Feasibility of Image-Guided SRS for Trigeminal Neuralgia with Novalis

Arizona’s First University.

Feasibility of Image-Guided SRS for Trigeminal Neuralgia with Novalis

Physicists (University of Arizona)RJ HamiltonCJ WatchmanJ GordonNJ Stanley

Radiation Oncologist (University of Arizona)B Stea

Neurosurgeon (Center for Neurosciences, Tucson, AZ)A Sanan

Feasibility of Image-Guided SRS forTrigeminal Neuralgia with Novalis

Accuracy of Frame-Based Novalis SRS for TGN

• Rahimian et al (J. Neurosurg S3 101:351-55, 2004)– Image Fusion 0.4 mm– MR 0.2 mm– Gantry 0.3 mm– Couch 0.6 mm– Overall 0.9 mm (0.6 mm w/o couch)

• Target position 0.2 mm• Laser 0.2 mm

• Phantom Radiographic Test 0.7 ± 0.1 mm

Frame-Based SRS for TGN

• Human Issues– Pins fix frame to skull

– Uncomfortable during treatment delivery too

– Patient remains in department for several hours waiting for treatment

– Difficult to abort procedure (machine problems)

– Scheduling: patient, neurosurgeon, radiation oncologist, physicist, treatment time, etc…

Frame-Based SRS for TGN

• Technical Issues– Frame-based systems require that the position of a

patient in the frame is unchanged from application of the pins through treatment completion

– The stereotactic frame (not the patient) is aligned with the machine isocenter for treatment

– Differential torque on the head ring deforms it

– Head ring is bolted to the table assembly, patient may be displaced relative to it.

Image-Based SRS for TGN

• Human Issues

– Plastic mask (BrainLab U-PLAST mask fixation system) helps patient remain still

– Scheduling: specialists (neurosurgeon, radiation oncologist) may work at their convenience, treatment time set for patient

Image-Based SRS for TGN

• Technical Issues

– Positioning accuracy • Depends on the accuracy of the registration of images

between the planning system (DRRs) and imaging system (ExacTrac)

• Depends on the calibration of the imaging system with the mechanical isocenter

– Variations of the position of the patient in the mask (stereotactic coordinate space) are not important provided that the imaging system can correct for it (ExacTrac X-Ray 6D and Robotics)

– Once positioned for treatment, the patient must remain fixed.

Feasibility of Image-Based SRS for TGN

• Demonstrate System Accuracy

– Anthropomorphic Phantom Experiment

• Demonstrate that variations of the patient position in the mask are not important

– Anthropomorphic Phantom Experiment

• Confirm for each patient that deviations of their head within the mask may be corrected by the ExacTrac X-ray 6D and robotics

– Perform planning CT scan plus two addition CT scans. The final scan is performed 1 hour prior to the scheduled treatment time.

• Confirm that the patient does not move during treatment

– Repeat the ExacTrac X-ray positioning 3-4 times (every 3rd couch position) during treatment delivery

Demonstrate System Accuracy

• Anthropomorphic Phantom – 0.25” BB embedded in head

– BrainLab U-PLAST mask constructed

– Axial CT scan (35 cm FOV 1.5mm slice) with BrainLab Head & Neck Localizer

– BrainScan Plan

– ExacTrac 6D X-Ray and Robotics

Anthropomorphic Phantom CT



Anthropomorphic Phantom Plan

Accuracy of Isocenter Determination

• Two planners contoured the BB in the planning system and then placed the isocenter at the center. Repeated the procedure 10 times each.

mean stdev mean stdev difference pixel size

AP -24.93 0.19 -24.65 0.22 -0.28 0.68Lateral -30.34 0.24 -29.93 0.25 -0.41 0.68Vertical 25.09 0.01 25.24 0.10 -0.15 1.50

Planner 1 Planner 2

Isocenter Coordinate Determination

Pixel SizePixel Size = 0.5 mm




0.5 mm

ExacTrac Calibration











Anthropomorphic Phantom Treatment

Head Holder



Select Isocenter


Automatic Preposition



Automatic Preposition Film Verification

Difference = ( 0.3, -0.2 ) = 0.4 mm

10 mm cone

BB shadow






Film Verification

Difference = ( 0.1, 0.1 ) = 0.1 mm

10 mm cone

BB shadow

Phantom Displacement Experiment

• Phantom moved by exactly 2.0 cm in all three directions as determined by a ruler

• Perform X-ray Alignment with ExacTrac

• Shifts Found by ExacTrac – AP 19.85 mm– Lateral 20.05 mm– Vertical 20.40 mm

Phantom Moved: Not in Mask !!!

Head Rotated


No Head Holder Head Lower and Rotated

Head Holder Removed


Position using standard procedures with array





Film Verification

Difference = ( 0.1, 0.1 ) = 0.1 mm

10 mm cone

BB shadow


Rotation too large for robotics


Uncorrected Rotations

• Isocenter position is still fine

• Uncorrected rotation changes geometry of beams relative to skull and therefore changes the calculated doses

Patient Repeat CT Scans

• Original CT scan includes mask fabrication

• Second CT scan scheduled between date of original scan and treatment day

• Third CT scan obtained one hour prior to scheduled treatment time

Patient Repeat CT Scans Results

• The mean observed isocenter displacements from repeat CT localizations in the mask

– AP 0.9 mm

– Lateral 0.6 mm

– Vertical 1.7 mm

Patient Motion During Treatment

• ExacTrac localization was preformed during treatment every three arcs and at the end of the procedure to evaluate the extent of patient movement in the mask during treatment.

Patient Motion During Treatment: Results

• The mean displacements from isocenter – AP 0.3 mm– Lateral 0.5mm– Vertical 0.3 mm

• The mean rotations were all less than 0.4 degrees.

Dosimetric Effects of Isocenter Misplacement

• Five patients treated to 90 Gy at the isocenter with a 4 mm cone

• Isocenters of original plans were displaced towards the brainstem by 0.5, 1.0, 1.5, and 2.0 mm (worst case scenarios).

• Maximum brainstem and CN5 doses computed

Isocenter Misplacement Results: CN5

Maximum CN5 Dose vs Isocenter Displacement Towards Brainstem

85

86

87

88

89

90

91

0 0.5 1 1.5 2

Displacement Towards Brainstem (mm)

Do

se (G

y)

P1

P2

P3

P4

P5

Isocenter Misplacement Results: Brainstem

Maximum Brainstem Dose vs Isocenter Displacement Towards Brainstem

0

30

60

0 0.5 1 1.5 2

Displacement Towards Brainstem (mm)

Do

se

(G

y)

P1

P2

P3

P4

P5

Conclusions

• Accuracy is better than image pixel size

• Mask reproducibility allows accurate positioning with ExacTrac 6D and Robotics

• Patients move minimally during treatment. Monitoring the position adds little to the overall treatment time and insures accuracy

• Patients tolerate the procedure well and have not experienced complications

Thank You!

Date post:	24-Jan-2016
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Feasibility of Image-Guided SRS for Trigeminal Neuralgia with Novalis

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