title: Treatment-planning study of prostate cancer intensity-modulated radiotherapy with a Varian Clinac operated without a flattening filter.
abstract: OBJECTIVE
  To assess the feasibility of intensity-modulated radiotherapy for prostate cancer using photon beams from an accelerator operated without a flattening filter; and to determine potential benefits and drawbacks of using unflattened beams for this type of treatment.
  
  
  METHODS
  Intensity-modulated radiotherapy plans were generated for 10 patients with early-stage prostate cancer. For each patient, four plans were generated: with and without the flattening filter, at 6 and 18 MV. The prescription dose was 75.6 Gy to 98% of the planning target volume. The number of beams, their orientations, and optimization constraints were the same for all plans. Plans were generated with Eclipse 8.0 (Varian Medical Systems).
  
  
  RESULTS
  All the plans developed with unflattened beams were clinically acceptable. In terms of patient dose distributions, plans with unflattened beams were similar to the corresponding plans with flattened beams. Plans with unflattened beams required fewer monitor units (MUs) per plan: on average, by a factor of 2.0 at 6 MV and 2.6 at 18 MV, assuming that removal of the flattening filter was not followed by recalibration of MUs.
  
  
  CONCLUSIONS
  Clinically acceptable intensity-modulated radiotherapy plans for prostate cancer can be developed with unflattened beams at both 6 and 18 MV. Dosimetrically, flattened and unflattened beams generated similar treatment plans. The plans with unflattened beams required substantially fewer MUs. The reduction in the number of MUs indicates corresponding reduction in beam-on time and in the amount of radiation outside the target.
authors.affiliation: Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston, TX 77030-4009, USA. onvassil@mdanderson.org
authors.forename: Oleg N
authors.initials: ON
authors.lastname: Vassiliev
authors.forename: Stephen F
authors.initials: SF
authors.lastname: Kry
authors.forename: Deborah A
authors.initials: DA
authors.lastname: Kuban
authors.forename: Mohammad
authors.initials: M
authors.lastname: Salehpour
authors.forename: Radhe
authors.initials: R
authors.lastname: Mohan
authors.forename: Uwe
authors.initials: U
authors.lastname: Titt
mesh_terms: D000465:Algorithms; D005240:Feasibility Studies; D005270:Femur Head / Q000528:radiation effects; D005374:Filtration / Q000295:instrumentation; D006801:Humans; D008297:Male; D010315:Particle Accelerators / Q000295:instrumentation; D017785:Photons / Q000627:therapeutic use*; D011471:Prostatic Neoplasms / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D011882:Radiotherapy, High-Energy / Q000295:instrumentation / Q000379:methods; D050397:Radiotherapy, Intensity-Modulated / Q000295:instrumentation / Q000379:methods*; D012007:Rectum / Q000528:radiation effects; D001743:Urinary Bladder / Q000528:radiation effects