title: Comparison of radiobiologic modeling for one- and two-isocenter dose distributions applied to ellipsoidal radiosurgery targets.
abstract: OBJECTIVE
  RTOG protocol 90-05 determined the maximum acutely tolerated dose of single-fraction radiosurgery in patients receiving prior fractionated large volume cranial irradiation. Data from 90-05 have suggested that patients treated with a gamma unit, compared to linac-based therapy, have a tumor control advantage and lower rates of severe complications. This study was performed to investigate the radiobiologic effect of using one vs. two isocenters in single-fraction radiosurgery of ellipsoidal targets.
  
  
  METHODS
  For a series of ellipsoidal targets that varied by volume and radiosensitivity, single and two-isocenter treatment plans were generated to approximate those typically employed for gamma unit and linac radiosurgery. Tumor control probabilities (TCP) and normal tissue complication probabilities (NTCP) were generated automatically by the treatment planning system based on established parameter values.
  
  
  RESULTS
  The modeling data showed that multiple-isocenter plans resulted in improved TCP with equivalent or lesser NTCP, particularly for larger, radioresistant targets. Multiple-isocenter plans reduce the amount of normal tissue that receives high dose. Also, areas within the tumor receive significantly higher doses than the prescription dose, which contributes to increased tumor cell inactivation.
  
  
  CONCLUSIONS
  For ellipsoidal targets, radiobiologic modeling data are consistent with the clinical findings of the RTOG 90-05 trial, as they predict improved outcome with a multiple-isocenter plan relative to a single-isocenter plan. The benefit is most apparent with increasing target volume and decreasing tumor radiosensitivity.
authors.affiliation: Department of Radiation Oncology, New England Medical Center and Tufts University School of Medicine, 750 Washington Street, Boston, MA 02111, USA. jmignano@lifespan.org
authors.forename: J E
authors.initials: JE
authors.lastname: Mignano
authors.forename: M J
authors.initials: MJ
authors.lastname: Engler
authors.forename: J S
authors.initials: JS
authors.lastname: Tsai
authors.forename: D E
authors.initials: DE
authors.lastname: Wazer
mesh_terms: D000465:Algorithms; D001932:Brain Neoplasms / Q000532:radiotherapy / Q000556:secondary / Q000601:surgery*; D016371:Cranial Irradiation; D006801:Humans; D008954:Models, Biological; D009364:Neoplasm Recurrence, Local / Q000601:surgery*; D011836:Radiation Tolerance; D011853:Radiobiology; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage