Figure 10. Contour values (dose) versus angle for all points on the extracted isodose contours

from treatment using the 35 mm applicator.

Table 2. Values of Unwrapped Isodose Lines with the 35 mm Applicator

Dose Average % Sigmas

9 Gy 8.21 Gy 2.08

7 Gy 6.55 Gy 2.48

5 Gy 4.79 Gy 2.28

4 Gy 3.88 Gy 2.32

3 Gy 2.94 Gy 2.17

2 Gy 1.99 Gy 2.16

Figure 8. Calibrated film image from treatment using the 35 mm

applicator. False color scale is at upper right.

Figure 9. Calibrated film image combined with isodose mask to

extract dose values on isodose contours.

-9.0

-0.0

-12.0

-0.4

-8.0

-0.0

-13.1

-0.4

Figure 3. Isodose image mask (created

from the BrachyVision treatment plan)

Figure 4. The isodose mask (Figure 3) was superimposed on the calibrated film image

from treatment with a 35 mm applicator.

False color scale is at upper right.

-3.5

-0.0

35 mm Applicator

� Similar results were seen with the 35 mm

applicator. Visual comparison of isodose

contours from the BrachyVision treatment

plan (Figure 3) and the calibrated film

image from treatment using the 35 mm

applicator (Figure 8) again showed

qualitatively very good agreement of the

delivered treatment with the plan (see

Figure 9).

� Further image processing quantified the

agreement (Figure 10).

� Average dose values were again close to

the plan values. Dose variation along

each contour was found to be about 5%

(2 sigma) for dose contours from 2 to 9

Gy. See Table 2.

Ionization Chamber Data

� Ionization chamber data at the

prescription depth of 5mm from the

surface showed that absolute dose values

in three runs with each applicator were

within 3% (2 sigma) of the plan values.