title: Determining the optimal block margin on the planning target volume for extracranial stereotactic radiotherapy.
abstract: OBJECTIVE
  To determine the block margin that minimizes normal tissue irradiation outside of the planning target volume (PTV) for body stereotactic radiotherapy (Body-SRT) of lung and liver tumors.
  
  
  METHODS
  Representative patient cases of lung and liver tumors were chosen for analysis. A PTV was constructed for each case and plans were generated which employed an array of block margins ranging from -2.5 mm to 10 mm at isocenter. Plans were generated for cerrobend blocks and for a multileaf collimator. The prescription isodose coverage was renormalized for each case and dose-volume histograms (DVH) and normal tissue complication probabilities (NTCP) were determined for each plan.
  
  
  CONCLUSIONS
  For the cases studied, the optimal block margin was in the 0.0 mm range. The ranking of plans was identical for both dose-volume based and biological based criteria. The method of blocking had no significant effect on treatment plans. The use of narrow margins for Body-SRT results in normal tissue sparing and creates significant target dose inhomogeneity which may be beneficial for tumor control.
authors.affiliation: Department of Radiation Oncology, Medical College of Virginia Hospitals, Virginia Commonwealth University, Richmond 23298-0058, USA.
authors.forename: R M
authors.initials: RM
authors.lastname: Cardinale
authors.forename: Q
authors.initials: Q
authors.lastname: Wu
authors.forename: S H
authors.initials: SH
authors.lastname: Benedict
authors.forename: B D
authors.initials: BD
authors.lastname: Kavanagh
authors.forename: E
authors.initials: E
authors.lastname: Bump
authors.forename: R
authors.initials: R
authors.lastname: Mohan
mesh_terms: D000328:Adult; D006801:Humans; D008113:Liver Neoplasms / Q000601:surgery*; D008175:Lung Neoplasms / Q000601:surgery*; D055585:Physical Phenomena; D010825:Physics; D016634:Radiosurgery / Q000295:instrumentation / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*