title: Investigation of Dose Falloff for Intact Brain Metastases and Surgical Cavities Using Hypofractionated Volumetric Modulated Arc Radiotherapy.
abstract: BACKGROUND
  Intact brain metastases tend to be small and spherical compared to postsurgery brain cavities, which tend to be large and irregular shaped and, as a result, a challenge with respect to treatment planning. The purpose of the present study is to develop guidelines for normal brain tissue dose and to investigate whether there is a dependence on target type for patients treated with hypofractionated volumetric modulated arc radiotherapy (HF-VMAT).
  
  
  METHODS
  Treatment plans from a total of 100 patients and 136 targets (55 cavity and 81 intact) were retrospectively reviewed. All targets were treated with HF-VMAT with total doses ranging between 20 and 30 gray (Gy) in 5 fractions. All plans met institutional objectives for organ-at-risk constraints and were clinically delivered. Dose falloff was quantified using gradient index (GI) and distance between the 100% and 50% isodose lines (R50). Additionally, the dose to normal brain tissue (brain contour excluding all gross tumor or clinical target volumes) was assessed using volume receiving specific doses (Vx) where x ranged from 5 to 30 Gy. Best-fit curves using power law relationships of the form y = ax(b) were generated for GI, R50, and Vx (normal brain tissue) versus target volume.
  
  
  RESULTS
  There was a statistically significant difference in planning target volume (PTV) for cavities versus intact metastases with mean volumes of 37.8 cm(3) and 9.5 cm(3), respectively (P < .0001). The GI and R50 were statistically different: 3.4 and 9.8 mm for cavities versus 4.6 and 8.3 mm for intact metastases (P < .0001). The R50 increased with PTV with power law coefficients (a, b) = (6.3, 0.12) and (5.9, 0.15) for cavities and intact, respectively. GI decreased with PTV with coefficients (a, b) = (5.9, -0.18) and (5.7, -0.14) for cavities and intact, respectively. The normal brain tissue Vx also exhibited power law relationships with PTV for x = 20 to 28.8 Gy. In conclusion, target volume is the main predictor of dose falloff. The results of the present study can be used for determining target volume-based thresholds for dose falloff and normal brain tissue dose-volume constraints.
authors: Department of Medical Physics, Sunnybrook Odette Cancer Centre, Toronto, Canada Department of Radiation Oncology, University of Toronto, Toronto, Canada mark.ruschin@sunnybrook.ca.
authors: Mark
authors: M
authors: Ruschin
authors: Department of Medical Physics, Sunnybrook Odette Cancer Centre, Toronto, Canada Department of Radiation Oncology, University of Toronto, Toronto, Canada.
authors: Young
authors: Y
authors: Lee
authors: Department of Medical Physics, Sunnybrook Odette Cancer Centre, Toronto, Canada Department of Radiation Oncology, University of Toronto, Toronto, Canada.
authors: David
authors: D
authors: Beachey
authors: Department of Medical Physics, Sunnybrook Odette Cancer Centre, Toronto, Canada Department of Radiation Oncology, University of Toronto, Toronto, Canada.
authors: Collins
authors: C
authors: Yeboah
authors: Department of Medical Physics, Sunnybrook Odette Cancer Centre, Toronto, Canada.
authors: Matt
authors: M
authors: Wronski
authors: Department of Medical Physics, Sunnybrook Odette Cancer Centre, Toronto, Canada.
authors: Steven
authors: S
authors: Babic
authors: Department of Medical Physics, Sunnybrook Odette Cancer Centre, Toronto, Canada.
authors: Fiona
authors: F
authors: Lochray
authors: Department of Medical Physics, Sunnybrook Odette Cancer Centre, Toronto, Canada.
authors: Anula
authors: A
authors: Nico
authors: Department of Radiation Oncology, University of Toronto, Toronto, Canada Department of Radiation Oncology, Sunnybrook Odette Cancer Centre, Toronto, Canada.
authors: Luluel
authors: L
authors: Khan
authors: Department of Radiation Oncology, University of Toronto, Toronto, Canada Department of Radiation Oncology, Sunnybrook Odette Cancer Centre, Toronto, Canada.
authors: Hany
authors: H
authors: Soliman
authors: Department of Radiation Oncology, University of Toronto, Toronto, Canada Department of Radiation Oncology, Sunnybrook Odette Cancer Centre, Toronto, Canada.
authors: Arjun
authors: A
authors: Sahgal
mesh_terms: D001921:Brain / Q000473:pathology; D001932:Brain Neoplasms / Q000473:pathology / Q000556:secondary / Q000601:surgery*; D019583:Dose Fractionation, Radiation; D004307:Dose-Response Relationship, Radiation; D006801:Humans; D059351:Organ Sparing Treatments; D058958:Organs at Risk; D016634:Radiosurgery; D011880:Radiotherapy Planning, Computer-Assisted; D050397:Radiotherapy, Intensity-Modulated; D047368:Tumor Burden
keywords: brain; hypofractionated VMAT; stereotactic radiosurgery; treatment planning