Patient Set-ups and Tumor Localizations

Transcription

1 Patient Set-ups and Tumor Localizations Amy S. Harrison Patient Positioning Prior to starting any localization or simulation procedure patients need to be positioned and immobilized Patients disease location and physical limitations must be taken into consideration

2 Patient Alignment Reproducible positioning of the entire patient, not just the treatment region is imperative A small angle change of the patient on the table can represent a significant change in the delivered treatment Exaggerated Patient Position Shift

3 Patient Positioning-Brain/H+N Immobilized by aquaplast masks over the head alone/ or head and shoulders The need for a bite block should be addressed Head holders should be selected for patient comfort and extension of neck Indexing of immobilization improves reproducibility of set up Head Immobilization

4 Head Immobilization Full Set Up Photo H+N Case

5 Lung Alpha cradle Arms up? Arms down? ABC device Abdominal Compression Device Leg immobilization-rubber band, plastic foot holder, angle sponge? Will it clear CT/MRI bore Stereotactic Lung

6 Lung Lung

7 Breast Immobilization Alpha cradle or wing board Opposite arm immobilized how? ABC device Leg immobilization-rubber band, plastic foot holder, angle sponge? Will it clear CT/MRI bore Breast

8 Breast Breast

9 Breast Prostate/Pelvis Pelvis-full aquaplast, alpha cradle or nothing Legs on angle sponge or flat Feet rubber bands or foot holder Arms?

10 Prostate/Pelvis Prostate/Pelvis

11 Prone Rectum Belly board, angle sponge? Feet? Patient on pillow or not? Rectum/Prone

12 Prone Rectum Prone Rectum

13 Immobilization vs. High Tech IGRT- cone beam or fiducials Tomotherapy Cyber Knife ExacTrac Localization Procedure where the target and critical structures are delineated with reference to the patient s external surface

14 Classical Localization The patient s external surface was attained by using solder wire or plaster of paris This surface was then drawn on a piece of paper Or pantograph 2D Contours

15 Classical Localization Tumor and critical structures were transcribed from hard copy CT studies or MD demarcation on the orthogonal films taken at the time of localization 2D Contours

16 2D Contours 2D Contours

17 2D Contours and Coordinates Simulation A procedure where the planned fields are verified by shooting diagnostic quality films in the simulator (a machine which mimics the geometry of the treatment unit))

18 The CT Simulator The advent of the CTsim dramatically modified the simulation and localization procedures. Localization could now be done at the time of Ctsim Target delineation was truly 3D Precision and accuracy greatly improved The Ct Sim For Patient Marking An integral functionality of the CT Simulator unit is the capability of placing reference marks on the patient to indicate the isocenter for the treatment fields. 1) Reference marks can be placed near the isocenter of the patient. This is an estimate of the final isocenter and does not require extensive contouring. The isocenter is found through a series of exact moves from the reference marks. 2) After detailed contouring, the final isocenter position is marked on the patient.

19 XIO FocalSim 3D Contours and Coordinates

20 3D Contours and Coordinates 3D CT Images Scanned = Transverse/ Axial Generated= Sagittal Generated= Coronal

21 Virtual Simulations Ct Sims allowed the verification simulation process to become virtual Traditional 2D fields could be set in the 3D dataset The patient could be scanned, marked and sent home given an appointment time for treatment XIO Focalsim

22 Advantages of the CT Simulator Virtual simulation/verification process Generating a new/conedown plan can be accomplished without having to bring the patient back for another simulation. Disadvantages of the CT Simulator Structure motion cannot easily be detected with a CT Simulator. Excluding 4D scanners. CT doughnut is usually restricted to 70-80cm in diameter.this can limit the patient's position for some treatments. For example, placing the patient's arms up can be a problem.

23 CT Simulators CT scan process allows 3D volumetric information to be gathered and carries out simulation as a digital process. Relies on construction of digitally reconstructed radiographs QA of CT-Simulators

24 HISTORICALLY: QA of CT Scanners CT scans for treatment planning are often done with a flat top insert on the CT table to reproduce the radiation therapy treatment couch top. laser system mimicking that used on the simulation and treatment units should be mounted in the CT suite and the alignment of the lasers should be checked daily. Such a system is an integral component for relating the patient s position during CT with that on the simulation and treatment machines. The correlation of CT numbers with electron densities and the variation of CT numbers with position and phantom size should be determined. Since this correlation is a function of the quality of the x-ray beam, it should be checked yearly. In addition, the CT scanner should be checked for image quality and other parameters described in the QA protocol provided by the manufacturer. QA of CT scanners (AAPM, 1977) Quality assurance for computed-tomography simulators and the computed tomographysimulation process: Report of the AAPM Radiation Therapy Committee Task Group No. 66

