1. An Enabling Technology for Creating Sculpted Brachytherapy Dose Patterns With The Xoft Axxent™ System
Steve Axelrod, PhD
Don Pettibone, PhD
Rob Neimeyer
Xoft, Inc., Sunnyvale CA

2. Background Xoft 50 kVp source spectrum amenable to attenuation
Selective attenuation allows for sculpting dose pattern
Spare healthy or sensitive tissue
Brachytherapy introduces challenges
TG43 based planning does not accommodate arbitrary asymmetry, nor varying beam quality
Stepping through multiple dwell points complicates things
Not as easy a thing to do as IMRT

3. Sample Dose Sculpting Application - APBI
Accelerated Partial Breast Irradiation performed with balloon inserted into lumpectomy cavity
Prescription is 3.4 Gy per fraction at 1 cm for 10 fractions
When balloon to skin distance is less than 10 mm, skin receives more than the prescription dose
Clinical effects have been seen when distance is ~< 7 mm

4. The Basic Idea of Sculpted Brachytherapy
Position an attenuator so as to lower dose in a specific region
Simple model of balloon and breast contours
Spacing between balloon and breast
Just right
Too much
If we had a means of attenuating like this…
We would wind up with this

5. Simplest Concept – “Shadow Dot”
Single dot of absorbing material placed on an insertable sheath
Sheath is inside the central balloon lumen
Consider a dwell point directly opposite the dot
Sheath
X-ray source anode
Shadow dot
Need to fully characterize the attenuation patterns to allow treatment planning

6. Azimuthal Scan Around Source
Ion chamber
X-ray source
PTW miniature ion chamber
Measurement 2 cm from source
~4 mm silver dot, 0.001” thick, ~3 mm from source center
Anode ~ 1+ mm in extent

7. Polar Measurements with 0.001” Ag Dot
Scans with dot placed at various Z positions relative to source
-3 mm to +4 mm shifts, ion chamber at 4 cm
Normalized as per TG43 on left
Differences from baseline on right
Ion chamber
X-ray source
Difference from reference
Angle, degrees
Angle, degrees

8. Beam Quality Considerations
50 kVp beam is hardened by attenuation
Especially problematic for TG43 treatment planning
But we work in an interesting area of the x-ray spectrum 1 3 2
Spectra:
1. Raw (measured)
2. After 1 cm water
3. After 1 HVL Silver

9. Predicted Radial Dose Functions
Radial Dose Functions in water
Calculated using mass-energy absorption curves and measured spectrum
Following HVL’s of several different materials, after 1 cm water
Silver and Molybdenum show much less beam hardening than other materials
In practice Silver works better than moly

10. Azimuthal Measurements at Multiple Distances
Scans at 2, 3, 4, 5, 6 and 7 cm from source
When ion chamber distance is varied…
Depth-dose changes only slightly – as expected
Shape remains ~constant
Difference from average, %
Angle, degrees

11. Film Studies of the Shadow Dot
Film layers
Source
Water tank
Provides high spatial precision for characterization
Films placed at 1, 3 and 5 cm from source
Exposed with and without dot, single dwell point
Films scanned and processed/calibrated
Difference image created (with dot minus without dot)
1 cm Image with Dot
Subtracted Image
Line plot along cursor

12. Film at 3 and 5 cm From Source
Effect gets harder to pull out of the noise at 5 cm, but attenuation fraction stays fairly steady
3 cm
5 cm

13. Film With Three Dwell Points
Cannot directly measure multi-dwell effect with ion chamber setup
Off-axis dwell points “fill in” and diminish attenuation
Need to plan for this effect
Minor ripples created from off-axis dwell points
Image with Dot
Subtracted Image
Line plot along cursor
Films at 1 cm shown here

14. MatLab Simulations of Breast Treatment
TG43-based TPS type calculations
Modeled attenuation using trapezoidal “shield functions”
40% attenuation
Gray area is the PTV
Upper area has a thinner “skin bridge”
Shadow dot has shifted isodose lines by 2-3 mm
Lower skin dose

15. Treatment Simulations with a Tracking Shield
What if the shadow dot moved as the source stepped?
Always in an optimal position w.r.t the source
Larger shift of isodose lines
Retains smooth behavior
Better conformality than a stronger attenuator

16. Challenges to Realization
Treatment planning is beyond current capabilities
Use traditional TPS, with post-planning “shadow functions”
Not an ideal work flow
Create a dedicated TPS that incorporates the physics
Verification
How to locate and verify action of shadow dot prior to treatment
Easy in a film fixture, but harder in a patient
Promising ideas are being pursued…

17. Laser Dot Finder for Pre-Treatment Verification
Simple concept to locate the shadow dot
Replace source with bright light
Measure light penetrating balloon and breast with sensor
Adjust sheath and dot until light is minimized

18. Got Bremsstrahlung?
Please stop by the Xoft booth
Brem Boy is back!

19. Bonus Slides
Like there's any way any time is left!

20. Polar Measurements with Dot Rotated
Sheath holding dot is rotated in 15° steps out of scan plane
0, 15 and 30° scans very similar
No attenuation at 45° and above
Difference from reference
Angle, degrees

21. Polar Plots of the Polar Data
Z displacements of dot, of 5, 2 and 0 mm
Narrowing of attenuation is evident when 5 mm off axis
5 mm offset
2 mm offset
0 mm offset

22. Variable Attenuation via Moving Dots
With motion capability, one can select “on” and “off” time
By extracting the dot for part of the dwell
At each dwell point, the dot is in shadowing position for a different fraction of the time
Allows further control over shape of the isodose lines