Steve Axelrod, PhD Don Pettibone, PhD Rob Neimeyer Xoft, Inc., Sunnyvale CA

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. Too much Just right 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 If we had a means of attenuating like this… We would wind up with this

5. Sheath X-ray source anode Shadow dot 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 • Need to fully characterize the attenuation patterns to allow treatment planning

6. X-ray source Ion chamber Azimuthal Scan Around 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. X-ray source Ion chamber 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 Difference from reference Angle, degrees Angle, degrees

8. 1 2 3 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 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. Water tank Film layers Source Film Studies of the Shadow Dot • 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