1. Stereotactic Radiosurgery (SRS)
王怡振(Yizhen Wang), Medical Physicist
Mississauga Halton/Central West Regional Cancer Program
Trillium Health Partners

2. Definition of Stereotactic Radiosurgery (SRS)
“A single high dose of radiation, stereotactically directed to an intra-cranial region of interest. May be from X-ray, gamma ray, protons or heavy particles.”
Lars Leksell, 1951
High dose of “ablative” radiation delivered to a target localized in 3-dimensions with overall end to end precision on the order of 1-2 mm delivered over 1-5 treatments
- Present

3. The Evolution of SRS
Courtesy of Dr. Timothy D. Solberg et al. Historical Development of Stereotactic Ablative Radiotherapy

4. The invention of Stereotactic technique
Stereotaxis: Method of locating points in brain using an external 3D frame.
Concept originated with Robert Clarke (engineer, physiologist and surgeon) in 1895.
First device built by Clarke and Victor Horsley (neurosurgeon) in 1905.
First experiment in 1906.
Fig a. original frame by Clarke and Horsley
Fig c. Leksell frame
Courtesy of Dr. Timothy D. Solberg et al. Historical Development of Stereotactic Ablative Radiotherapy

5. The invention of stereotactic radiosurgery (SRS)
1947, Ernest Speigel and Henry Wycis performed first successful human cranial stereotactic surgery. (Temple university, Philladelphia)
1949, Lars Leksell built his first stereotactic frame.
1951, Leksell would use his frame to target narrow radiation beams.
1955, Leksell and Bjore Larsson treated SRS patients using 280 KV X-ray. (Uppsala, Sweden)
Late 1950s and 1960s, Uppsala group, Berkeley group (CA) and Cambridge group (MA) used proton facilities for physics research to treat SRS patients.
1967, Leksell, Larsson, Lidén and Walstam built “Gamma Knife I”.
1980, Leksell and Jernberg developed CT compatible frame

6. Gamma Knife Development
Gamma Knife I:
Co sources
-rectangle collimator
U Gamma knife:
Co sources
- Cone collimator
Model B:
- Simpler source change
Model 4C:
-Allow 3D planning
- More efficient helmet change
Courtesy of Dr. Timothy D. Solberg et al. Historical Development of Stereotactic Ablative Radiotherapy

7. Gamma Knife Development

8. Leksell Gamma Knife Perfexion
• 8 independent sectors.
24 60Co sources/each sector
• 4, 8, and 16 mm collimators can
be combined within each sector. Automatic collimator adjustment.

9. Linac Based SRS Techniques- Early Development
1982, Derechinsky and Betti. Buenos Aris, Argentina

10. Linac Based SRS Techniques- Early Development

11. Linac Based SRS Techniques- Early Development
1986, McGill SRS system. Montreal, QC

12. Modern Dedicated SRS Machines - Cyberknife
110 beam angles

13. Modern Dedicated SRS Machines - Novalis

14. Modern Dedicated SRS Machines - Varian EDGE
Flattening Filter Free (FFF) mode up to 2400 MU/min
HD-MLC
VisionRT for patient setup and real time tracking
PerfectpitchTM couch

15. Comparison of Various Techniques
Gamma Knife:
- high accuracy (submillimeter)
- for cranial disease only
- cone collimation better for circular target
- source change & radiation safety concern
Cyberknife:
- SRS/SBRT
- cone collimation (MLC under development)
- long treatment time
Linac:
- Deliver all types of radiation treatment
- capable of conforming to all target shapes (cone & MLC)
- less cost
- accuracy at a few millimeters.

16. Clinical Aspects of SRS

17. What SRS Treats
Malignant tumor: Brain metastases, Glioma, Glioblastoma (GBM), Astrocytoma, Oligodendroglioma
Benign tumor: meningioma, schwannoma, pituitary adenoma, acoustic neuroma,
Functional disorder or other benign conditions: trigeminal neuralgia, arteriovenous malformations (AVM), Epilepsy, Parkinson’s Disease

18. Brain Metastases Most commonly grey/white junction
80% in cerebrum, 15% in cerebellum, 5% in brain stem
Multi sites more often.

