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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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…

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

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

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

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

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

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