Direct Segment Optimization Optimizing conformal plans without IMRT Jennifer M. Steers 1,2, Martha M. Matuszak 1,2, Benedick A. Fraass 1 Departments of Radiation Oncology 1 and Nuclear Engineering & Radiological Sciences 2 University of Michigan, Ann Arbor, Michigan

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

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 What is DSO?

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

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

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

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

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) – D 95 (for PTVs) – 3D dose comparisons

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)

7-Field, Non-coplanar Plan – Brain11

Normal Brain

PTV Normal Brain Chiasm

Maximum Doses (Gy) * = Max dose to 0.5 cc; ** = Max dose to 0.1 cc PTV * Chiasm ** R Eye ** L Eye ** Normal Brain * DSO x1 Beamlets Mean Doses (Gy) PTVChiasmR EyeL Eye Normal Brain DSO x1 Beamlets Brain11 PTV D 95 (Gy) DSO x1 Beamlets 56.2

Plan Comparison Brain16 Brain11 MU/FxSegments Estimated beam-on time/Fx (min) DSO x1 Beamlets

1x1 beamlet planDSO plan IMRT vs. DSO Brain11

1x1 beamlet planDSO plan IMRT vs. DSO Brain11

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

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

HeartCord Esophagus PTV

HeartCord Esophagus PTV

Maximum Doses (Gy) * = Max dose to 0.5 cc PTV * Cord * Esophagus * Heart * Lung-P2 * DSO x1 Beamlets Mean Doses (Gy) PTVCordEsophagusHeartLung-P2 DSO x1 Beamlets Lung2-123 PTV D 95 (Gy) DSO x1 Beamlets 80.7

Plan Comparison Lung2-123 Lung2-123 MU/FxSegments Estimated beam-on time/Fx (min) DSO x1 Beamlets

5-field Axial plan – Liver4

Cord Normal Liver PTV

Cord Normal Liver PTV

Cord Normal Liver PTV

Maximum Doses (Gy) * = Max dose to 0.5 cc PTV * Normal Liver * R Kidney * L Kidney * Cord * DSO – 5 Segments DSO – 7 Segments x1 Beamlets Mean Doses (Gy) PTV Normal Liver R KidneyL KidneyCord DSO – 5 Segments DSO – 7 Segments x1 Beamlets Liver4 PTV D 95 (Gy) DSO86.5 1x1 Beamlets88.5

Plan Comparison Liver4 MU/FxSegments Estimated beam-on time/Fx (min) DSO – 5 segments DSO – 7 segments x1 Beamlets Liver4

1x1 beamlet planDSO plan IMRT vs. DSO

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

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)

Questions?