19. The Sources (Primary Cancers) of Brain Metastases
Chart created based on Memorial Sloan-Kettering study

20. Clinical Indication for Brain Mets
RTOG 90-05, as an example:
- ≤ 2.0 cm, 24 Gy
cm, 18 Gy
cm, 15 Gy
Less dose -> higher local recurrence
Higher dose -> more toxicities (nausea, vomiting, dizziness, seizure, headaches)

21. Outcome of SRS
Studies from RTOG (95-08), EORTC ( ) and MD Anderson reveal:
Radiation can reduce the risk of recurrence
SRS produce similar results as surgery+WBRT in terms of local control
WBRT provide better control on distant intracranial recurrence compared to SRS
SRS has much better neurocognitive outcome compared to WBRT
SRS/WBRT has little impact on total survival rate

22. Technique Requirements for SRS

23. 1. Reliable patient immobilization
Headrings
Relocatable frame based masks

24. 2. Accurate target localization
CT localizer: Z = ?

25. 2. Accurate target localization
CT:
Good Geometry integrity,
e- density information,
but limited soft tissue contrast.
Good for treatment planning
MRI:
Geometry distortion, no e- density information, but excellent soft tissue contrast.
Good for target delineation

26. 2. Accurate target localization
Target positioner

27. 3. Dose conformal to target.

28. 3. Dose conformal to target.
Micro multileaf collimators (Micro MLC):
Attached to or built-in a Linac
- While projecting at isocenter, 14 leaf pairs of 3 mm width, 6 leaf pairs of 4.5 mm width, and 6 leaf pairs of 5.5 mm width.
Can conform to any shape

29. 3. Dose conformal to target.
Conical cone collimator:
Better for spherical or oval targets
Sharp penumbra

30. Safety events involving radiosurgery
Courtesy of Dr. Kelly Younge

31. SRS involves extremely small fields and very high doses
Very little opportunity to correct mistakes
Some incidents have been highly publicized:
Radiosurgery involves extremely small fields and very high doses compared to standard 3D conformal treatments. Because of the way these treatments are delivered, there is very little opportunity to make corrections if things go wrong. Some of you may have heard of or seen the series of articles published in the New York Times about radiation accidents. This article was specific to radiosurgery, and described in gruesome detail what happened to three different patients who were treated incorrectly with SRS.

32. Incorrect calibration of accelerator output
77 patients in Florida, 145 patients in Toulouse, France, 152 patients in Springfield, MO
25-100% overdoses in these patients
Lesson: use the right detector, and use more than one
This is unfortunately not an uncommon mistake, and many patients have gotten unintended overdoses. The lesson that we can learn from this is to use the right kind of detector, and to use more than one in case we make a mistake with one.

33. Backup jaws set incorrectly
A Pinpoint Beam Strays Invisibly, Harming Instead of Healing, W. Bogdanich and K. Rebelo, NYT, Dec. 28, 2010
These patients received an SRS dose to a significant area of normal brain
Multiple people were treated this way before the mistake was realized
Another mistake that has happened is having the backup jaws set incorrectly. This is the mount that secures the cone to the gantry. (Show mount) The mount slides in to the accessory tray. The issue with this is that it covers up the light field, so it is impossible to tell if the jaws are in the right place by looking at the patient. If the jaws are open past the edge of the cone, then that radiation will leak out and hit the patient.

