title: Treatment planning for pulsed reduced dose-rate radiotherapy in helical tomotherapy.
abstract: OBJECTIVE
  Pulsed reduced dose-rate radiotherapy (PRDR) is a valuable method of reirradiation because of its potential to reduce late normal tissue toxicity while still yielding significant tumoricidal effect. A typical method using a conventional linear accelerator (linac) is to deliver a series of 20-cGy pulses separated by 3-min intervals to give an effective dose-rate of just under 7 cGy/min. Such a strategy is fraught with difficulties when attempted on a helical tomotherapy unit. We investigated various means to overcome this limitation.
  
  
  METHODS
  Phantom and patient cases were studied. Plans were generated with varying combinations of field width (FW), pitch, and modulation factor (MF) to administer 200 cGy per fraction to the planning target in eight subfractions, thereby mimicking the technique used on conventional linacs. Plans were compared using dose-volume histograms, homogeneity indices, conformation numbers, and treatment time. Plan delivery quality assurance was performed to assess deliverability.
  
  
  RESULTS
  It was observed that for helical tomotherapy, intrinsic limitations in leaf open time in the multileaf collimator deteriorate plan quality and deliverability substantially when attempting to deliver very low doses such as 20-40 cGy. The various permutations evaluated revealed that the combination of small FW (1.0 cm), small MF (1.3-1.5), and large pitch (∼0.86), along with the half-gantry-angle-blocked scheme, can generate clinically acceptable plans with acceptable delivery accuracy (±3%).
  
  
  CONCLUSIONS
  Pulsed reduced dose-rate radiotherapy can be accurately delivered using helical tomotherapy for tumor reirradiation when the appropriate combination of FW, MF, and pitch is used.
authors.affiliation: Department of Human Oncology, University of Wisconsin-Madison, Madison, WI, USA.
authors.forename: Yi
authors.initials: Y
authors.lastname: Rong
authors.forename: Bhudatt
authors.initials: B
authors.lastname: Paliwal
authors.forename: Steven P
authors.initials: SP
authors.lastname: Howard
authors.forename: James
authors.initials: J
authors.lastname: Welsh
mesh_terms: D000465:Algorithms; D001365:Axilla; D001479:Basal Ganglia; D001932:Brain Neoplasms / Q000532:radiotherapy; D019583:Dose Fractionation, Radiation; D005240:Feasibility Studies; D006321:Heart / Q000528:radiation effects; D006801:Humans; D008168:Lung / Q000528:radiation effects; D008545:Melanoma / Q000532:radiotherapy / Q000556:secondary; D009364:Neoplasm Recurrence, Local / Q000532:radiotherapy*; D010315:Particle Accelerators; D019047:Phantoms, Imaging; D011785:Quality Assurance, Health Care; D011832:Radiation Injuries / Q000517:prevention & control; D011836:Radiation Tolerance / Q000528:radiation effects; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods* / Q000592:standards; D019233:Retreatment / Q000379:methods; D013116:Spinal Cord / Q000528:radiation effects