title: On the impact of dose rate variation upon RapidArc implementation of volumetric modulated are therapy.
abstract: OBJECTIVE
  A study was carried out to evaluate the robustness and mutual interplay of two variables concurring to generate modulation patterns of the RapidArc (RapidArc) implementation of volumetric modulated are therapy. Dose rate (DR) and gantry speed (GS) are free parameters optimized alongside field aperture shape by the RapidArc engine; however, they are limited by machine constraints and mutually compensate in order to deliver the proper MU/deg during the gantry rotation.
  
  
  METHODS
  Four test cases (one geometrical and three clinical) were selected and RapidArc plans were optimized using maximum allowed dose rates from 100 to 600 MU/min. The maximum gantry speed was fixed at 4.8 deg/s. Qualitative analysis of DR and GS patterns from these cases was summarized together with quantitative assessment of delivery parameters. Pretreatment quality assurance measurements and scoring of plan quality aimed to determine whether preferable initial conditions might be identified or the optimization engine might be invariant to those variables and capable of providing adequate plans independently from the limits applied.
  
  
  RESULTS
  The results of the study were: (i) High dynamic range in MU/deg is achievable across all dose rates by means of gantry speed modulation; (ii) there is a robust compensation mechanism between the two variables; (iii) from a machine delivery point-of-view, slightly improved accuracy is achieved when lower DRs are applied; however, this does not have practical consequences since measurements and plan evaluation showed a lack of clinically relevant deviation; and (iv) reduced total treatment time is a major advantage of high DR.
  
  
  CONCLUSIONS
  A trend toward improved plan quality for clinical cases was observed with high DR but cannot be generalized, due to the limited amount of cases investigated and the consequent limited significance of the observed differences. As a minimum benefit, the reduced total treatment time should be considered as well.
authors.affiliation: Medical Physics Unit, Oncology Institute of Southern Switzerland, CH-6500 Bellinzona, Switzerland. giorgia.nicolini@eoc.ch
authors.forename: Giorgia
authors.initials: G
authors.lastname: Nicolini
authors.forename: Alessandro
authors.initials: A
authors.lastname: Clivio
authors.forename: Luca
authors.initials: L
authors.lastname: Cozzi
authors.forename: Antonella
authors.initials: A
authors.lastname: Fogliata
authors.forename: Eugenio
authors.initials: E
authors.lastname: Vanetti
mesh_terms: D006801:Humans; D009369:Neoplasms / Q000532:radiotherapy; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D011881:Radiotherapy, Computer-Assisted / Q000379:methods*