34. Commissioning of SRS
Dosimetry (small field dosimetry very challenging)
Mechanicals
Imaging
IGRT
Margin design
Secondary dosimetry check system
End-2-end tests
External audit

35. Commissioning: Dosimetry
PDDs, profiles, output factors

36. Commissioning: Dosimetry
Multiple detectors to ensure accuracy:
Stereotactic Diode or Electron Diode
Micro ionization
chamber
Edge Diode
Film
Diamond dosimeter
Liquid ion chamber 36

37. Commissioning: Dosimetry
Detector selection:
Need to select dosimeters with high resolution
Use at least two types of detectors to verify with each other
Ion chambers: volume ≤ ~0.01 cc (cc01, A16)
Diodes: unshielded diodes (SRS diodes, electron diodes)
Radiochromic film: Handling process dependent, better for verification only.

38. Commissioning: Dosimetry
PDD measurement:
Follow TPS’ requirement (e.g. depth > 30cm, F.S. covers clinical need), verify 10x10 PDD
Can use micro IC at vertical orientation (chamber parallel to beam)
Diodes

39. Commissioning: Dosimetry
Profile measurement:
Air-filled ion chamber too large
Use diodes
Verify with Film

40. Commissioning: Dosimetry

41. IAEA/AAPM Working Group’s Recommendation (and upcoming TG-155*):
Commissioning: Dosimetry
IAEA/AAPM Working Group’s Recommendation (and upcoming TG-155*):
Reference dosimetry:
Corrects for differences between the conditions of field size, geometry, phantom material, and beam quality
Output factor:
Accounts for detector response difference
Alfonso, et al. "A new formalism for reference dosimetry of small and non-standard fields,"Med Phys 35, (2008)
*TG-155: Small fields and non-equilibrium condition photon beam dosimetry

42. Commissioning: Dosimetry

43. Commissioning: Dosimetry

44. Commissioning: Dosimetry
Absolute dose machine calibration:
Use standard ion chamber
Calibrated at reference field size (not always 10x10).

45. Commissioning: Dosimetry
Dosimetry verification of TPS configured
Deliver a set of fields and/or plans and perform measurements. Measurement and calculation should agree.

46. Commissioning: Mechanicals
Linac mechanical/radiation isocenter: ≤1mm
Couch position accuracy: ≤1mm
Collimator position accuracy:
- cone alignment
- MLC position accuracy (fence test)
Laser alignment: Winston Lutz (WL) test (<1mm)

47. Commissioning: Mechanicals
Winston Lutz (WL) test (cone or MLC)

48. Commissioning: Imaging
CT imaging:
- good geometry integrity
- for reference image, simulation, planning,
- prefer thin slice(<), axial scan, with contrast
MRI imaging:
- good soft tissue contrast
- use for target delineation
- need T1, T2, Axial, Sagittal, Coronal, 3D T1 with contrast most useful
Angiogram:
- orthogonal X-rays, for Arteriovenous Malformation (AVM, 动静脉畸形)

49. Commissioning: Imaging
CT imaging

50. Commissioning: Imaging
MRI imaging

51. Commissioning: Imaging
MRI imaging: T1 weighted, before and after contrast

52. Commissioning: Imaging
MRI imaging: T2 weighted,

53. Commissioning: Imaging
CT/MRI image fusion

54. Commissioning: Imaging
Angiogram
Angiogram/CT registration

55. Commissioning: IGRT Dedicated IGRT system Cone beam CT (CBCT)
On board orthobonal X-rays
Need to determine the accuracy of the imaging system

56. Commissioning: IGRT Dedicated IGRT system Mounted on floor and ceiling
Submillimeter accuracy
Realtime imaging at any couch angle
ExacTrac®

57. Commissioning: IGRT
CBCT/OBI
Accuracy up to 1 mm
Only at couch = 0

58. Commissioning: Margin Design
Need to include:
Size of mechanical and radiation isocenters
CT/MRI slice thickness
Image registration
Contouring accuracy
Patient setup accuracy (IGRT accuracy if imaging used as primary target positioning)

59. Commissioning: Secondary dosimetry check system
Commercial monitor unit (MU) calculation software
Hand MU calculation table or spreadsheet (not for IMRT or VMAT)
Patient specific QA measurement (IMRT or Vmat)

