A presentation by Paul Jursinic, PhD, West Michigan Cancer Center, at the Vision RT dinner at AAPM 2017.

1. Comparison of head
immobilization with a metal
frame and two different models
of face masks
Paul A. Jursinic, Ph.D.
West Michigan Cancer Center
Kalamazoo, MI

2. Subject of the Talk
• Describe three types of immobilization
• Our stereotactic clinical procedures
• Use of AlignRT to measure patient motion
• Motion with each type of immobilization
• Immobilization method used in our clinic

3. 4 pins into the skull
Head ring is mounted to a
3D positioner (pitch
and roll ±10º)
Fiducial plate establishes a
secondary coordinate
system for patient
positioning
Conventional method (BRW head frame)

4. Form fit to face
Open mask so that face can
be observed optically
Mask mounted to extension
plate and 3D positioner
(pitch and roll ±10º)
Patient’s face establishes a
primary coordinate
system for patient
positioning
Thermoplastic mask system
(Orfit Industries America,
Jericho, NY)

5. Two types of thermoplastic mask were used
Model 33759: 3-point Hybrid Open-face Mask
Model 33759/16MI/12MI+N: 3-point Hybrid Open-face Mask
with Nanor reinforcement
This mask has thicker plastic webbing
This mask can be reheated and reformed

6. Can you see me now?
A paradigm shift in the coordinate reference system
Immobilization
method
Coordinate system Order of reference
system
Head ring Fiducial plate Secondary
Open face mask Patient’s face Primary

7. Head ring
Fiducial plate
The fiducial plate has
six infrared emitters
Infrared
emitter

9. AlignRT pattern on the face mask
There are 20,000
optical points on the
patient’s face.
This allows precise
3D alignment

10. GateCT pod
Wide bore CT
Arrangement of
equipment in the
CT room

11. WMCC workflow for SRS CT
Fabricate mask
Evaluate motion with GateCT
Motion < 0.5 mm
< 0.5 mm
Finish CT
Increase target
margins
Reform mask
yes no
Finish CT

12. GateCT data screen
Monitor
point on
face
Motion of
face versus
time
Adequacy of the
mask is judged by
motion of the face
(anterior-posterior)
Generally < 0.5 mm

13. AlignRT pods
Three pods
compensate for
obstructed view
as the gantry
rotates
Arrangement of equipment in the treatment room

14. WMCC workflow for SRS treatment
Select patient file in AlignRT software
Preliminary align patient, roll and pitch,
with lead spheres and room lasers
Refine alignment with AlignRT reference
image from treatment planning
Do cone beam CT (CBCT)
Optical
space
X-ray
space

15. Lead spheres aligned to room lasers

16. WMCC workflow for SRS treatment
Adjust patient alignment based on CBCT
Capture a reference image with AlignRT
Monitor patient motion with AlignRT
X-ray
space
Optical
space

17. WMCC workflow for SRS treatment
Keep treating
Stop treatment
Motion < 0.5 mm
< 0.5 mm
Adjust patient alignment based
on AlignRT reference image
noyes
Resume treating

18. ( ) ( ) ( )222
rtltinfsuppostantvector −+−+−=
Patient motion
Vector is sampled every 0.2 s
A motion file for the entire treatment is generated by
special software available from VisionRT

19. Optical noise is interpreted as motion
by AlignRT
Test this by measuring a fixed object

20. Skull phantom
secured to treatment
couch
A fixed object
Skull
phantom
3D
positioner

21. Fixed skull phantom apparent-motion vector versus time

22. Histogram analysis of skull phantom apparent-motion vector
Average apparent-motion = 0.05 mm, 90% of time < 0.07 mm

23. Typical patient motion vector versus time (Nanor mask)
Noise

24. Histogram analysis of typical patient motion vector
Average motion = 0.33 mm, 90% of time < 0.47 mm
Apparent-motion due to noise is 7 fold smaller (0.05 mm)

25. Patient
Couch
angle,
º
Average
motion, mm
Motion for 90%
of the time, mm
1, arc 1 0 0.32 0.47
1, arc 2 90 0.22 0.34
1, arc 3 45 0.31 0.46
1, arc 4 315 0.21 0.32
Average 0.27±0.06 0.40±0.08
2, arc 1 0 0.52 0.73
2, arc 2 90 0.32 0.43
2, arc 3 45 0.31 0.42
2, arc 4 315 0.52 0.78
Average 0.42±0.12 0.59±0.19
3, arc 1 295 0.32 0.46
3, arc 2 340 0.29 0.43
3, arc 3 20 0.19 0.27
3, arc 4 65 0.33 0.47
Average 0.28±0.06 0.41±0.09
Histogram analysis of motion of three patients,
multiple arcs, Nanor reinforced masks
Patients have
different amount of
motion
Motion in the first arc
characterizes the
overall motion for of
a patient

26. Histogram analysis of motion of 52 patients
26 patients, Orfit mask,
model 33579
Average
motion,
mm
Motion for
90% of the
time, mm
Ave 0.34 0.51
Std dev 0.13 0.21
Coef var 37.9% 41.1%
Average
motion,
mm
Motion for
90% of the
time, mm
Ave 0.32 0.49
Std dev 0.18 0.30
Coef var 54.1% 61.9%
26 patients, Orfit mask,
Nanor reinforced
There is no significant difference between the two
types of masks

27. Histogram analysis of motion of two trigeminal
neuralgia patients, head frame immobilization
Patient
Couch
angle,º
Average
motion,
mm
Motion for 90%
of the time,
mm
1, arc 1 50 0.8 1.4
2, arc 1 50 0.6 0.9
2, arc 2 285 1.0 1.5
2, arc 3 80 1.2 1.7
2, arc 4 65 1.1 1.5
2, arc 5 270 0.9 1.4
Average, mm 0.93 1.40
Std dev, mm 0.22 0.27
Coef var, % 23.1 19.2
Average motion with a head ring is about 3 fold greater
than with an open face mask: 0.93 versus 0.32 mm

28. The ring can flex on
the cantilevered
mount
This causes target
motion:
Superior-inferior
Anterior-posterior

29. The plate can flex on
the positioner mount
This causes target
motion:
Anterior-posterior
Superior-inferior

30. Not all patients have < 0.5 mm motion
(When a good mask is not good enough)

31. A patient with significant breathing motion (Nanor
mask)

32. Average apparent-motion = 0.27 mm, 90% of time < 0.52 mm
A patient with significant breathing motion (Nanor
mask)

33. A Fourier analysis of the data indicates a periodicity of
18 cycles (breaths) per minute
A patient with significant breathing motion (Nanor
mask)

34. A patient with significant back pain (Nanor mask)

35. A patient with significant back pain (Nanor mask)
Average apparent-motion = 1.68 mm, 90% of time < 2.79 mm

36. A patient with significant back pain (Nanor mask)
• Tumor contour margins were 2 mm
• This patient was given pain medication
Motion decreased to < 1 mm
• Treatment proceeded with many pauses to correct patient position

37. Conclusions
1.Both types of open-face masks immobilize patients to ≤ 0.4
mm of motion. This is acceptable for stereotactic
radiosurgery.
2. The Nanor reinforced mask provides no benefit compared
to the model 33579 mask.
3. The head ring allows 3 fold more patient motion than an
open-face mask.
4. Open-face masks are preferable to the head ring for
immobilization of stereotactic radiosurgery patients.
5. The AlignRT optical guidance system is a useful tool for
monitoring motion of stereotactic radiosurgery patients.

38. Am I counting angels on the head of a pin ?