Op#miza#on of an On- Board Imaging System for Rapid Radiotherapy. Erica Kemmerling, Meng Wu, He Yang, and Rebecca Fahrig

Transcription

1 Op#miza#on of an On- Board Imaging System for Rapid Radiotherapy Erica Kemmerling, Meng Wu, He Yang, and Rebecca Fahrig

2 Problem: mo#on during treatment Two overlapping thorax views during normal breathing liver lung spine lung K.K. Brock, Imaging and Image- Guided Radia7on Therapy in Liver Cancer, 2011

3 Fix: PHASER PHASER = Pluridirec#onal High- Energy Agile Scanning Electron Radiotherapy Basic idea: deliver en#re radia#on treatment in a few seconds to effec#vely freeze mo#on Mechanism: use high- energy electrons, which can be steered electronically (no moving parts) 100 MeV electrons can deliver a dose with similar accuracy to 2 MeV photons (mean energy)

4 PHASER Setup

5 Why do we need on- board imaging? The pa#ent changes between treatment planning and treatment delivery (interfrac#on mo#on/ deforma#on) Must align pa#ent and treatment plan CT system works best for this Fast imaging Hardware won t interfere with the accelerator hardware Q: What kind of CT system should we use? A: Whatever allows us to accurately register the treatment plan to the pa#ent s current posi#on

6 Work Flow Acquire helical scan data: high quality, 40 cm axial FOV ReconstrucEon RegistraEon, choosing plan, verifying choice Deliver therapy Not shown: DiagnosEc- quality CT taken with clinical system for treatment planning axial scan and tumor posieon verificaeon: FAST, 10 cm axial FOV

7 CT Detector design op#miza#on Limi#ng our imaging system to include ONE custom part will reduce overall cost Detector is the most expensive part of the system Possible detector designs Low resolu#on: 48 rows, all 4x4 mm pixels Medium resolu#on: 64 rows, 16 4x4 mm pixels x2 mm pixels x4 mm pixels (beber resolu#on in center, use only high- quality sec#on for helical scan) High resolu#on: 192 rows, all 1x1 mm pixels

8 Op#miza#on Procedure Make three numerical anthropomorphic phantoms with realis#c deforma#ons Generate realis#c CT data for each imaging step Register helical data to clinical data and axial data to helical data Evaluate goodness of registra#on to determine which detector designs meet image quality requirements

9 Phantoms Pa#ent on day of clinical scan Pa#ent on day of treatment Different breathing phase Full stomach Full bladder Sample phantom difference image axial slices Bowel has moved to accommodate organ expansion Lungs and liver have deformed

10 Liver Lesions Ques#on: Can we accurately register small liver lesions with deforma#on? Spherical liver lesions of different densi#es in original phantom Spherical liver lesions in deformed phantom (axial distance between top and bobom slices is 2 cm)

11 Primary Simula#on Sohware generates cone beam projec#on images, includes tools for 3D volume reconstruc#on, DOES NOT include scaber X- ray abenua#on, one energy, one material I(L) = I 0 exp( µl) X- ray abenua#on, polyenerge#c, several materials E max I(x tot ) = I 0,E E min n exp( µie L i )de i=1 Polychroma#c X- ray source Abenua#on caused by passage through material Detector captures abenuated rays

12 Scaber Simula#on Monte Carlo simula#on accounts for random physical processes responsible for scabering MC- GPU open- source scaber simula#on sohware Scaber can be reduced with an an#- scaber grid Source 2D an#- scaber grid, Philips Curved detector with an#- scaber grid

13 Sample Data Registration ROI Phase 1 Diagnostic Registration Registration Phase 2 Helical Low Resolution Medium Resolution High Resolution Phase 3 Axial (on-axis) Registration Registration Registration Phase 3 Axial (off-axis)

14 Gold Standard for Registra#on We want to evaluate the imaging system, not the registra#on algorithm We don t have the ground truth deforma#on field anyway Solu#on: Convert different imaging phase phantoms to HU Register them using the same workflow Compare gold standard registra#on deforma#on field with deforma#on fields

15 Diagnos#c- Quality/Helical Registra#on Gold Standard Registration Low Resolution Medium Resolution High Resolution Deformation field scale: -15 mm 20 mm 20 mm Deformation error scale: -2 mm 2 mm 2 mm

16 Helical/Axial Registra#on Gold Standard Registration Low Resolution Medium Resolution High Resolution Deformation field scale: -3 mm 3 mm 5 mm Deformation error scale: -0.4 mm 0.4 mm 0.7 mm

17 Global Registra#on Results Distances are between real data registra#on deforma#on field and gold standard data registra#on deforma#on field D a,b (x, y, z) = (a x (x, y, z) b x (x, y, z)) 2 + (a y (x, y, z) b y (x, y, z)) 2 + (a z (x, y, z) b z (x, y, z)) 2 Detector DiagnosEc/helical registraeon mean pointwise distance (mm) Helical/axial registraeon mean pointwise distance (mm) Low Resolu#on Medium Resolu#on High Resolu#on

18 Liver Lesion Registra#on Results Distances are between real data registra#on deforma#on field and gold standard data registra#on deforma#on field Detector DiagnosEc/helical registraeon mean pointwise distance (mm) Helical/axial registraeon mean pointwise distance (mm) Low Resolu#on Medium Resolu#on High Resolu#on

19 Conclusions Low resolu#on detector is worse than the other two by preby much every metric Medium and high resolu#on detectors are virtually interchangeable Use of medium resolu#on detector is recommended because it is much more economical than high resolu#on detector

20 Goals for Future Work Inves#gate possible dose reduc#on in imaging system Develop algorithm for fast treatment plan adapta#on Build custom CT and integrate it with other PHASER hardware

21 Acknowledgements Thanks to Meng Wu and He Yang for their work on this project Thanks to the Fahrig group and the PHASER team Thanks to SCIT for suppor#ng me Thanks to the Weston Havens Founda#on, the Department of Radia#on Oncology, the School of Medicine, and the Office of the Provost of Stanford University for suppor#ng PHASER

22 Thank you, RSL! New job: faculty posi#on at Tuhs University New address: When you look me up, don t forget that my name is Erica Kemmerling, not Erica Cherry!!

23 Ques#ons?

24 EXTRA SLIDES

25 Custom CT Parameter Summary Parameter Clinical Scanner On board CBCT Custom scanner Source to detector distance (mm) 1040 (too small can t shih pa#ent) Typical scan #me (sec) In- plane field of view (mm) (too slow pa#ent must breathe) (too small trunca#on ar#facts) 10 helical, 1 axial 500 Maximum axial field of view at isocenter (mm) 32 (too small can t acquire data quickly enough)

26 Scaber- to- primary ra#o (SPR) No ane- scauer grid 1- cm- tall 2D ane- scauer grid SPR 0 2