Somvilai Mayurasakorn, MD.
Division of Therapeutic Radiology and Oncology,
Faculty of Medicine, Chiang Mai University
Machine characteristics
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6 MV accelerator
3.5 MV: MVCT
No flattening filter
Field Size in width: 40 cm
Treatment length: 160 cm
6 MV LINAC
64 leaf MLC
Pitch = Distance Traveled Per Rotation
Slice Width
MVCT detector array
Machine characteristics
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TOMO Binary multileaf collimator
Optical sensors and pneumatic driven
Binary 64 leaf MLC
MLC leaf width: 6.25 mm (ISO)
Field Size in length: 1, 2.5 and 5 cm
by a set of moveable jaws
100 times faster MLC Speed
MLC leaves that move at
250 cm/s to open or shut in
20 milliseconds
Machine characteristics
• Treatment is delivered through the full 360°
• This is approximated in the planning process
by 51 different beam directions
• With a spacing of just over 7°
• The rate of movement of the couch, relative to the
thickness of the fan beam, is referred to as the pitch
Allows treatment of very long
volumes without field junctions
Slice width
Helical Tomotherapy Process
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Imaging / Contouring
Planning / Optimizing
In-room megavoltage CT imaging
Image Registration (IGRT)
Treatment delivery
Adaptive RT:
Dose Guidance
(Recalculation / Reconstruction )
Dose Modification
Helical Tomotherapy
• The complex integrated technologies
• With multiprofessional team
Physicist/dosimetrist
Radiation oncologist
Radiation therapist
Radiographer
Helical Tomotherapy
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Clinical use : Rationale for patient selection
Treat patients who were considered likely to
benefit from
IGRT : to inform repositioning
IMRT : complex target volume shapes,
adjacent critical normal tissue structures
Combined IGRT/IMRT approach
University of Cambridge /Burnet et al. Clinical Oncology 2010;22:294–312
Helical Tomotherapy
University of Cambridge /Burnet et al. Clinical Oncology 2010;22:294–312
Helical Tomotherapy
Treatment planning
• Inverse planned IMRT concepts
• Dose volume constraints : typically same as
conventional IMRT planning
– For all target and normal tissue structures
Helical Tomotherapy
Treatment planning
Tomotherapy Planning Parameters
1. Field Size in length : Jaw size
– 5, 2.5 or 1 cm
2. Modulation factor
– ratio of maximum leaf opening time to mean
leaf opening time
3. Pitch
Somvilai Mayurasakorn, MD.
Division of Therapeutic Radiology and Oncology,
Faculty of Medicine, Chiang Mai University
Helical Tomotherapy
Tomotherapy Planning Parameters
• Smaller field widths, smaller pitches and
higher modulation factors
More conformal plans
But increased treatment time !!
Helical Tomotherapy
Image acquisition and matching
• MVCT fan-beam images
• In-plane resolution of 0.78 mm at the bore
center ; mean beam energy = 0.75 MeV
• Set up slices thickness of 2, 4 or 6 mm
– ‘fine’, ‘normal’ and ‘coarse’ settings
• The scan length is kept as short as possible
– always within the region of the PTV
Helical Tomotherapy
Image matching