Direct Segment Optimization Optimizing conformal plans without IMRT

Jennifer M. SteersJennifer M. Steers1,21,2 , Martha M. Matuszak , Martha M. Matuszak1,21,2 , Benedick A. Fraass , Benedick A. Fraass11

Departments of Radiation OncologyDepartments of Radiation Oncology11 and and

Nuclear Engineering & Radiological SciencesNuclear Engineering & Radiological Sciences22

University of Michigan, Ann Arbor, MichiganUniversity 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?

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)

– D95 (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 63.5 15.1 1.0 1.0 63.4

1x1 Beamlets 69.2 17.9 1.0 1.0 62.9

Mean Doses (Gy)

PTV Chiasm R Eye L EyeNormal Brain

DSO 59.67 8.52 0.59 0.73 25.60

1x1 Beamlets 59.71 8.53 0.60 0.63 24.56

Brain11

PTV D95 (Gy)

DSO 56.1

1x1 Beamlets 56.2

Plan Comparison Brain16

Brain11

MU/Fx SegmentsEstimated beam-on

time/Fx (min)

DSO 237 7 0.4

1x1 Beamlets 845 351 6.6

1x1 beamlet plan DSO plan

IMRT vs. DSO

Brain11

1x1 beamlet plan DSO plan

IMRT vs. DSO

Brain11

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)

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

Heart Cord

Esophagus

PTV

Heart Cord

Esophagus

PTV

Maximum Doses (Gy) * = Max dose to 0.5 cc

PTV* Cord* Esophagus* Heart* Lung-P2*

DSO 90.4 44.0 83.6 88.1 89.3

1x1 Beamlets 89.4 35.4 82.8 88.9 91.5

Mean Doses (Gy)

PTV Cord Esophagus Heart Lung-P2

DSO 84.2 3.7 14.0 14.0 11.1

1x1 Beamlets 84.4 5.0 15.1 15.8 12.6

Lung2-123

PTV D95 (Gy)

DSO 78.5

1x1 Beamlets 80.7

Plan ComparisonLung2-123

Lung2-123

MU/Fx SegmentsEstimated beam-on

time/Fx (min)

DSO 492 14 0.8

1x1 Beamlets 680 300 5.3

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 94.2 92.9 2.0 1.3 15.5

DSO – 7 Segments 92.7 92.9 1.8 1.4 12.8

1x1 Beamlets 92.2 95.1 1.5 1.0 11.3

Mean Doses (Gy)

PTVNormal

LiverR Kidney L Kidney Cord

DSO – 5 Segments 89.9 29.4 1.1 0.7 2.1

DSO – 7 Segments 90.1 28.3 1.1 0.7 2.0

1x1 Beamlets 90.0 25.6 0.2 0.3 2.5

Liver4

PTV D95 (Gy)

DSO 86.5

1x1 Beamlets 88.5

Plan Comparison Liver4

MU/Fx SegmentsEstimated beam-on

time/Fx (min)

DSO – 5 segments

257 5 0.43

DSO – 7 segments

267 7 0.45

1x1 Beamlets 586 303 5.24

Liver4

Liver4

1x1 beamlet plan DSO 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?