TomoTherapy (Helical Radiotherapy)
Integrates a 6 MV linear accelerator into a CT-style rotating gantry. The patient couch moves continuously through the bore while the gantry rotates and the MLC modulates — delivering radiation helically, analogous to how CT acquires helically. Produces highly conformal dose distributions for long targets (craniospinal), large targets (total marrow, total body), and complex geometries.
What makes TomoTherapy distinct architecturally
- Ring gantry — closed torus, similar form factor to CT. Not a C-arm.
- Helical delivery — continuous couch motion + continuous gantry rotation + MLC modulation
- Integrated MVCT — megavoltage (treatment-beam-energy) CT for IGRT before each fraction. Lower soft-tissue contrast than kV CBCT but immune to most metal artifacts
- Binary MLC — 64-leaf MLC, each leaf either fully in or fully out (no intermediate positions). Modulation achieved by rapid leaf motion during rotation
- 6 MV FFF beam — only energy, no electrons
Clinical niches
TomoTherapy excels in specific geometries:
- Craniospinal — entire neuroaxis (brain + spinal cord) in one continuous target. Conventional linacs require multiple junction-matched fields; tomo delivers in one pass.
- Total marrow irradiation (TMI) — replaces total-body irradiation (TBI) in bone-marrow transplant conditioning. Conforms dose to marrow-containing bones, spares OARs.
- Total lymphoid irradiation
- Head and neck — long, complex targets with many adjacent OARs
- Large-field pediatric — neuroblastoma, Wilms tumor
For conventional shorter-field IMRT (prostate, breast, lung, single-site treatment), TomoTherapy's throughput is slower than a Varian/Elekta C-arm linac — not the best use of the platform.
Key specs
- Energy — 6 MV FFF
- Dose rate — ~850 cGy/min at isocenter
- Binary MLC — 64 leaves, 6.25 mm width at isocenter
- Ring rotation — 10–60 sec per full rotation (selectable)
- Bore diameter — 85 cm
- Field length — up to 135 cm continuous
- Couch motion — continuous through gantry
Systems
- TomoTherapy Hi-Art (legacy)
- Accuray Radixact (current generation)
Competitive positioning
vs Varian Halcyon — both are ring-gantry; Halcyon delivers IMRT/VMAT in conventional paradigm (not helical), simpler + faster for routine cases.
vs conventional linacs — TomoTherapy wins on long or complex-geometry targets; loses on throughput for routine fractionated IMRT.
Regulatory
Same as linac — state radiation facility license, QMP, RSO, TG-142 QA adapted for helical delivery.