25 AAPM Task Group #66 Mechanicals Common Sense Applies +/- 2mm most items Table indexing and motions are 1mm

26 Spatial integrity QA goals: CT-simulation images should accurately reproduce true patient anatomy within 1 mm without spatial distortions in the entire scan field. This should be verified for both head and body scan protocols using a phantom of known dimensions. Spatial resolution Characterizes the imaging system s ability to distinguish between two very small objects placed closely together. Spatial resolution is frequently referred to as high contrast resolution

27 High contrast resolution most commonly measured using either a resolution pattern ~line pair phantom with a range of spatial frequencies!, or by the modulation transfer function ~MTF! method. The line pair pattern in following slide ranges in frequency from 1 lp/cm to 21 lp/cm. Note the Bead in the phantom phantom which is a high-density, tungsten carbide bead which is used to create an impulse, or point source, from which the MTF can be calculated. Manufacturers often specify the limiting spatial resolution at the 5% or lower point on the MTF curve. The limiting spatial resolution ~lp/cm measured with MTF, and specified at the 5%value, is typically higher than the resolution that can be observed with a line pair phantom. CT Scanner Line Pairs Slide Images from

28 MTF Plots the contrast against the resolution Completely characterizes the high-contrast resolution of the scan mode Slide Images from High Contrast Low Contrast

29 Contrast resolution Contrast resolution can be defined as the CTscanner s ability to distinguish relatively large objects which differ only slightly in density from background. QA goals: Quality assurance should demonstrate that the CT-scanner meets or exceeds manufacturer specifications for low contrast resolution Sensitivity and Profiles Slice sensitivity is a curve showing the effect of broadening of the CT slice Thickness along patient in helical CT Slide Images from

30 Image Performance When referencing manufacturers specifications tolerances are set to the acceptance criteria and can then be called the baseline measurement CT Sim Software: Image input test Structure delineation (contouring) Multimodality image registration Machine definition Isocenter calculation and movement Image reconstruction Evaluation of digitally reconstructed radiographs

31 Evaluation of digitally reconstructed radiographs Spatial and contrast resolution: It is generally understood that smaller slice thickness and spacing produces better spatial resolution DRRs. Geometric and spatial accuracy: Magnification should be within 1 mm of expected. Spatial errors ~e.g., collimator, table rotation, incorrect jaw setting, etc.! can also cause errors which may not be detected from patient port films. The QA for the CTsimulation process should include evaluation of DRR geometric errors. Hardcopy quality: Printing of standard test patterns and comparison with baseline data can reveal potential problems EVALUATION OF THE CT-SIMULATION PROCESS Overall process tests: Patient positioning and immobilization, Scan limits, Scan protocol, Contrast, Special considerations and instructions, Data acquisition, Localization/marking, Virtual simulation, DRR and setup documentation

32 DRR Digitally constructed images from the 3D dataset Generated with the same geometry as a divergent radiograph produced with a point source of radiation. Created by combining the influence of the CT pixel elements from a CT dataset along divergent ray lines. DRR Tools Computer generated films allow selection of the bony anatomy needing to be imaged DRR s can be adjusted by using a window/leveling tool Drr s can be generated for any treatment angle there are no collision issues in virtual space

33 DRR Region of Interest DRR Quality # of Slices CT # accuracy Slice thickness Scan technique used Reconstruction algorithm

34 DRR Artifacts Contrast Agents Prosthesis Respiratory Motion Anatomy (inadequate scan technique) Scanner Types First Generation: Translate/Rotate Second Generation: Translate/Rotate Third Generation: Rotate/Rotate Fourth Generation: Rotate/Fixed Spiral CT (3rd or 4th generation type) Cine Ct

35 Retrieved from

36 Single Slice Spiral CT Pitch Pitch = (table increment distance (mm) per 360 gantry rotation) / slice thickness (mm) slice thickness 5mm, table motion 7.5mm/rotation, Pitch = 1.5 Pitch of 1=adjacent rotations Pitch>1 = gaps between x-ray beams from adjacent rotations Multi Slice Spiral CT Beam Pitch = (table increment distance (mm) per 360 gantry rotation) / slice thickness (mm) X n (number of slices acquired) slice thickness 5mm, on a 4 slice scanner, table motion 15 mm/rotation Beam pitch = 15 / 4 x 5 =.75

37 Slice Sensitivity Image Reconstruction Iterative Solvers-slower but better when missing data Analytic Methods-Fourier Analysis Filtered back projection Can begin with first data acquired Can be hard wired into system (speed)

