title: A novel dose-based positioning method for CT image-guided proton therapy.
abstract: OBJECTIVE
  Proton dose distributions can potentially be altered by anatomical changes in the beam path despite perfect target alignment using traditional image guidance methods. In this simulation study, the authors explored the use of dosimetric factors instead of only anatomy to set up patients for proton therapy using in-room volumetric computed tomographic (CT) images.
  
  
  METHODS
  To simulate patient anatomy in a free-breathing treatment condition, weekly time-averaged four-dimensional CT data near the end of treatment for 15 lung cancer patients were used in this study for a dose-based isocenter shift method to correct dosimetric deviations without replanning. The isocenter shift was obtained using the traditional anatomy-based image guidance method as the starting position. Subsequent isocenter shifts were established based on dosimetric criteria using a fast dose approximation method. For each isocenter shift, doses were calculated every 2 mm up to ± 8 mm in each direction. The optimal dose alignment was obtained by imposing a target coverage constraint that at least 99% of the target would receive at least 95% of the prescribed dose and by minimizing the mean dose to the ipsilateral lung.
  
  
  RESULTS
  The authors found that 7 of 15 plans did not meet the target coverage constraint when using only the anatomy-based alignment. After the authors applied dose-based alignment, all met the target coverage constraint. For all but one case in which the target dose was met using both anatomy-based and dose-based alignment, the latter method was able to improve normal tissue sparing.
  
  
  CONCLUSIONS
  The authors demonstrated that a dose-based adjustment to the isocenter can improve target coverage and/or reduce dose to nearby normal tissue.
authors: Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, 1515 Holcombe Boulevard, Unit 94, Houston, Texas 77030, USA.
authors: Joey P
authors: JP
authors: Cheung
authors: Peter C
authors: PC
authors: Park
authors: Laurence E
authors: LE
authors: Court
authors: X
authors: X
authors: Ronald Zhu
authors: Rajat J
authors: RJ
authors: Kudchadker
authors: Steven J
authors: SJ
authors: Frank
authors: Lei
authors: L
authors: Dong
mesh_terms: D006801:Humans; D008175:Lung Neoplasms / Q000000981:diagnostic imaging / Q000532:radiotherapy; D061766:Proton Therapy / Q000379:methods*; D011829:Radiation Dosage; D011879:Radiotherapy Dosage; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D014057:Tomography, X-Ray Computed
references: Phys Med Biol. 2012 Jun 7;57(11):3555-69
references: 22588165
references: Int J Radiat Oncol Biol Phys. 2009 Jun 1;74(2):583-92
references: 19345518
references: Med Phys. 2009 Jan;36(1):149-63
references: 19235384
references: Int J Radiat Oncol Biol Phys. 2012 Jul 15;83(4):e557-62
references: 22632772
references: Med Phys. 2003 May;30(5):979-85
references: 12773007
references: Int J Radiat Oncol Biol Phys. 2007 Mar 1;67(3):906-14
references: 17293240
references: Int J Radiat Oncol Biol Phys. 2008 Sep 1;72(1):268-77
references: 18722277
references: Int J Radiat Oncol Biol Phys. 2008 Aug 1;71(5):1581-90
references: 18640501
references: Phys Med Biol. 1984 May;29(5):553-66
references: 6330772
references: Med Phys. 2003 Oct;30(10):2750-7
references: 14596313
references: Int J Radiat Oncol Biol Phys. 2010 Mar 1;76(3 Suppl):S70-6
references: 20171521
references: Int J Radiat Oncol Biol Phys. 2008 Sep 1;72(1):210-9
references: 18722272
references: Int J Radiat Oncol Biol Phys. 2012 Dec 1;84(5):1093-100
references: 22543217
references: Phys Med Biol. 2012 Nov 7;57(21):6807-26
references: 23032638
references: Int J Radiat Oncol Biol Phys. 2012 Feb 1;82(2):e329-36
references: 21703781
references: Radiother Oncol. 1999 Jun;51(3):257-71
references: 10435821
references: Int J Radiat Oncol Biol Phys. 2008 Dec 1;72(5):1385-95
references: 18486357
references: Med Phys. 2002 Oct;29(10):2305-18
references: 12408305
references: Radiat Oncol. 2009 Jan 27;4:4
references: 19173738
references: Int J Radiat Oncol Biol Phys. 2006 Jul 15;65(4):1087-96
references: 16682145
references: Phys Med Biol. 2005 Jun 21;50(12):2887-905
references: 15930609
references: Int J Radiat Oncol Biol Phys. 2012 Nov 1;84(3):e427-33
references: 22672753