60. Commissioning: End-to-end tests
Localization/positioning end-to-end test: hidden target
Curtesy: Dr. Kelly Younge

61. Commissioning: End-to-end tests
Dosimetry end-to-end test:
follow clinical procedure using clinical mode and R&V system
measuring with film (plus ion chamber if possible)
Curtesy: Dr. Kelly Younge 61

62. Commissioning: External Audit
Perform external audit and/or invite external physicist with experience to review the program, if possible.
SRS phantom from IROC, MD Anderson, Houston 62

63. SRS Planning and Evaluation
Techniques available:
Non-coplaner arcs
McGill dynamic arc
Static beams
IMRT
Vmat

64. SRS Planning and Evaluation
Cone planning:
Sharp penumbra but can be tricky
on GK, CK and maybe Linac, prefer spherical target

65. SRS Planning and Evaluation
Conformal index:
CI= Rx dose Volume/ PTV, Range: 0 - ∞, ideally 1. but,
Paddick Index: Range: 0 – 1, ideally 1
RTOG wants CI <2, dose homegeneity index, DHI = Max Dose/Rx dose <2. For most cases better indices are achievable.

66. Quality Assurance of SRS Program
Equipment QA:
Daily QA
Winston-Lutz test (mechanical / rad isocenter)
Verification imaging isocenter
Monthly
Winston-Lutz
Couch position
Annual
End-to-end test (including Localization/dosimetry)
Patient/Process QA: Apply checklist

67. Quality Assurance of SRS Program: Daily QA
From AAPM TG-142

68. Quality Assurance of SRS Program: Daily QA
From Astro white paper, “Quality and safety consideration in SRS/SBRT”

69. Quality Assurance of SRS Program: Daily QA
From Astro white paper, “Quality and safety consideration in SRS/SBRT”

70. Quality Assurance of SRS Program: Monthly QA
From AAPM TG-142 70

71. Quality Assurance of SRS Program: Monthly QA
From Astro white paper, “Quality and safety consideration in SRS/SBRT” 71

72. Quality Assurance of SRS Program: Annual QA
From AAPM TG-142 72

73. Quality Assurance of SRS Program: Annual QA
From Astro white paper, “Quality and safety consideration in SRS/SBRT” 73

74. Quality Assurance of SRS Program: Patient QA
From Astro white paper, “Quality and safety consideration in SRS/SBRT” 74

75. Quality Assurance of SRS Program: Patient QA
Checklist Example
From Astro white paper, “Quality and safety consideration in SRS/SBRT” 75

76. Quality Assurance of SRS Program: Patient QA
Checklist Example
From Astro white paper, “Quality and safety consideration in SRS/SBRT” 76

77. Quality Assurance of SRS Program: Patient QA
Checklist Example
From Astro white paper, “Quality and safety consideration in SRS/SBRT” 77

78. Quality Assurance of SRS Program: Patient QA
Checklist Example
From Astro white paper, “Quality and safety consideration in SRS/SBRT” 78

79. Quality Assurance of SRS Program: Patient QA
Checklist Example
From Astro white paper, “Quality and safety consideration in SRS/SBRT” 79

80. Quality Assurance of SRS Program: Patient QA
Checklist Example
From Astro white paper, “Quality and safety consideration in SRS/SBRT” 80

81. References:
“Stereotactic Radiosurgery”, Dr. Michael Schell, et al. AAPM TG-42 (report 54)
“ Quality and Safety Considerations in Stereotactic Radiosurgery and Stereotactic Body Radiation Therapy”, Dr. Timothy D. Solberg, et al. Practical Radiation Oncology: August (Astro white paper)
“Stereotactic body radiation therapy”, Dr. Stanley Benedict, et al. The report of AAPM Task Group 101
“Small fields and non-equilibrium condition photon beam dosimetry”, AAPM TG-155 (in progress)
“Intracranial stereotactic positioning systems”, AAPM TG-68
“Use of MRI in treatment planning and stereotactic procedures”, AAPM TG-117 (in progress)