Anthropomorphic Verification of the Eclipse AAA Algorithm for Spine SBRT Treatments Involving Titanium Implants

Transcription

1 Anthropomorphic Verification of the Eclipse AAA Algorithm for Spine SBRT Treatments Involving Titanium Implants Mike Grams Clinical Medical Physicist Mayo Clinic, Rochester, MN NCCAAPM Meeting, April 17,

2 Background Spine SBRT treatments are technically challenging High doses in a single fraction Very steep dose gradients

3 1400 cgy/cm

4 Background Spine SBRT treatments are technically challenging High doses in a single fraction Very steep dose gradients Stringent test of the entire treatment process! Imaging Treatment planning Machine/IMRT QA Treatment delivery

5 Background So as if this weren t already hard enough

6 Background The presence of metal introduces additional complexity What do we do about the artifacts? What HU values should we assign to the metal? Standard or Extended CT Scale? How well does the treatment planning system calculate dose around a big chunk of metal?

7 The goals of this study Recreate as accurately as possible a typical spine SBRT treatment with hardware Use an actual human spine with surgically implanted titanium hardware Measure a planar dose distribution within the spinal column and compare to TPS calculations

8 Insert sheet of Gafchromic film here

9 Phantom Design The original plan was to use beeswax for the phantom material Nearly tissue equivalent Solid, reproducible, easy to handle

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11 Phantom Design Just use a water tank already! Need to build spine holders

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13 40 cm Width = 32 cm 21 cm

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15 Treatment planning I tried to duplicate the main features of a typical spine SBRT treatment in our clinic and used Eclipse AAA 3 Full Arcs High dose PTV Low dose PTV Maximum sparing of spinal cord Used 24 Gy in 1 fx with a 6 MV FFF beam

16 PTV Low PTV High Placement of Gafchromic film

17 Location of dose profiles shown later

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19 Contouring Original images

20 Contouring Auto contour hardware but subtract 1 mm

21 Contouring Add 1 mm margin and override to water

22 Contouring Clean up the remaining artifact, override to water

23 What about the hardware itself? Use either Standard or Extended CT Scales for treatment planning Standard Scale HU = to 3071 HU override Extended Scale HU = -10,000 to 30,710 No override necessary

24 Treatment planning scenarios Standard CT Scale Contour hardware and override so RED = 3.73, contour artifact and override such that RED = 1 Extended CT Scale Contour hardware but do NOT override the RED, contour artifact and override such that RED = 1 No Artifact Correction ( Do Nothing Approach ) Do not contour hardware or artifact, do not override anything, use the standard CT scale

25 Film Measurements Films analyzed using FilmQA Pro software Made with EBT-XD Gafchromic film Relatively new film (Jan 2015) Built for high doses and SBRT/SRS applications

26 The QA Process In order to mimic a clinical scenario, each plan underwent IMRT QA in a homogeneous phantom

27 The QA Process Used only the standard CT scale for QA measurements All treatment plans were evaluated via gamma passing rates with a 2%/2 mm criterion The T spine plan with hardware passed at 99.5% and the L spine plan without hardware at 99.9%

28 QA T Spine 2%/2 mm

29 QA L Spine 2%/2 mm

30 Treatment delivery Position spine with CBCT and 6 DOF couch Deliver each plan 3 times to assess repeatability of measurements

31 Film Measurements T spine with metal

32 Film Measurements L spine without metal

33 L Spine Measurements (no metal) Use Standard and Extended CT scale No artifacts or hardware to contour Gamma passing rates (2%/2mm): 99.3%-99.7%

34 Standard CT Scale Extended CT Scale Average Measurement Dose (cgy) Position (mm)

35 T Spine (with metal) Standard CT Scale Scenario Contour metal, assign it HU = 5585 (RED = 3.73) Contour the streak artifact and assign it HU = 0 (RED = 1) Gamma passing rates (2%/2mm): 99.1% 99.0% 99.3%

36 Standard CT Scale Average Measurement Dose (cgy) Position (mm)

37 T Spine (with metal) Extended CT Scale Scenario Contour metal, do not override Contour the streak artifact and assign it HU = 0 (RED = 1) Gamma passing rates (2%/2mm): 99.3% 98.6% 98.9%

38 Standard CT Scale Extended CT Scale Average Measurement Dose (cgy) Position (mm)

39 T Spine (with metal) No Artifact Correction ( Do nothing approach ) Do not contour metal, do not override Do not contour the streak artifact, do not override Gamma passing rates (2%/2mm): 99.3% 98.6% 99.0%

40 Standard CT Scale Extended CT Scale No Artifact Correction Average Measurement Dose (cgy) Position (mm)

41 What about a less forgiving beam arrangement?

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43 200 Dose (cgy) Head of Screw Standard CT Scale Extended CT Scale No Artifact Correction Measurement Tip of Screw Position (mm)

44 Conclusions Eclipse AAA does a reasonably good job handling titanium Standard and extended CT scales give similar results Photons and VMAT are forgiving

45 Acknowledgement We would like to acknowledge the noble generosity of the whole body donors associated with Mayo Clinic s Anatomical Bequest program whose altruistic gift made this study possible

46 Acknowledgements ROREC Small Grants Program Terry Regnier Tyler King Michelle Clarke, MD Luis Fong de los Santos, PhD Deb Brinkmann, PhD John Antolak, PhD TJ Whitaker, PhD Ken Olivier, MD Sean Park, MD, PhD

47 Thanks

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49 Dose (cgy) Standard CT Scale Extended CT Scale No Artifact Correction Measurement Acuros Position (mm)

50 T Spine Flat Beam Dose (cgy) Standard CT Scale Extended CT Scale Acuros Average Measurement Position (mm)

51 3000 T Spine FFF Dose (cgy) Position (mm) Standard CT Scale Extended CT Scale Acuros Average Measurement

52 Dose (cgy) Position (mm) Standard CT Scale Extended CT Scale No Artifact Correction Average Measurement Extended CT Scale 0.1 cm grid

53 Dose (cgy) Extended CT Scale Average Measurement Position (mm) Extended CT Scale 0.1 cm grid

54 Treatment delivery Position spine with CBCT and 6 DOF couch

55 Why didn t you just use a pig?

56 Why didn t you just use a pig? Because pigs aren t human! Because no one likes boneless porkchops! Because no one wants to cut up a pig for you! Because no one wants to store cut up pig parts for you! Because our surgeon didn t want to put borrowed hardware in a pig!

57 Other options? $14,750