Radiation characteristics of helical tomotherapy

• Published in: Medical physics (Lancaster) Vol. 31; no. 2; pp. 396 - 404
• Main Authors: Jeraj, Robert, Mackie, Thomas R., Balog, John, Olivera, Gustavo, Pearson, Dave, Kapatoes, Jeff, Ruchala, Ken, Reckwerdt, Paul
• Format: Journal Article
• Language: English
• Published: United States American Association of Physicists in Medicine 01-02-2004
• Subjects: beam characteristics; collimators; electron beam applications; Electron beams; Electrons; helical tomotherapy; Humans; In field scattering; Intensity modulated radiation therapy; intensity modulation; Medical imaging; Monte Carlo Method; Monte Carlo methods; Monte Carlo simulations; Particle Accelerators; Photons; Radiation safety; radiation therapy; Radiation treatment; Radiotherapy Dosage; Radiotherapy Planning, Computer-Assisted - methods; Radiotherapy sources; Radiotherapy, Conformal - methods; safety; Sensitivity and Specificity; shielding; Software; Tomography - methods; helical tomotherapy; beam characteristics; Monte Carlo simulations
• ISSN: 0094-2405; 2473-4209
• DOI: 10.1118/1.1639148

Helical tomotherapy is a dedicated intensity modulated radiation therapy (IMRT) system with on-board imaging capability (MVCT) and therefore differs from conventional treatment units. Different design goals resulted in some distinctive radiation field characteristics. The most significant differences in the design are the lack of flattening filter, increased shielding of the collimators, treatment and imaging operation modes and narrow fan beam delivery. Radiation characteristics of the helical tomotherapy system, sensitivity studies of various incident electron beam parameters and radiation safety analyses are presented here. It was determined that the photon beam energy spectrum of helical tomotherapy is similar to that of more conventional radiation treatment units. The two operational modes of the system result in different nominal energies of the incident electron beam with approximately 6 MeV and 3.5 MeV in the treatment and imaging modes, respectively. The off-axis mean energy dependence is much lower than in conventional radiotherapy units with less than 5% variation across the field, which is the consequence of the absent flattening filter. For the same reason the transverse profile exhibits the characteristic conical shape resulting in a 2-fold increase of the beam intensity in the center. The radiation leakage outside the field was found to be negligible at less than 0.05% because of the increased shielding of the collimators. At this level the in-field scattering is a dominant source of the radiation outside the field and thus a narrow field treatment does not result in the increased leakage. The sensitivity studies showed increased sensitivity on the incident electron position because of the narrow fan beam delivery and high sensitivity on the incident electron energy, as common to other treatment systems. All in all, it was determined that helical tomotherapy is a system with some unique radiation characteristics, which have been to a large extent optimized for intensity modulated delivery.