Dosimetric Validation of Clinical Serial Tomotherapy Delivery

1
Dosimetric Validation of Clinical Serial
Tomotherapy Delivery

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• R. Sanford, Ph.D., M. Al-Ghazi, Ph.D., H. Chung,
  B.S., and R. Yakoob, M.S.
• Department of Radiation Oncology
• University of California, Irvine

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Introduction

• Due to complex nature of Serial Tomotherapy (ST),
  conventional dose verification procedures are
  impractical
• _at_ UCI treated over 130 patients with sites in
  head, neck, thorax, abdomen, and pelvis using
  NOMOS Corp. ST treatment planning and delivery
  system
• Incorporates computer-optimized 3D inverse
  planning (Peacock) and computer-controlled
  dynamic multi-leaf collimator with up to 40
  independent, intensity modulated beamlets during
  arc therapy (MIMiC)

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Delivery Equipment
MIMiC leaves in checker pattern
Arc 1
Arc 2
Intensity pattern in 10o increments
Varian 600C with NOMOS MIMiC multi-leaf
collimator. Crane used for precise couch
indexing (0.01mm)
Intensity pattern
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Introduction (continued)

• Any ST verification procedure is time consuming
  (1fx can take gt15 minutes)
• Several schemes have been described
• Limited to specific target sites (e.g. head/neck)
• TLDs (use of gt60) time consuming
• Gel dosimetry requires special equipment (e.g.
  MR or laser absorption imaging)
• Our validation procedure is generalized to all
  sites using common QA tools with a relatively low
  time demand
• Validated gt130 clinical ST patient treatments

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Materials

• NOMOS polystyrene cubic film phantom
• Calibrated 0.125 cm3 IC-10 ion chamber w/
  Keithley electrometer
• provides precise absolute dose measurements
• Kodak X-OMAT film
• provides accurate relative dose distributions
• Wellhofer film scanning equipment and software

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Materials (continued)
NOMOS film phantom with drilled insert
Phantom with ion chamber inserted
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Procedure

• Copy patient plan to phantom CTs using NOMOS
  software tool
• provides patient dose distribution within phantom
• Treat phantom to 1 fraction with ion chamber
  inserted _at_ known position
• precise setup and couch movements
• collect total charge and convert to dose
  (requires prior calibration of ion
  chamber/electrometer)

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Procedure (continued)

• Treat phantom to 2nd fraction with films inserted
• generally use 6 films (6mm spacing)
• scale MU/arc such that total dose 1Gy/fraction
• scan films using densitometer
• normalize scans to maximum gray scale and print
  out isodose contours to full scale

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Comparison of Measured and Calculated Doses

• Ion Chamber
• calculated dose at ion chamber location
• Directly compare measured and calculated doses
• Film
• Compare relative doses by overlaying
  corresponding measured and calculated dose
  distributions

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Results

• Used this technique to validate 133 patients

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Relative Dose Distribution Validations
1
Calculated (NOMOS)
Measured (film)
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Relative Dose Distribution Validations
2
Calculated (NOMOS)
Measured (film)
13
Relative Dose Distribution Validations
3
Calculated (NOMOS)
Measured (film)
14
Relative Dose Distribution Validations
4
Calculated (NOMOS)
Measured (film)
15
Relative Dose Distribution Validations
5
Calculated (NOMOS)
Measured (film)
16
Relative Dose Distribution Validations
6
Calculated (NOMOS)
Measured (film)
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Absolute Dose Validations
Entries 133 Mean 99.3 RMS 3.4
Gaussian Curve Fit Constant 15.8 Mean
98.8 Sigma 2.9
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Conclusions

• Combination of ion chamber and film dosimetries
  provides time/cost effective validation
• Procedure is generalized to targets in head,
  neck, thorax, abdomen, and pelvis
• Validated 133 patients
• spatial resolution is better than 3 mm in
  diameter
• errors in absolute dose can be detected w/in
  tolerance of 6 (2s at the 95 confidence level)