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Direct Segment Optimization Optimizing conformal plans without IMRT
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  1. Direct Segment Optimization Optimizing conformal plans without IMRT Jennifer M. Steers1,2 , Martha M. Matuszak1,2 , Benedick A. Fraass1 Departments of Radiation Oncology1 and Nuclear Engineering & Radiological Sciences2 University of Michigan, Ann Arbor, Michigan

  2. Outline • Introduction • What is DSO? • Why are in we interested in DSO? • Goal • Methods and Materials • Results • IMRT vs. DSO comparisons • Conclusions

  3. What is DSO? • Direct segment optimization (DSO) • form of direct aperture optimization (DAO) but based on flat fields, not on beamlet distribution • Utilizes user-defined cost functions to optimize the following: • Beam weights • MLC positions

  4. What is DSO?

  5. Why are we interested in DSO? • Plans are optimized conformal plans • Can reduce delivery time over IMRT • Can result in fewer MUs when compared to IMRT • Plans do not require IMRT QA - plans could be started or adapted much quicker • It may make tweaking and optimizing leaf positions in conformal plans, such as SBRT, much quicker

  6. Goal • Can DSO produce simpler plans comparable in quality to IMRT plans with the same beam angles and cost function?

  7. Methods and Materials:Features of DSO • MLCs and beam weights can be optimized separately or together • Search strategy options • Ordered and random searches • Step sizes • DSO offers fewer degrees of freedom per beam when compared to IMRT • User must create segments in a plan before optimizing

  8. IMRT vs. DSO Setup • Planning goal: Minimize dose to OARs and normal tissues without compromising target uniformity • Same gantry angles were used between the IMRT and DSO plans except when needed segments were added to the DSO case • The same cost function was used to optimize both the IMRT and DSO plans

  9. Evaluation of Comparisons • Both plans were optimized and evaluated with the following metrics • DVHs • MU/fx and beam-on time/fx • Mean doses to structures • Max structure doses (to 0.5cc or 0.1 cc) • D95 (for PTVs) • 3D dose comparisons

  10. Brain Planning Goals • Target : 60 Gy (Min: 59, max: 61) • Normal brain: minimize dose (threshold 0, power 2) • Optic structures: minimize dose (threshold 0, power 2)

  11. 7-Field, Non-coplanar Plan – Brain11

  12. Normal Brain

  13. PTV Normal Brain Chiasm

  14. Brain11

  15. Plan Comparison Brain16 Brain11

  16. IMRT vs. DSO 1x1 beamlet plan DSO plan Brain11

  17. IMRT vs. DSO 1x1 beamlet plan DSO plan Brain11

  18. Lung Planning Goals • Using adaptive protocol (2007-123) • PTV: 85 Gy (Min: 84 Gy, max: 86 Gy) • Esophagus: NTCP < 47% (Veff = 33%) • Heart: NTCP < 5% • Normal Lung: NTCP < 17.2% • All normal structures: minimize dose (threshold 0, power 2)

  19. 7-field, Non-coplanar Plan – Lung2-123

  20. PTV Esophagus Heart Cord

  21. PTV Esophagus Heart Cord

  22. Lung2-123

  23. Plan ComparisonLung2-123 Lung2-123

  24. 5-field Axial plan – Liver4

  25. PTV Normal Liver Cord

  26. PTV Normal Liver Cord

  27. PTV Normal Liver Cord

  28. Liver4

  29. Plan Comparison Liver4 Liver4

  30. IMRT vs. DSO 1x1 beamlet plan DSO plan Liver4

  31. Conclusions • DSO can successfully optimize several types of planning scenarios while reducing treatment time • Optimizing with DSO can produce plans comparable in quality to IMRT with less MUs and segments • No need for the type of QA associated with IMRT plans • Overall plan complexity is less

  32. Conclusions • In the future: • DSO may be more time efficient while yielding similar quality plans as IMRT in an adaptive re-planning scenario • Since DSO produces flat-field plans, it may be useful for cases with inter- and intra-fraction motion • Looking at other sites that could benefit from twiddling (prostate, SBRT, pediatric)

  33. Questions?