title: Intensity modulated proton therapy treatment planning using single-field optimization: the impact of monitor unit constraints on plan quality.
abstract: OBJECTIVE
  To investigate the effect of monitor unit (MU) constraints on the dose distribution created by intensity modulated proton therapy (IMPT) treatment planning using single-field optimization (SFO).
  
  
  METHODS
  Ninety-four energies between 72.5 and 221.8 MeV are available for scanning beam IMPT delivery at our institution. The minimum and maximum MUs for delivering each pencil beam (spot) are 0.005 and 0.04, respectively. These MU constraints are not considered during optimization by the treatment planning system; spots are converted to deliverable MUs during postprocessing. Treatment plans for delivering uniform doses to rectangular volumes with and without MU constraints were generated for different target doses, spot spacings, spread-out Bragg peak (SOBP) widths, and ranges in a homogeneous phantom. Four prostate cancer patients were planned with and without MU constraints using different spot spacings. Rounding errors were analyzed using an in-house software tool.
  
  
  RESULTS
  From the phantom study, the authors have found that both the number of spots that have rounding errors and the magnitude of the distortion of the dose distribution from the ideally optimized distribution increases as the field dose, spot spacing, and range decrease and as the SOBP width increases. From our study of patient plans, it is clear that as the spot spacing decreases the rounding error increases, and the dose coverage of the target volume becomes unacceptable for very small spot spacings.
  
  
  CONCLUSIONS
  Constraints on deliverable MU for each spot could create a significant distortion from the ideally optimized dose distributions for IMPT fields using SFO. To eliminate this problem, the treatment planning system should incorporate the MU constraints in the optimization process and the delivery system should reliably delivery smaller minimum MUs.
authors.affiliation: Department of Radiation Physics, The University of Texas M. D. Anderson Cancer Center, Houston, Texas 77030, USA. xrzhu@mdanderson.org
authors.forename: X R
authors.initials: XR
authors.lastname: Zhu
authors.forename: N
authors.initials: N
authors.lastname: Sahoo
authors.forename: X
authors.initials: X
authors.lastname: Zhang
authors.forename: D
authors.initials: D
authors.lastname: Robertson
authors.forename: H
authors.initials: H
authors.lastname: Li
authors.forename: S
authors.initials: S
authors.lastname: Choi
authors.forename: A K
authors.initials: AK
authors.lastname: Lee
authors.forename: M T
authors.initials: MT
authors.lastname: Gillin
mesh_terms: D016477:Artifacts; D004867:Equipment Design; D019544:Equipment Failure Analysis; D011785:Quality Assurance, Health Care / Q000379:methods*; D011874:Radiometry / Q000295:instrumentation* / Q000379:methods; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000295:instrumentation* / Q000379:methods; D020266:Radiotherapy, Conformal / Q000295:instrumentation* / Q000379:methods; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
references.citation: Med Phys. 2001 Mar;28(3):317-24
references.pmid: 11318312
references.citation: Med Phys. 2009 Sep;36(9):4068-83
references.pmid: 19810479
references.citation: Int J Radiat Oncol Biol Phys. 2006 Nov 1;66(3):883-91
references.pmid: 17011461
references.citation: Phys Med Biol. 1999 Jan;44(1):185-205
references.pmid: 10071883
references.citation: Med Phys. 2004 Nov;31(11):3150-7
references.pmid: 15587667
references.citation: Med Phys. 2002 Jun;29(6):1007-18
references.pmid: 12094970
references.citation: Radiat Oncol. 2007 Sep 06;2:33
references.pmid: 17822529
references.citation: Int J Radiat Oncol Biol Phys. 2007 Feb 1;67(2):620-9
references.pmid: 17236979
references.citation: Med Phys. 1998 May;25(5):656-61
references.pmid: 9608475
references.citation: Int J Radiat Oncol Biol Phys. 2001 Apr 1;49(5):1429-38
references.pmid: 11286851
references.citation: Phys Med Biol. 2008 Feb 21;53(4):1027-42
references.pmid: 18263956
references.citation: Med Phys. 2010 Jan;37(1):154-63
references.pmid: 20175477
references.citation: Int J Radiat Oncol Biol Phys. 2010 Oct 1;78(2):428-34
references.pmid: 20100641
references.citation: Med Phys. 1995 Jan;22(1):37-53
references.pmid: 7715569
references.citation: Phys Med Biol. 2008 Feb 21;53(4):1043-56
references.pmid: 18263957