title: Improved power steering with double and triple ring waveguide systems: the impact of the operating frequency.
abstract: BACKGROUND
  Regional hyperthermia systems with 3D power steering have been introduced to improve tumour temperatures. The 3D 70-MHz AMC-8 system has two rings of four waveguides. The aim of this study is to evaluate whether T(90) will improve by using a higher operating frequency and whether further improvement is possible by adding a third ring.
  
  
  METHODS
  Optimised specific absorption rate (SAR) distributions were evaluated for a centrally located target in tissue-equivalent phantoms, and temperature optimisation was performed for five cervical carcinoma patients with constraints to normal tissue temperatures. The resulting T(90) and the thermal iso-effect dose (i.e. the number of equivalent min at 43°C) were evaluated and compared to the 2D 70-MHz AMC-4 system with a single ring of four waveguides. FDTD simulations were performed at 2.5 × 2.5 × 5 mm(3) resolution. The applied frequencies were 70, 100, 120, 130, 140 and 150 MHz.
  
  
  RESULTS
  Optimised SAR distributions in phantoms showed an optimal SAR distribution at 140 MHz. For the patient simulations, an optimal increase in T(90) was observed at 130 MHz. For a two-ring system at 70 MHz the gain in T(90) was about 0.5°C compared to the AMC-4 system, averaged over the five patients. At 130 MHz the average gain in T(90) was ~1.5°C and ~2°C for a two and three-ring system, respectively. This implies an improvement of the thermal iso-effect dose with a factor ~12 and ~30, respectively.
  
  
  CONCLUSIONS
  Simulations showed that a 130-MHz two-ring waveguide system yields significantly higher tumour temperatures compared to 70-MHz single-ring and double-ring waveguide systems. Temperatures were further improved with a 130-MHz triple-ring system.
authors.affiliation: Department of Radiation Oncology, Academic Medical Center, University of Amsterdam, Meibergdreef 9, Amsterdam, The Netherlands. H.P.Kok@amc.uva.nl
authors.forename: H P
authors.initials: HP
authors.lastname: Kok
authors.forename: M
authors.initials: M
authors.lastname: de Greef
authors.forename: P P
authors.initials: PP
authors.lastname: Borsboom
authors.forename: A
authors.initials: A
authors.lastname: Bel
authors.forename: J
authors.initials: J
authors.lastname: Crezee
mesh_terms: D003198:Computer Simulation; D005260:Female; D006801:Humans; D006979:Hyperthermia, Induced / Q000295:instrumentation* / Q000379:methods*; D019047:Phantoms, Imaging; D013696:Temperature; D002583:Uterine Cervical Neoplasms / Q000532:radiotherapy*