38 Image Reconstruction Ramp Filters

39 Cone Beam CT Planar images are acquired with the kv or MV imaging system. Volumetric image reconstruction is performed Houndsfield Units The relative attenuation coefficient ( ) is usually expressed in HU aka CT numbers HU= 1000 x ( x - water )/ water where x is the attenuation coefficient of material x and water is the attenuation coefficient of water

40 HU vs CT CT Numbers are based on manufacturer constant K CT = K x ( x - water )/ water where x is the attenuation coefficient of material x and water is the attenuation coefficient of water Houndsfield Units

41 Window and Level Lung Windows

42 Soft Tissue Windows Relation of FOV, Matrix Size and Pixels

43 Beam Hardening Barrett J F, Keat N Radiographics 2004;24: Figure 15b. CT images of a patient with metal spine implants, reconstructed without any correction (a) and with metal artifact reduction (b) by Radiological Society of North America

44 Partial Volume Effects Motion

45 Typical Doses Magnetic Resonance Imaging Study of the magnetic properties of the nucleus Nuclei under a strong magnetic field absorb energy which is then released at a later time This time period is unique to the nuclei and surrounding area T1 and T2 are time values

46 T1 Images T2 Images

47 MRI: The Pros and Cons Pros: Cons: 1. Better soft tissue imaging 2. Multiplane imaging 3. Data unaffected by bones 1. Image distortion 2. No electron density information-cannot be used for dose calculation w/o CT fusion Positron Emission Tomography Functional images: provides information about physiology instead of anatomy Generates transverse images depicting the distribution of positron emitting nuclides MUST be fused with CT images for treatment planning

48 PET Continued When positron annihilates it emits two 511keV photons in nearly opposite directions; these photons interact with the annihilation coincidence detectors and obtain projections of the activity distributed in the patient Image Fusion 4 Techniques 1. Coordinate transformation Fiducial markers/stereotactic frames 2. Surfaced based registration The surfaces of one or more structures are matched and used for computation and minimizing mismatch of the data set. Useful with skull or pelvis.

49 Image Fusion Continued 3. Image Based Registration Grayscale data is used directly to measure mismatch or similarity between datasets (Mutual Informationmeasurement of redundant data) 4. Interactive Techniques Effective in cases with a limited number of degrees of freedom. Verified visually. Can be used to limit the amount of time needed for calculation based fusion. Fusion Once the datasets are fused structures may be mapped from one dataset to another So target volumes may be delineated on an MRI or PET and transferred to CT data for planning

50 MRI/CT Mutual Information MRI/CT Grayscale Visual Check

51 PET/CT Pre-Fusion PET/Ct Post Fusion

52 What is the fourth dimension? Time and therefore motion 4D CT Scan Measures Lung Cancer Motion

53 4D CT scan GE Lightspeed with Varian RPM system captures repeat CT images at each couch position during respiratory cycle CT sample interval images/slice position Pan et al, Med Phys 31, 333 (2004); Med Phys 34, 4499 (2007) 4D thoracic CT imaging Vedam et al PMB 2003

54 What use are 4D CT scans? Determine tumor motion/screening Motion inclusive treatment Respiratory gated treatment 4D radiotherapy All video images on 4d treatment techniques are curtesy of Paul Keall 4D CT in radiotherapy Scenario 1: No respiratory motion management devices

55 Inhale & exhale CT phases Tumor Motion encompassing volume Tumor Exhale Inhale Motion inclusive treatment

56 Scenario 2: Respiratory gating Acquire 4D CT Select respiratory phase(s) Delineate GTV/CTV on chosen phase(s) Create PTV Plan and treat with gating Gating tumor tumor tumor Beam ON Beam OFF Beam ON

57 Respiratory gated treatment Accuray Works in Progress 4D Radiotherapy Dynamic MLC motion to match target motion Dynamic table motion

58 Things to Chew Over Dynamic delivery will require planning of each phase of respiration What will the QA of the delivery devices look like 2010 question- What if the patient sneezes?!? 2011 answer- 4D conebeam D CBCT Elekta XVI 4.5 Symmetry Slow gantry motions about 3 minutes for a 200 degree rotation Software auto correlates data by surface or internal motions Motion induced blur of structures reduced Streaking artifacts common-more visible in axial images Have had patients not treated due variations in respirations usually caused by coughing from illness-returned next day with cough suppressant

59 Sonke et al, Med Phys 32, 1176 (2005) 4D-CBCT Streaking Streaking artifacts can be Reduced with slower gantry Rotations = increased times Li & Xing, IJROPBP 67, 1211, 2007

60 4D CBCT Pre-Treatment Verification Thank you so much for your time and consideration Good luck on all your future physics endeavors